Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar: "The main changes are: - Documentation updates, including fixes to the design-level requirements documentation and a fixed version of the design-level data-structure documentation. These fixes include removing cartoons and getting rid of the html/htmlx duplication. - Further improvements to the new-age expedited grace periods. - Miscellaneous fixes. - Torture-test changes, including a new rcuperf module for measuring RCU grace-period performance and scalability, which is useful for the expedited-grace-period changes" * 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (56 commits) rcutorture: Add boot-time adjustment of leaf fanout rcutorture: Add irqs-disabled test for call_rcu() rcutorture: Dump trace buffer upon shutdown rcutorture: Don't rebuild identical kernel rcutorture: Add OS-jitter capability documentation: Add documentation for RCU's major data structures rcutorture: Convert test duration to seconds early torture: Kill qemu, not parent process torture: Clarify refusal to run more than one torture test rcutorture: Consider FROZEN hotplug notifier transitions rcutorture: Remove redundant initialization to zero rcuperf: Do not wake up shutdown wait queue if "shutdown" is false. rcutorture: Add largish-system rcuperf scenario rcutorture: Avoid RCU CPU stall warning and RT throttling rcutorture: Add rcuperf holdoff boot parameter to reduce interference rcutorture: Make scripts analyze rcuperf trace data, if present rcutorture: Make rcuperf collect expedited event-trace data rcutorture: Print measure of batching efficiency rcutorture: Set rcuperf writer kthreads to real-time priority rcutorture: Bind rcuperf reader/writer kthreads to CPUs ...
author: Linus Torvalds <torvalds@linux-foundation.org> 2016-05-16 12:02:08 -0700
committer: Linus Torvalds <torvalds@linux-foundation.org> 2016-05-16 12:02:08 -0700
commit: a3871bd434cfc313cdcd1edc433507298f4aa928 (patch)
tree: e47f2796fb8381219aff3cec54328589c948d683
parent: 0052af4411b048eb6c0b0adb73d0fb4803ba1794 (diff)
parent: 41ed943d855c3fa2bf6fecc33250b294bd568b7a (diff)
download: linux-a3871bd434cfc313cdcd1edc433507298f4aa928.tar.bz2
47 files changed, 10417 insertions, 3693 deletions
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diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
new file mode 100644
index 000000000000..7eb47ac25ad7
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@@ -0,0 +1,1333 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+           <p>January 27, 2016</p>
+           <p>This article was contributed by Paul E.&nbsp;McKenney</p>
+
+<h3>Introduction</h3>
+
+This document describes RCU's major data structures and their relationship
+to each other.
+
+<ol>
+<li>	<a href="#Data-Structure Relationships">
+	Data-Structure Relationships</a>
+<li>	<a href="#The rcu_state Structure">
+	The <tt>rcu_state</tt> Structure</a>
+<li>	<a href="#The rcu_node Structure">
+	The <tt>rcu_node</tt> Structure</a>
+<li>	<a href="#The rcu_data Structure">
+	The <tt>rcu_data</tt> Structure</a>
+<li>	<a href="#The rcu_dynticks Structure">
+	The <tt>rcu_dynticks</tt> Structure</a>
+<li>	<a href="#The rcu_head Structure">
+	The <tt>rcu_head</tt> Structure</a>
+<li>	<a href="#RCU-Specific Fields in the task_struct Structure">
+	RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
+<li>	<a href="#Accessor Functions">
+	Accessor Functions</a>
+</ol>
+
+At the end we have the
+<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+
+<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
+
+<p>RCU is for all intents and purposes a large state machine, and its
+data structures maintain the state in such a way as to allow RCU readers
+to execute extremely quickly, while also processing the RCU grace periods
+requested by updaters in an efficient and extremely scalable fashion.
+The efficiency and scalability of RCU updaters is provided primarily
+by a combining tree, as shown below:
+
+</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
+
+</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
+containing a tree of <tt>rcu_node</tt> structures.
+Each leaf node of the <tt>rcu_node</tt> tree has up to 16
+<tt>rcu_data</tt> structures associated with it, so that there
+are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
+one for each possible CPU.
+This structure is adjusted at boot time, if needed, to handle the
+common case where <tt>nr_cpu_ids</tt> is much less than
+<tt>NR_CPUs</tt>.
+For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
+which results in a three-level <tt>rcu_node</tt> tree.
+If the actual hardware has only 16 CPUs, RCU will adjust itself
+at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
+
+</p><p>The purpose of this combining tree is to allow per-CPU events
+such as quiescent states, dyntick-idle transitions,
+and CPU hotplug operations to be processed efficiently
+and scalably.
+Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
+and other events are recorded by the leaf-level <tt>rcu_node</tt>
+structures.
+All of these events are combined at each level of the tree until finally
+grace periods are completed at the tree's root <tt>rcu_node</tt>
+structure.
+A grace period can be completed at the root once every CPU
+(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
+has passed through a quiescent state.
+Once a grace period has completed, record of that fact is propagated
+back down the tree.
+
+</p><p>As can be seen from the diagram, on a 64-bit system
+a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
+of 64 at the root and a fanout of 16 at the leaves.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why isn't the fanout at the leaves also 64?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because there are more types of events that affect the leaf-level
+	<tt>rcu_node</tt> structures than further up the tree.
+	Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
+	64, the contention on these structures' <tt>-&gt;structures</tt>
+	becomes excessive.
+	Experimentation on a wide variety of systems has shown that a fanout
+	of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
+	</font>
+
+	<p><font color="ffffff">Of course, further experience with
+	systems having hundreds or thousands of CPUs may demonstrate
+	that the fanout for the non-leaf <tt>rcu_node</tt> structures
+	must also be reduced.
+	Such reduction can be easily carried out when and if it proves
+	necessary.
+	In the meantime, if you are using such a system and running into
+	contention problems on the non-leaf <tt>rcu_node</tt> structures,
+	you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
+	parameter to reduce the non-leaf fanout as needed.
+	</font>
+
+	<p><font color="ffffff">Kernels built for systems with
+	strong NUMA characteristics might also need to adjust
+	<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
+	<tt>rcu_node</tt> structures align with hardware boundaries.
+	However, there has thus far been no need for this.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
+a 32-bit system), then RCU will automatically add more levels to the
+tree.
+For example, if you are crazy enough to build a 64-bit system with 65,536
+CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
+
+</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
+
+</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
+accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
+32-bit systems.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
+as small as 2 if you wish, which would permit only 16 CPUs, which
+is useful for testing.
+
+</p><p>This multi-level combining tree allows us to get most of the
+performance and scalability
+benefits of partitioning, even though RCU grace-period detection is
+inherently a global operation.
+The trick here is that only the last CPU to report a quiescent state
+into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
+structure at the next level up the tree.
+This means that at the leaf-level <tt>rcu_node</tt> structure, only
+one access out of sixteen will progress up the tree.
+For the internal <tt>rcu_node</tt> structures, the situation is even
+more extreme:  Only one access out of sixty-four will progress up
+the tree.
+Because the vast majority of the CPUs do not progress up the tree,
+the lock contention remains roughly constant up the tree.
+No matter how many CPUs there are in the system, at most 64 quiescent-state
+reports per grace period will progress all the way to the root
+<tt>rcu_node</tt> structure, thus ensuring that the lock contention
+on that root <tt>rcu_node</tt> structure remains acceptably low.
+
+</p><p>In effect, the combining tree acts like a big shock absorber,
+keeping lock contention under control at all tree levels regardless
+of the level of loading on the system.
+
+</p><p>The Linux kernel actually supports multiple flavors of RCU
+running concurrently, so RCU builds separate data structures for each
+flavor.
+For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
+rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
+
+</p><p>Energy efficiency is increasingly important, and for that
+reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
+turns off the scheduling-clock interrupts on idle CPUs, which in
+turn allows those CPUs to attain deeper sleep states and to consume
+less energy.
+CPUs whose scheduling-clock interrupts have been turned off are
+said to be in <i>dyntick-idle mode</i>.
+RCU must handle dyntick-idle CPUs specially
+because RCU would otherwise wake up each CPU on every grace period,
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
+RCU uses the <tt>rcu_dynticks</tt> structure to track
+which CPUs are in dyntick idle mode, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
+
+</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
+for all flavors of RCU.
+Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
+CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
+that <tt>rcu_dynticks</tt>, as shown in the figure.
+
+</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
+rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
+
+</p><p>RCU updaters wait for normal grace periods by registering
+RCU callbacks, either directly via <tt>call_rcu()</tt> and
+friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+there being a separate interface per flavor of RCU)
+or indirectly via <tt>synchronize_rcu()</tt> and friends.
+RCU callbacks are represented by <tt>rcu_head</tt> structures,
+which are queued on <tt>rcu_data</tt> structures while they are
+waiting for a grace period to elapse, as shown in the following figure:
+
+</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
+
+</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
+<tt>PREEMPT_RCU</tt>'s major data structures are related.
+Lesser data structures will be introduced with the algorithms that
+make use of them.
+
+</p><p>Note that each of the data structures in the above figure has
+its own synchronization:
+
+<p><ol>
+<li>	Each <tt>rcu_state</tt> structures has a lock and a mutex,
+	and some fields are protected by the corresponding root
+	<tt>rcu_node</tt> structure's lock.
+<li>	Each <tt>rcu_node</tt> structure has a spinlock.
+<li>	The fields in <tt>rcu_data</tt> are private to the corresponding
+	CPU, although a few can be read and written by other CPUs.
+<li>	Similarly, the fields in <tt>rcu_dynticks</tt> are private
+	to the corresponding CPU, although a few can be read by
+	other CPUs.
+</ol>
+
+<p>It is important to note that different data structures can have
+very different ideas about the state of RCU at any given time.
+For but one example, awareness of the start or end of a given RCU
+grace period propagates slowly through the data structures.
+This slow propagation is absolutely necessary for RCU to have good
+read-side performance.
+If this balkanized implementation seems foreign to you, one useful
+trick is to consider each instance of these data structures to be
+a different person, each having the usual slightly different
+view of reality.
+
+</p><p>The general role of each of these data structures is as
+follows:
+
+</p><ol>
+<li>	<tt>rcu_state</tt>:
+	This structure forms the interconnection between the
+	<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+	tracks grace periods, serves as short-term repository
+	for callbacks orphaned by CPU-hotplug events,
+	maintains <tt>rcu_barrier()</tt> state,
+	tracks expedited grace-period state,
+	and maintains state used to force quiescent states when
+	grace periods extend too long,
+<li>	<tt>rcu_node</tt>: This structure forms the combining
+	tree that propagates quiescent-state
+	information from the leaves to the root, and also propagates
+	grace-period information from the root to the leaves.
+	It provides local copies of the grace-period state in order
+	to allow this information to be accessed in a synchronized
+	manner without suffering the scalability limitations that
+	would otherwise be imposed by global locking.
+	In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
+	of tasks that have blocked while in their current
+	RCU read-side critical section.
+	In <tt>CONFIG_PREEMPT_RCU</tt> with
+	<tt>CONFIG_RCU_BOOST</tt>, it manages the
+	per-<tt>rcu_node</tt> priority-boosting
+	kernel threads (kthreads) and state.
+	Finally, it records CPU-hotplug state in order to determine
+	which CPUs should be ignored during a given grace period.
+<li>	<tt>rcu_data</tt>: This per-CPU structure is the
+	focus of quiescent-state detection and RCU callback queuing.
+	It also tracks its relationship to the corresponding leaf
+	<tt>rcu_node</tt> structure to allow more-efficient
+	propagation of quiescent states up the <tt>rcu_node</tt>
+	combining tree.
+	Like the <tt>rcu_node</tt> structure, it provides a local
+	copy of the grace-period information to allow for-free
+	synchronized
+	access to this information from the corresponding CPU.
+	Finally, this structure records past dyntick-idle state
+	for the corresponding CPU and also tracks statistics.
+<li>	<tt>rcu_dynticks</tt>:
+	This per-CPU structure tracks the current dyntick-idle
+	state for the corresponding CPU.
+	Unlike the other three structures, the <tt>rcu_dynticks</tt>
+	structure is not replicated per RCU flavor.
+<li>	<tt>rcu_head</tt>:
+	This structure represents RCU callbacks, and is the
+	only structure allocated and managed by RCU users.
+	The <tt>rcu_head</tt> structure is normally embedded
+	within the RCU-protected data structure.
+</ol>
+
+<p>If all you wanted from this article was a general notion of how
+RCU's data structures are related, you are done.
+Otherwise, each of the following sections give more details on
+the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
+and <tt>rcu_dynticks</tt> data structures.
+
+<h3><a name="The rcu_state Structure">
+The <tt>rcu_state</tt> Structure</a></h3>
+
+<p>The <tt>rcu_state</tt> structure is the base structure that
+represents a flavor of RCU.
+This structure forms the interconnection between the
+<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+tracks grace periods, contains the lock used to
+synchronize with CPU-hotplug events,
+and maintains state used to force quiescent states when
+grace periods extend too long,
+
+</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+The more specialized fields are covered in the discussion of their
+use.
+
+<h5>Relationship to rcu_node and rcu_data Structures</h5>
+
+This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node node[NUM_RCU_NODES];
+  2   struct rcu_node *level[NUM_RCU_LVLS + 1];
+  3   struct rcu_data __percpu *rda;
+</pre>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	You said that the <tt>rcu_node</tt> structures formed a tree,
+	but they are declared as a flat array!
+	What gives?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	The tree is laid out in the array.
+	The first node In the array is the head, the next set of nodes in the
+	array are children of the head node, and so on until the last set of
+	nodes in the array are the leaves.
+	</font>
+
+	<p><font color="ffffff">See the following diagrams to see how
+	this works.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>rcu_node</tt> tree is embedded into the
+<tt>-&gt;node[]</tt> array as shown in the following figure:
+
+</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
+
+</p><p>One interesting consequence of this mapping is that a
+breadth-first traversal of the tree is implemented as a simple
+linear scan of the array, which is in fact what the
+<tt>rcu_for_each_node_breadth_first()</tt> macro does.
+This macro is used at the beginning and ends of grace periods.
+
+</p><p>Each entry of the <tt>-&gt;level</tt> array references
+the first <tt>rcu_node</tt> structure on the corresponding level
+of the tree, for example, as shown below:
+
+</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
+
+</p><p>The zero<sup>th</sup> element of the array references the root
+<tt>rcu_node</tt> structure, the first element references the
+first child of the root <tt>rcu_node</tt>, and finally the second
+element references the first leaf <tt>rcu_node</tt> structure.
+
+</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
+rather than array-shaped, it is easy to draw a planar representation:
+
+</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
+
+</p><p>Finally, the <tt>-&gt;rda</tt> field references a per-CPU
+pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
+
+</p><p>All of these fields are constant once initialization is complete,
+and therefore need no protection.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gpnum;
+  2   unsigned long completed;
+</pre>
+
+<p>RCU grace periods are numbered, and
+the <tt>-&gt;gpnum</tt> field contains the number of the grace
+period that started most recently.
+The <tt>-&gt;completed</tt> field contains the number of the
+grace period that completed most recently.
+If the two fields are equal, the RCU grace period that most recently
+started has already completed, and therefore the corresponding
+flavor of RCU is idle.
+If <tt>-&gt;gpnum</tt> is one greater than <tt>-&gt;completed</tt>,
+then <tt>-&gt;gpnum</tt> gives the number of the current RCU
+grace period, which has not yet completed.
+Any other combination of values indicates that something is broken.
+These two fields are protected by the root <tt>rcu_node</tt>'s
+<tt>-&gt;lock</tt> field.
+
+</p><p>There are <tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt> fields
+in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
+as well.
+The fields in the <tt>rcu_state</tt> structure represent the
+most current values, and those of the other structures are compared
+in order to detect the start of a new grace period in a distributed
+fashion.
+The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
+(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
+
+<h5>Miscellaneous</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_max;
+  2   char abbr;
+  3   char *name;
+</pre>
+
+<p>The <tt>-&gt;gp_max</tt> field tracks the duration of the longest
+grace period in jiffies.
+It is protected by the root <tt>rcu_node</tt>'s <tt>-&gt;lock</tt>.
+
+<p>The <tt>-&gt;name</tt> field points to the name of the RCU flavor
+(for example, &ldquo;rcu_sched&rdquo;), and is constant.
+The <tt>-&gt;abbr</tt> field contains a one-character abbreviation,
+for example, &ldquo;s&rdquo; for RCU-sched.
+
+<h3><a name="The rcu_node Structure">
+The <tt>rcu_node</tt> Structure</a></h3>
+
+<p>The <tt>rcu_node</tt> structures form the combining
+tree that propagates quiescent-state
+information from the leaves to the root and also that propagates
+grace-period information from the root down to the leaves.
+They provides local copies of the grace-period state in order
+to allow this information to be accessed in a synchronized
+manner without suffering the scalability limitations that
+would otherwise be imposed by global locking.
+In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
+of tasks that have blocked while in their current
+RCU read-side critical section.
+In <tt>CONFIG_PREEMPT_RCU</tt> with
+<tt>CONFIG_RCU_BOOST</tt>, they manage the
+per-<tt>rcu_node</tt> priority-boosting
+kernel threads (kthreads) and state.
+Finally, they record CPU-hotplug state in order to determine
+which CPUs should be ignored during a given grace period.
+
+</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Combining Tree</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node *parent;
+  2   u8 level;
+  3   u8 grpnum;
+  4   unsigned long grpmask;
+  5   int grplo;
+  6   int grphi;
+</pre>
+
+<p>The <tt>-&gt;parent</tt> pointer references the <tt>rcu_node</tt>
+one level up in the tree, and is <tt>NULL</tt> for the root
+<tt>rcu_node</tt>.
+The RCU implementation makes heavy use of this field to push quiescent
+states up the tree.
+The <tt>-&gt;level</tt> field gives the level in the tree, with
+the root being at level zero, its children at level one, and so on.
+The <tt>-&gt;grpnum</tt> field gives this node's position within
+the children of its parent, so this number can range between 0 and 31
+on 32-bit systems and between 0 and 63 on 64-bit systems.
+The <tt>-&gt;level</tt> and <tt>-&gt;grpnum</tt> fields are
+used only during initialization and for tracing.
+The <tt>-&gt;grpmask</tt> field is the bitmask counterpart of
+<tt>-&gt;grpnum</tt>, and therefore always has exactly one bit set.
+This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
+structure in its parent's bitmasks, which are described later.
+Finally, the <tt>-&gt;grplo</tt> and <tt>-&gt;grphi</tt> fields
+contain the lowest and highest numbered CPU served by this
+<tt>rcu_node</tt> structure, respectively.
+
+</p><p>All of these fields are constant, and thus do not require any
+synchronization.
+
+<h5>Synchronization</h5>
+
+<p>This field of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   raw_spinlock_t lock;
+</pre>
+
+<p>This field is used to protect the remaining fields in this structure,
+unless otherwise stated.
+That said, all of the fields in this structure can be accessed without
+locking for tracing purposes.
+Yes, this can result in confusing traces, but better some tracing confusion
+than to be heisenbugged out of existence.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gpnum;
+  2   unsigned long completed;
+</pre>
+
+<p>These fields are the counterparts of the fields of the same name in
+the <tt>rcu_state</tt> structure.
+They each may lag up to one behind their <tt>rcu_state</tt>
+counterparts.
+If a given <tt>rcu_node</tt> structure's <tt>-&gt;gpnum</tt> and
+<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_node</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> structure,
+the <tt>-&gt;gpnum</tt> field will be one greater than the
+<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
+indicating which grace period this <tt>rcu_node</tt> believes
+is still being waited for.
+
+</p><p>The <tt>&gt;gpnum</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning
+of each grace period, and the <tt>-&gt;completed</tt> fields are
+updated at the end of each grace period.
+
+<h5>Quiescent-State Tracking</h5>
+
+<p>These fields manage the propagation of quiescent states up the
+combining tree.
+
+</p><p>This portion of the <tt>rcu_node</tt> structure has fields
+as follows:
+
+<pre>
+  1   unsigned long qsmask;
+  2   unsigned long expmask;
+  3   unsigned long qsmaskinit;
+  4   unsigned long expmaskinit;
+</pre>
+
+<p>The <tt>-&gt;qsmask</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current normal grace period.
+Such children will have a value of 1 in their corresponding bit.
+Note that the leaf <tt>rcu_node</tt> structures should be
+thought of as having <tt>rcu_data</tt> structures as their
+children.
+Similarly, the <tt>-&gt;expmask</tt> field tracks which
+of this <tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current expedited grace period.
+An expedited grace period has
+the same conceptual properties as a normal grace period, but the
+expedited implementation accepts extreme CPU overhead to obtain
+much lower grace-period latency, for example, consuming a few
+tens of microseconds worth of CPU time to reduce grace-period
+duration from milliseconds to tens of microseconds.
+The <tt>-&gt;qsmaskinit</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children cover for at least
+one online CPU.
+This mask is used to initialize <tt>-&gt;qsmask</tt>,
+and <tt>-&gt;expmaskinit</tt> is used to initialize
+<tt>-&gt;expmask</tt> and the beginning of the
+normal and expedited grace periods, respectively.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why are these bitmasks protected by locking?
+	Come on, haven't you heard of atomic instructions???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Lockless grace-period computation!  Such a tantalizing possibility!
+	</font>
+
+	<p><font color="ffffff">But consider the following sequence of events:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">CPU&nbsp;0 has been in dyntick-idle
+		mode for quite some time.
+		When it wakes up, it notices that the current RCU
+		grace period needs it to report in, so it sets a
+		flag where the scheduling clock interrupt will find it.
+		</font><p>
+	<li>	<font color="ffffff">Meanwhile, CPU&nbsp;1 is running
+		<tt>force_quiescent_state()</tt>,
+		and notices that CPU&nbsp;0 has been in dyntick idle mode,
+		which qualifies as an extended quiescent state.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0's scheduling clock
+		interrupt fires in the
+		middle of an RCU read-side critical section, and notices
+		that the RCU core needs something, so commences RCU softirq
+		processing.
+		</font>
+		<p>
+	<li>	<font color="ffffff">CPU&nbsp;0's softirq handler
+		executes and is just about ready
+		to report its quiescent state up the <tt>rcu_node</tt>
+		tree.
+		</font><p>
+	<li>	<font color="ffffff">But CPU&nbsp;1 beats it to the punch,
+		completing the current
+		grace period and starting a new one.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0 now reports its quiescent
+		state for the wrong
+		grace period.
+		That grace period might now end before the RCU read-side
+		critical section.
+		If that happens, disaster will ensue.
+		</font>
+	</ol>
+
+	<p><font color="ffffff">So the locking is absolutely required in
+	order to coordinate
+	clearing of the bits with the grace-period numbers in
+	<tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h5>Blocked-Task Management</h5>
+
+<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
+midst of their RCU read-side critical sections, and these tasks
+must be tracked explicitly.
+The details of exactly why and how they are tracked will be covered
+in a separate article on RCU read-side processing.
+For now, it is enough to know that the <tt>rcu_node</tt>
+structure tracks them.
+
+<pre>
+  1   struct list_head blkd_tasks;
+  2   struct list_head *gp_tasks;
+  3   struct list_head *exp_tasks;
+  4   bool wait_blkd_tasks;
+</pre>
+
+<p>The <tt>-&gt;blkd_tasks</tt> field is a list header for
+the list of blocked and preempted tasks.
+As tasks undergo context switches within RCU read-side critical
+sections, their <tt>task_struct</tt> structures are enqueued
+(via the <tt>task_struct</tt>'s <tt>-&gt;rcu_node_entry</tt>
+field) onto the head of the <tt>-&gt;blkd_tasks</tt> list for the
+leaf <tt>rcu_node</tt> structure corresponding to the CPU
+on which the outgoing context switch executed.
+As these tasks later exit their RCU read-side critical sections,
+they remove themselves from the list.
+This list is therefore in reverse time order, so that if one of the tasks
+is blocking the current grace period, all subsequent tasks must
+also be blocking that same grace period.
+Therefore, a single pointer into this list suffices to track
+all tasks blocking a given grace period.
+That pointer is stored in <tt>-&gt;gp_tasks</tt> for normal
+grace periods and in <tt>-&gt;exp_tasks</tt> for expedited
+grace periods.
+These last two fields are <tt>NULL</tt> if either there is
+no grace period in flight or if there are no blocked tasks
+preventing that grace period from completing.
+If either of these two pointers is referencing a task that
+removes itself from the <tt>-&gt;blkd_tasks</tt> list,
+then that task must advance the pointer to the next task on
+the list, or set the pointer to <tt>NULL</tt> if there
+are no subsequent tasks on the list.
+
+</p><p>For example, suppose that tasks&nbsp;T1, T2, and&nbsp;T3 are
+all hard-affinitied to the largest-numbered CPU in the system.
+Then if task&nbsp;T1 blocked in an RCU read-side
+critical section, then an expedited grace period started,
+then task&nbsp;T2 blocked in an RCU read-side critical section,
+then a normal grace period started, and finally task&nbsp;3 blocked
+in an RCU read-side critical section, then the state of the
+last leaf <tt>rcu_node</tt> structure's blocked-task list
+would be as shown below:
+
+</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
+
+</p><p>Task&nbsp;T1 is blocking both grace periods, task&nbsp;T2 is
+blocking only the normal grace period, and task&nbsp;T3 is blocking
+neither grace period.
+Note that these tasks will not remove themselves from this list
+immediately upon resuming execution.
+They will instead remain on the list until they execute the outermost
+<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
+section.
+
+<p>
+The <tt>-&gt;wait_blkd_tasks</tt> field indicates whether or not
+the current grace period is waiting on a blocked task.
+
+<h5>Sizing the <tt>rcu_node</tt> Array</h5>
+
+<p>The <tt>rcu_node</tt> array is sized via a series of
+C-preprocessor expressions as follows:
+
+<pre>
+ 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1        (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2        (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3        (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4        (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS &lt;= RCU_FANOUT_1
+27 #  define RCU_NUM_LVLS        1
+28 #  define NUM_RCU_LVL_0        1
+29 #  define NUM_RCU_NODES        NUM_RCU_LVL_0
+30 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
+31 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
+32 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
+33 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
+34 #elif NR_CPUS &lt;= RCU_FANOUT_2
+35 #  define RCU_NUM_LVLS        2
+36 #  define NUM_RCU_LVL_0        1
+37 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
+41 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS &lt;= RCU_FANOUT_3
+44 #  define RCU_NUM_LVLS        3
+45 #  define NUM_RCU_LVL_0        1
+46 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS &lt;= RCU_FANOUT_4
+54 #  define RCU_NUM_LVLS        4
+55 #  define NUM_RCU_LVL_0        1
+56 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 #  define NUM_RCU_LVL_3        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+</pre>
+
+<p>The maximum number of levels in the <tt>rcu_node</tt> structure
+is currently limited to four, as specified by lines&nbsp;21-24
+and the structure of the subsequent &ldquo;if&rdquo; statement.
+For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
+should be sufficient for the next few years at least.
+For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
+should see us through the next decade or so.
+This four-level tree also allows kernels built with
+<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
+which might be useful in very large systems having eight CPUs per
+socket (but please note that no one has yet shown any measurable
+performance degradation due to misaligned socket and <tt>rcu_node</tt>
+boundaries).
+In addition, building kernels with a full four levels of <tt>rcu_node</tt>
+tree permits better testing of RCU's combining-tree code.
+
+</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
+are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
+If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
+it is set based on the word size of the system, which is also
+the Kconfig default.
+
+</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
+handled by each leaf <tt>rcu_node</tt> structure.
+Experience has shown that allowing a given leaf <tt>rcu_node</tt>
+structure to handle 64 CPUs, as permitted by the number of bits in
+the <tt>-&gt;qsmask</tt> field on a 64-bit system, results in
+excessive contention for the leaf <tt>rcu_node</tt> structures'
+<tt>-&gt;lock</tt> fields.
+The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
+limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
+If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
+selected is based on the word size of the system, just as for
+<tt>CONFIG_RCU_FANOUT</tt>.
+Lines&nbsp;11-19 perform this computation.
+
+</p><p>Lines&nbsp;21-24 compute the maximum number of CPUs supported by
+a single-level (which contains a single <tt>rcu_node</tt> structure),
+two-level, three-level, and four-level <tt>rcu_node</tt> tree,
+respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
+and <tt>RCU_FANOUT_LEAF</tt>.
+These numbers of CPUs are retained in the
+<tt>RCU_FANOUT_1</tt>,
+<tt>RCU_FANOUT_2</tt>,
+<tt>RCU_FANOUT_3</tt>, and
+<tt>RCU_FANOUT_4</tt>
+C-preprocessor variables, respectively.
+
+</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
+statement spanning lines&nbsp;26-66 that computes the number of
+<tt>rcu_node</tt> structures required for each level of the tree,
+as well as the number of levels required.
+The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
+C-preprocessor variable by lines&nbsp;27, 35, 44, and&nbsp;54.
+The number of <tt>rcu_node</tt> structures for the topmost level
+of the tree is always exactly one, and this value is unconditionally
+placed into <tt>NUM_RCU_LVL_0</tt> by lines&nbsp;28, 36, 45, and&nbsp;55.
+The rest of the levels (if any) of the <tt>rcu_node</tt> tree
+are computed by dividing the maximum number of CPUs by the
+fanout supported by the number of levels from the current level down,
+rounding up.  This computation is performed by lines&nbsp;37,
+46-47, and&nbsp;56-58.
+Lines&nbsp;31-33, 40-42, 50-52, and&nbsp;62-63 create initializers
+for lockdep lock-class names.
+Finally, lines&nbsp;64-66 produce an error if the maximum number of
+CPUs is too large for the specified fanout.
+
+<h3><a name="The rcu_data Structure">
+The <tt>rcu_data</tt> Structure</a></h3>
+
+<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
+corresponding flavor of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+This structure is the
+focus of quiescent-state detection and RCU callback queuing.
+It also tracks its relationship to the corresponding leaf
+<tt>rcu_node</tt> structure to allow more-efficient
+propagation of quiescent states up the <tt>rcu_node</tt>
+combining tree.
+Like the <tt>rcu_node</tt> structure, it provides a local
+copy of the grace-period information to allow for-free
+synchronized
+access to this information from the corresponding CPU.
+Finally, this structure records past dyntick-idle state
+for the corresponding CPU and also tracks statistics.
+
+</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Other Data Structures</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int cpu;
+  2   struct rcu_state *rsp;
+  3   struct rcu_node *mynode;
+  4   struct rcu_dynticks *dynticks;
+  5   unsigned long grpmask;
+  6   bool beenonline;
+</pre>
+
+<p>The <tt>-&gt;cpu</tt> field contains the number of the
+corresponding CPU, the <tt>-&gt;rsp</tt> pointer references
+the corresponding <tt>rcu_state</tt> structure (and is most frequently
+used to locate the name of the corresponding flavor of RCU for tracing),
+and the <tt>-&gt;mynode</tt> field references the corresponding
+<tt>rcu_node</tt> structure.
+The <tt>-&gt;mynode</tt> is used to propagate quiescent states
+up the combining tree.
+<p>The <tt>-&gt;dynticks</tt> pointer references the
+<tt>rcu_dynticks</tt> structure corresponding to this
+CPU.
+Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
+structure is shared among all flavors of RCU.
+These first four fields are constant and therefore require not
+synchronization.
+
+</p><p>The <tt>-&gt;grpmask</tt> field indicates the bit in
+the <tt>-&gt;mynode-&gt;qsmask</tt> corresponding to this
+<tt>rcu_data</tt> structure, and is also used when propagating
+quiescent states.
+The <tt>-&gt;beenonline</tt> flag is set whenever the corresponding
+CPU comes online, which means that the debugfs tracing need not dump
+out any <tt>rcu_data</tt> structure for which this flag is not set.
+
+<h5>Quiescent-State and Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long completed;
+  2   unsigned long gpnum;
+  3   bool cpu_no_qs;
+  4   bool core_needs_qs;
+  5   bool gpwrap;
+  6   unsigned long rcu_qs_ctr_snap;
+</pre>
+
+<p>The <tt>completed</tt> and <tt>gpnum</tt>
+fields are the counterparts of the fields of the same name
+in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
+They may each lag up to one behind their <tt>rcu_node</tt>
+counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
+<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
+arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
+will catch up upon exit from dyntick-idle mode).
+If a given <tt>rcu_data</tt> structure's <tt>-&gt;gpnum</tt> and
+<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_data</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
+structure,
+the <tt>-&gt;gpnum</tt> field will be one greater than the
+<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
+indicating which grace period this <tt>rcu_data</tt> believes
+is still being waited for.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	All this replication of the grace period numbers can only cause
+	massive confusion.
+	Why not just keep a global pair of counters and be done with it???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because if there was only a single global pair of grace-period
+	numbers, there would need to be a single global lock to allow
+	safely accessing and updating them.
+	And if we are not going to have a single global lock, we need
+	to carefully manage the numbers on a per-node basis.
+	Recall from the answer to a previous Quick Quiz that the consequences
+	of applying a previously sampled quiescent state to the wrong
+	grace period are quite severe.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>-&gt;cpu_no_qs</tt> flag indicates that the
+CPU has not yet passed through a quiescent state,
+while the <tt>-&gt;core_needs_qs</tt> flag indicates that the
+RCU core needs a quiescent state from the corresponding CPU.
+The <tt>-&gt;gpwrap</tt> field indicates that the corresponding
+CPU has remained idle for so long that the <tt>completed</tt>
+and <tt>gpnum</tt> counters are in danger of overflow, which
+will cause the CPU to disregard the values of its counters on
+its next exit from idle.
+Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
+cases where a given operation has resulted in a quiescent state
+for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>.
+
+<h5>RCU Callback Handling</h5>
+
+<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
+the same CPU that registered them.
+This is strictly a cache-locality optimization: callbacks can and
+do get invoked on CPUs other than the one that registered them.
+After all, if the CPU that registered a given callback has gone
+offline before the callback can be invoked, there really is no other
+choice.
+
+</p><p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_head *nxtlist;
+ 2 struct rcu_head **nxttail[RCU_NEXT_SIZE];
+ 3 unsigned long nxtcompleted[RCU_NEXT_SIZE];
+ 4 long qlen_lazy;
+ 5 long qlen;
+ 6 long qlen_last_fqs_check;
+ 7 unsigned long n_force_qs_snap;
+ 8 unsigned long n_cbs_invoked;
+ 9 unsigned long n_cbs_orphaned;
+10 unsigned long n_cbs_adopted;
+11 long blimit;
+</pre>
+
+<p>The <tt>-&gt;nxtlist</tt> pointer and the
+<tt>-&gt;nxttail[]</tt> array form a four-segment list with
+older callbacks near the head and newer ones near the tail.
+Each segment contains callbacks with the corresponding relationship
+to the current grace period.
+The pointer out of the end of each of the four segments is referenced
+by the element of the <tt>-&gt;nxttail[]</tt> array indexed by
+<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period),
+<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period),
+<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next
+grace period), and
+<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated
+with a specific grace period)
+respectively, as shown in the following figure.
+
+</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
+
+</p><p>In this figure, the <tt>-&gt;nxtlist</tt> pointer references the
+first
+RCU callback in the list.
+The <tt>-&gt;nxttail[RCU_DONE_TAIL]</tt> array element references
+the <tt>-&gt;nxtlist</tt> pointer itself, indicating that none
+of the callbacks is ready to invoke.
+The <tt>-&gt;nxttail[RCU_WAIT_TAIL]</tt> array element references callback
+CB&nbsp;2's <tt>-&gt;next</tt> pointer, which indicates that
+CB&nbsp;1 and CB&nbsp;2 are both waiting on the current grace period.
+The <tt>-&gt;nxttail[RCU_NEXT_READY_TAIL]</tt> array element
+references the same RCU callback that <tt>-&gt;nxttail[RCU_WAIT_TAIL]</tt>
+does, which indicates that there are no callbacks waiting on the next
+RCU grace period.
+The <tt>-&gt;nxttail[RCU_NEXT_TAIL]</tt> array element references
+CB&nbsp;4's <tt>-&gt;next</tt> pointer, indicating that all the
+remaining RCU callbacks have not yet been assigned to an RCU grace
+period.
+Note that the <tt>-&gt;nxttail[RCU_NEXT_TAIL]</tt> array element
+always references the last RCU callback's <tt>-&gt;next</tt> pointer
+unless the callback list is empty, in which case it references
+the <tt>-&gt;nxtlist</tt> pointer.
+
+</p><p>CPUs advance their callbacks from the
+<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
+<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
+as grace periods advance.
+The CPU advances the callbacks in its <tt>rcu_data</tt> structure
+whenever it notices that another RCU grace period has completed.
+The CPU detects the completion of an RCU grace period by noticing
+that the value of its <tt>rcu_data</tt> structure's
+<tt>-&gt;completed</tt> field differs from that of its leaf
+<tt>rcu_node</tt> structure.
+Recall that each <tt>rcu_node</tt> structure's
+<tt>-&gt;completed</tt> field is updated at the end of each
+grace period.
+
+</p><p>The <tt>-&gt;nxtcompleted[]</tt> array records grace-period
+numbers corresponding to the list segments.
+This allows CPUs that go idle for extended periods to determine
+which of their callbacks are ready to be invoked after reawakening.
+
+</p><p>The <tt>-&gt;qlen</tt> counter contains the number of
+callbacks in <tt>-&gt;nxtlist</tt>, and the
+<tt>-&gt;qlen_lazy</tt> contains the number of those callbacks that
+are known to only free memory, and whose invocation can therefore
+be safely deferred.
+The <tt>-&gt;qlen_last_fqs_check</tt> and
+<tt>-&gt;n_force_qs_snap</tt> coordinate the forcing of quiescent
+states from <tt>call_rcu()</tt> and friends when callback
+lists grow excessively long.
+
+</p><p>The <tt>-&gt;n_cbs_invoked</tt>,
+<tt>-&gt;n_cbs_orphaned</tt>, and <tt>-&gt;n_cbs_adopted</tt>
+fields count the number of callbacks invoked,
+sent to other CPUs when this CPU goes offline,
+and received from other CPUs when those other CPUs go offline.
+Finally, the <tt>-&gt;blimit</tt> counter is the maximum number of
+RCU callbacks that may be invoked at a given time.
+
+<h5>Dyntick-Idle Handling</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int dynticks_snap;
+  2   unsigned long dynticks_fqs;
+</pre>
+
+The <tt>-&gt;dynticks_snap</tt> field is used to take a snapshot
+of the corresponding CPU's dyntick-idle state when forcing
+quiescent states, and is therefore accessed from other CPUs.
+Finally, the <tt>-&gt;dynticks_fqs</tt> field is used to
+count the number of times this CPU is determined to be in
+dyntick-idle state, and is used for tracing and debugging purposes.
+
+<h3><a name="The rcu_dynticks Structure">
+The <tt>rcu_dynticks</tt> Structure</a></h3>
+
+<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
+for the corresponding CPU.
+Unlike the other structures, <tt>rcu_dynticks</tt> is not
+replicated over the different flavors of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+Its fields are as follows:
+
+<pre>
+  1   int dynticks_nesting;
+  2   int dynticks_nmi_nesting;
+  3   atomic_t dynticks;
+</pre>
+
+<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
+nesting depth of normal interrupts.
+In addition, this counter is incremented when exiting dyntick-idle
+mode and decremented when entering it.
+This counter can therefore be thought of as counting the number
+of reasons why this CPU cannot be permitted to enter dyntick-idle
+mode, aside from non-maskable interrupts (NMIs).
+NMIs are counted by the <tt>-&gt;dynticks_nmi_nesting</tt>
+field, except that NMIs that interrupt non-dyntick-idle execution
+are not counted.
+
+</p><p>Finally, the <tt>-&gt;dynticks</tt> field counts the corresponding
+CPU's transitions to and from dyntick-idle mode, so that this counter
+has an even value when the CPU is in dyntick-idle mode and an odd
+value otherwise.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why not just count all NMIs?
+	Wouldn't that be simpler and less error prone?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	It seems simpler only until you think hard about how to go about
+	updating the <tt>rcu_dynticks</tt> structure's
+	<tt>-&gt;dynticks</tt> field.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>Additional fields are present for some special-purpose
+builds, and are discussed separately.
+
+<h3><a name="The rcu_head Structure">
+The <tt>rcu_head</tt> Structure</a></h3>
+
+<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
+These structures are normally embedded within RCU-protected data
+structures whose algorithms use asynchronous grace periods.
+In contrast, when using algorithms that block waiting for RCU grace periods,
+RCU users need not provide <tt>rcu_head</tt> structures.
+
+</p><p>The <tt>rcu_head</tt> structure has fields as follows:
+
+<pre>
+  1   struct rcu_head *next;
+  2   void (*func)(struct rcu_head *head);
+</pre>
+
+<p>The <tt>-&gt;next</tt> field is used
+to link the <tt>rcu_head</tt> structures together in the
+lists within the <tt>rcu_data</tt> structures.
+The <tt>-&gt;func</tt> field is a pointer to the function
+to be called when the callback is ready to be invoked, and
+this function is passed a pointer to the <tt>rcu_head</tt>
+structure.
+However, <tt>kfree_rcu()</tt> uses the <tt>-&gt;func</tt>
+field to record the offset of the <tt>rcu_head</tt>
+structure within the enclosing RCU-protected data structure.
+
+</p><p>Both of these fields are used internally by RCU.
+From the viewpoint of RCU users, this structure is an
+opaque &ldquo;cookie&rdquo;.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Given that the callback function <tt>-&gt;func</tt>
+	is passed a pointer to the <tt>rcu_head</tt> structure,
+	how is that function supposed to find the beginning of the
+	enclosing RCU-protected data structure?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In actual practice, there is a separate callback function per
+	type of RCU-protected data structure.
+	The callback function can therefore use the <tt>container_of()</tt>
+	macro in the Linux kernel (or other pointer-manipulation facilities
+	in other software environments) to find the beginning of the
+	enclosing structure.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="RCU-Specific Fields in the task_struct Structure">
+RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
+
+<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
+additional fields in the <tt>task_struct</tt> structure:
+
+<pre>
+ 1 #ifdef CONFIG_PREEMPT_RCU
+ 2   int rcu_read_lock_nesting;
+ 3   union rcu_special rcu_read_unlock_special;
+ 4   struct list_head rcu_node_entry;
+ 5   struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8   unsigned long rcu_tasks_nvcsw;
+ 9   bool rcu_tasks_holdout;
+10   struct list_head rcu_tasks_holdout_list;
+11   int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+</pre>
+
+<p>The <tt>-&gt;rcu_read_lock_nesting</tt> field records the
+nesting level for RCU read-side critical sections, and
+the <tt>-&gt;rcu_read_unlock_special</tt> field is a bitmask
+that records special conditions that require <tt>rcu_read_unlock()</tt>
+to do additional work.
+The <tt>-&gt;rcu_node_entry</tt> field is used to form lists of
+tasks that have blocked within preemptible-RCU read-side critical
+sections and the <tt>-&gt;rcu_blocked_node</tt> field references
+the <tt>rcu_node</tt> structure whose list this task is a member of,
+or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
+read-side critical section.
+
+<p>The <tt>-&gt;rcu_tasks_nvcsw</tt> field tracks the number of
+voluntary context switches that this task had undergone at the
+beginning of the current tasks-RCU grace period,
+<tt>-&gt;rcu_tasks_holdout</tt> is set if the current tasks-RCU
+grace period is waiting on this task, <tt>-&gt;rcu_tasks_holdout_list</tt>
+is a list element enqueuing this task on the holdout list,
+and <tt>-&gt;rcu_tasks_idle_cpu</tt> tracks which CPU this
+idle task is running, but only if the task is currently running,
+that is, if the CPU is currently idle.
+
+<h3><a name="Accessor Functions">
+Accessor Functions</a></h3>
+
+<p>The following listing shows the
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_for_each_leaf_node()</tt> function and macros:
+
+<pre>
+  1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+  2 {
+  3   return &amp;rsp-&gt;node[0];
+  4 }
+  5
+  6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+  7   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+  8        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+  9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+ 12        (rnp) &lt; (rsp)-&gt;level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15   for ((rnp) = (rsp)-&gt;level[NUM_RCU_LVLS - 1]; \
+ 16        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+</pre>
+
+<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
+first element of the specified <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, which is the root <tt>rcu_node</tt>
+structure.
+
+</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
+macro takes advantage of the layout of the <tt>rcu_node</tt>
+structures in the <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, performing a breadth-first traversal by
+simply traversing the array in order.
+The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
+similarly, but traverses only the first part of the array, thus excluding
+the leaf <tt>rcu_node</tt> structures.
+Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+the last part of the array, thus traversing only the leaf
+<tt>rcu_node</tt> structures.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+	<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+	contains only a single node?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In the single-node case,
+	<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
+	and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
+which contains a combining tree of <tt>rcu_node</tt> and
+<tt>rcu_data</tt> structures.
+Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
+state is tracked by an <tt>rcu_dynticks</tt> structure.
+
+If you made it this far, you are well prepared to read the code
+walkthroughs in the other articles in this series.
+
+<h3><a name="Acknowledgments">
+Acknowledgments</a></h3>
+
+I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
+Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
+for helping me get this document into a more human-readable state.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+</body></html>
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+++ /dev/null
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deleted file mode 100644
index ebcbeee391ed..000000000000
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-       rx="0"
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-       id="rect12" />
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="900"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text14"
-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3017">Read-Mostly, Stale &amp;</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="1350"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
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-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3019">Inconsistent Data OK</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="1800"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text18"
-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3021">(RCU Works Great!!!)</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="3825"
-       font-style="normal"
-       font-weight="normal"
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-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3023">(RCU Works Well)</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="3375"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text22"
-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3025">Read-Mostly, Need Consistent Data</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="5175"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text24"
-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3027">Read-Write, Need Consistent Data</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="6975"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text26"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-       sodipodi:linespacing="125%">Update-Mostly, Need Consistent Data</text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="5625"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text28"
-       sodipodi:linespacing="125%"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
-         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-         id="tspan3029">(RCU Might Be OK...)</tspan></text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="7875"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text30"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-       sodipodi:linespacing="125%">(1) Provide Existence Guarantees For Update-Friendly Mechanisms</text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="8325"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text32"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-       sodipodi:linespacing="125%">(2) Provide Wait-Free Read-Side Primitives for Real-Time Use)</text>
-    <!-- Text -->
-    <text
-       xml:space="preserve"
-       x="7200"
-       y="7425"
-       font-style="normal"
-       font-weight="normal"
-       font-size="324"
-       id="text34"
-       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
-       sodipodi:linespacing="125%">(RCU is Very Unlikely to be the Right Tool For The Job, But it Can:</text>
-  </g>
-</svg>
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index a725f9900ec8..e7e24b3e86e2 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1,5 +1,3 @@
-<!-- DO NOT HAND EDIT. -->
-<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' -->
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
         "http://www.w3.org/TR/html4/loose.dtd">
         <html>
@@ -65,8 +63,8 @@ All that aside, here are the categories of currently known RCU requirements:
 
 <p>
 This is followed by a <a href="#Summary">summary</a>,
-which is in turn followed by the inevitable
-<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+however, the answers to each quick quiz immediately follows the quiz.
+Select the big white space with your mouse to see the answer.
 
 <h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
 
@@ -153,13 +151,27 @@ Therefore, the outcome:
 </blockquote>
 cannot happen.
 
-<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a>
-Wait a minute!
-You said that updaters can make useful forward progress concurrently
-with readers, but pre-existing readers will block
-<tt>synchronize_rcu()</tt>!!!
-Just who are you trying to fool???
-<br><a href="#qq1answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	You said that updaters can make useful forward progress concurrently
+	with readers, but pre-existing readers will block
+	<tt>synchronize_rcu()</tt>!!!
+	Just who are you trying to fool???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	First, if updaters do not wish to be blocked by readers, they can use
+	<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
+	be discussed later.
+	Second, even when using <tt>synchronize_rcu()</tt>, the other
+	update-side code does run concurrently with readers, whether
+	pre-existing or not.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 This scenario resembles one of the first uses of RCU in
@@ -210,9 +222,20 @@ to guarantee that <tt>do_something()</tt> never runs concurrently
 with <tt>recovery()</tt>, but with little or no synchronization
 overhead in <tt>do_something_dlm()</tt>.
 
-<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a>
-Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
-<br><a href="#qq2answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Without that extra grace period, memory reordering could result in
+	<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
+	concurrently with the last bits of <tt>recovery()</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 In order to avoid fatal problems such as deadlocks,
@@ -332,12 +355,27 @@ It also prevents any number of &ldquo;interesting&rdquo; compiler
 optimizations, for example, the use of <tt>gp</tt> as a scratch
 location immediately preceding the assignment.
 
-<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a>
-But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
-two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
-from being reordered.
-Can't that also cause problems?
-<br><a href="#qq3answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
+	two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
+	from being reordered.
+	Can't that also cause problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, it cannot.
+	The readers cannot see either of these two fields until
+	the assignment to <tt>gp</tt>, by which time both fields are
+	fully initialized.
+	So reordering the assignments
+	to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
+	cause any problems.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 It is tempting to assume that the reader need not do anything special
@@ -494,11 +532,42 @@ The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
 	code protected by the corresponding update-side lock.
 </ol>
 
-<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a>
-Without the <tt>rcu_dereference()</tt> or the
-<tt>rcu_access_pointer()</tt>, what destructive optimizations
-might the compiler make use of?
-<br><a href="#qq4answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Without the <tt>rcu_dereference()</tt> or the
+	<tt>rcu_access_pointer()</tt>, what destructive optimizations
+	might the compiler make use of?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Let's start with what happens to <tt>do_something_gp()</tt>
+	if it fails to use <tt>rcu_dereference()</tt>.
+	It could reuse a value formerly fetched from this same pointer.
+	It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
+	manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
+	mash-up of two distince pointer values.
+	It might even use value-speculation optimizations, where it makes
+	a wrong guess, but by the time it gets around to checking the
+	value, an update has changed the pointer to match the wrong guess.
+	Too bad about any dereferences that returned pre-initialization garbage
+	in the meantime!
+	</font>
+
+	<p><font color="ffffff">
+	For <tt>remove_gp_synchronous()</tt>, as long as all modifications
+	to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
+	the above optimizations are harmless.
+	However,
+	with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
+	<tt>sparse</tt> will complain if you
+	define <tt>gp</tt> with <tt>__rcu</tt> and then
+	access it without using
+	either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 In short, RCU's publish-subscribe guarantee is provided by the combination
@@ -571,17 +640,156 @@ systems with more than one CPU:
 	<tt>synchronize_rcu()</tt> migrates in the meantime.
 </ol>
 
-<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a>
-Given that multiple CPUs can start RCU read-side critical sections
-at any time without any ordering whatsoever, how can RCU possibly tell whether
-or not a given RCU read-side critical section starts before a
-given instance of <tt>synchronize_rcu()</tt>?
-<br><a href="#qq5answer">Answer</a>
-
-<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a>
-The first and second guarantees require unbelievably strict ordering!
-Are all these memory barriers <i> really</i> required?
-<br><a href="#qq6answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Given that multiple CPUs can start RCU read-side critical sections
+	at any time without any ordering whatsoever, how can RCU possibly
+	tell whether or not a given RCU read-side critical section starts
+	before a given instance of <tt>synchronize_rcu()</tt>?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	If RCU cannot tell whether or not a given
+	RCU read-side critical section starts before a
+	given instance of <tt>synchronize_rcu()</tt>,
+	then it must assume that the RCU read-side critical section
+	started first.
+	In other words, a given instance of <tt>synchronize_rcu()</tt>
+	can avoid waiting on a given RCU read-side critical section only
+	if it can prove that <tt>synchronize_rcu()</tt> started first.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	The first and second guarantees require unbelievably strict ordering!
+	Are all these memory barriers <i> really</i> required?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Yes, they really are required.
+	To see why the first guarantee is required, consider the following
+	sequence of events:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>rcu_read_lock()</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>q = rcu_dereference(gp);
+		/* Very likely to return p. */</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>list_del_rcu(p);</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>synchronize_rcu()</tt> starts.
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>do_something_with(q-&gt;a);
+		/* No smp_mb(), so might happen after kfree(). */</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>rcu_read_unlock()</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>synchronize_rcu()</tt> returns.
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>kfree(p);</tt>
+		</font>
+	</ol>
+
+	<p><font color="ffffff">
+	Therefore, there absolutely must be a full memory barrier between the
+	end of the RCU read-side critical section and the end of the
+	grace period.
+	</font>
+
+	<p><font color="ffffff">
+	The sequence of events demonstrating the necessity of the second rule
+	is roughly similar:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">CPU 0: <tt>list_del_rcu(p);</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> starts.
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>rcu_read_lock()</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>q = rcu_dereference(gp);
+		/* Might return p if no memory barrier. */</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> returns.
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>kfree(p);</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>rcu_read_unlock()</tt>
+		</font>
+	</ol>
+
+	<p><font color="ffffff">
+	And similarly, without a memory barrier between the beginning of the
+	grace period and the beginning of the RCU read-side critical section,
+	CPU&nbsp;1 might end up accessing the freelist.
+	</font>
+
+	<p><font color="ffffff">
+	The &ldquo;as if&rdquo; rule of course applies, so that any
+	implementation that acts as if the appropriate memory barriers
+	were in place is a correct implementation.
+	That said, it is much easier to fool yourself into believing
+	that you have adhered to the as-if rule than it is to actually
+	adhere to it!
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+	generate absolutely no code in some kernel builds.
+	This means that the compiler might arbitrarily rearrange consecutive
+	RCU read-side critical sections.
+	Given such rearrangement, if a given RCU read-side critical section
+	is done, how can you be sure that all prior RCU read-side critical
+	sections are done?
+	Won't the compiler rearrangements make that impossible to determine?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+	generate absolutely no code, RCU infers quiescent states only at
+	special locations, for example, within the scheduler.
+	Because calls to <tt>schedule()</tt> had better prevent calling-code
+	accesses to shared variables from being rearranged across the call to
+	<tt>schedule()</tt>, if RCU detects the end of a given RCU read-side
+	critical section, it will necessarily detect the end of all prior
+	RCU read-side critical sections, no matter how aggressively the
+	compiler scrambles the code.
+	</font>
+
+	<p><font color="ffffff">
+	Again, this all assumes that the compiler cannot scramble code across
+	calls to the scheduler, out of interrupt handlers, into the idle loop,
+	into user-mode code, and so on.
+	But if your kernel build allows that sort of scrambling, you have broken
+	far more than just RCU!
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 Note that these memory-barrier requirements do not replace the fundamental
@@ -626,9 +834,19 @@ inconvenience can be avoided through use of the
 <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
 described later in this document.
 
-<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
-But how does the upgrade-to-write operation exclude other readers?
-<br><a href="#qq7answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But how does the upgrade-to-write operation exclude other readers?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	It doesn't, just like normal RCU updates, which also do not exclude
+	RCU readers.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 This guarantee allows lookup code to be shared between read-side
@@ -714,9 +932,20 @@ to do significant reordering.
 This is by design:  Any significant ordering constraints would slow down
 these fast-path APIs.
 
-<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
-Can't the compiler also reorder this code?
-<br><a href="#qq8answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Can't the compiler also reorder this code?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, the volatile casts in <tt>READ_ONCE()</tt> and
+	<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
+	this particular case.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
 
@@ -769,10 +998,28 @@ new readers can start immediately after <tt>synchronize_rcu()</tt>
 starts, and <tt>synchronize_rcu()</tt> is under no
 obligation to wait for these new readers.
 
-<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
-Suppose that synchronize_rcu() did wait until all readers had completed.
-Would the updater be able to rely on this?
-<br><a href="#qq9answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Suppose that synchronize_rcu() did wait until <i>all</i>
+	readers had completed instead of waiting only on
+	pre-existing readers.
+	For how long would the updater be able to rely on there
+	being no readers?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	For no time at all.
+	Even if <tt>synchronize_rcu()</tt> were to wait until
+	all readers had completed, a new reader might start immediately after
+	<tt>synchronize_rcu()</tt> completed.
+	Therefore, the code following
+	<tt>synchronize_rcu()</tt> can <i>never</i> rely on there being
+	no readers.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
 Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
@@ -969,11 +1216,24 @@ grace period.
 As a result, an RCU read-side critical section cannot partition a pair
 of RCU grace periods.
 
-<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a>
-How long a sequence of grace periods, each separated by an RCU read-side
-critical section, would be required to partition the RCU read-side
-critical sections at the beginning and end of the chain?
-<br><a href="#qq10answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	How long a sequence of grace periods, each separated by an RCU
+	read-side critical section, would be required to partition the RCU
+	read-side critical sections at the beginning and end of the chain?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In theory, an infinite number.
+	In practice, an unknown number that is sensitive to both implementation
+	details and timing considerations.
+	Therefore, even in practice, RCU users must abide by the
+	theoretical rather than the practical answer.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <h3><a name="Disabling Preemption Does Not Block Grace Periods">
 Disabling Preemption Does Not Block Grace Periods</a></h3>
@@ -1109,12 +1369,27 @@ These classes is covered in the following sections.
 <h3><a name="Specialization">Specialization</a></h3>
 
 <p>
-RCU is and always has been intended primarily for read-mostly situations, as
-illustrated by the following figure.
-This means that RCU's read-side primitives are optimized, often at the
+RCU is and always has been intended primarily for read-mostly situations,
+which means that RCU's read-side primitives are optimized, often at the
 expense of its update-side primitives.
+Experience thus far is captured by the following list of situations:
 
-<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
+<ol>
+<li>	Read-mostly data, where stale and inconsistent data is not
+	a problem:   RCU works great!
+<li>	Read-mostly data, where data must be consistent:
+	RCU works well.
+<li>	Read-write data, where data must be consistent:
+	RCU <i>might</i> work OK.
+	Or not.
+<li>	Write-mostly data, where data must be consistent:
+	RCU is very unlikely to be the right tool for the job,
+	with the following exceptions, where RCU can provide:
+	<ol type=a>
+	<li>	Existence guarantees for update-friendly mechanisms.
+	<li>	Wait-free read-side primitives for real-time use.
+	</ol>
+</ol>
 
 <p>
 This focus on read-mostly situations means that RCU must interoperate
@@ -1127,9 +1402,43 @@ synchronization primitives be legal within RCU read-side critical sections,
 including spinlocks, sequence locks, atomic operations, reference
 counters, and memory barriers.
 
-<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a>
-What about sleeping locks?
-<br><a href="#qq11answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What about sleeping locks?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	These are forbidden within Linux-kernel RCU read-side critical
+	sections because it is not legal to place a quiescent state
+	(in this case, voluntary context switch) within an RCU read-side
+	critical section.
+	However, sleeping locks may be used within userspace RCU read-side
+	critical sections, and also within Linux-kernel sleepable RCU
+	<a href="#Sleepable RCU"><font color="ffffff">(SRCU)</font></a>
+	read-side critical sections.
+	In addition, the -rt patchset turns spinlocks into a
+	sleeping locks so that the corresponding critical sections
+	can be preempted, which also means that these sleeplockified
+	spinlocks (but not other sleeping locks!)  may be acquire within
+	-rt-Linux-kernel RCU read-side critical sections.
+	</font>
+
+	<p><font color="ffffff">
+	Note that it <i>is</i> legal for a normal RCU read-side
+	critical section to conditionally acquire a sleeping locks
+	(as in <tt>mutex_trylock()</tt>), but only as long as it does
+	not loop indefinitely attempting to conditionally acquire that
+	sleeping locks.
+	The key point is that things like <tt>mutex_trylock()</tt>
+	either return with the mutex held, or return an error indication if
+	the mutex was not immediately available.
+	Either way, <tt>mutex_trylock()</tt> returns immediately without
+	sleeping.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 It often comes as a surprise that many algorithms do not require a
@@ -1160,10 +1469,7 @@ some period of time, so the exact wait period is a judgment call.
 One of our pair of veternarians might wait 30 seconds before pronouncing
 the cat dead, while the other might insist on waiting a full minute.
 The two veternarians would then disagree on the state of the cat during
-the final 30 seconds of the minute following the last heartbeat, as
-fancifully illustrated below:
-
-<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
+the final 30 seconds of the minute following the last heartbeat.
 
 <p>
 Interestingly enough, this same situation applies to hardware.
@@ -1343,7 +1649,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor
 <tt>synchronize_rcu_expedited()</tt> would be legal,
 including within preempt-disable code, <tt>local_bh_disable()</tt> code,
 interrupt-disable code, and interrupt handlers.
-However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers
+and from idle and offline CPUs.
 The callback function (<tt>remove_gp_cb()</tt> in this case) will be
 executed within softirq (software interrupt) environment within the
 Linux kernel,
@@ -1354,12 +1661,27 @@ write an RCU callback function that takes too long.
 Long-running operations should be relegated to separate threads or
 (in the Linux kernel) workqueues.
 
-<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a>
-Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
-After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
-structure, which would interact badly with concurrent insertions.
-Doesn't this mean that <tt>rcu_dereference()</tt> is required?
-<br><a href="#qq12answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
+	After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
+	structure, which would interact badly with concurrent insertions.
+	Doesn't this mean that <tt>rcu_dereference()</tt> is required?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
+	any changes, including any insertions that <tt>rcu_dereference()</tt>
+	would protect against.
+	Therefore, any insertions will be delayed until after
+	<tt>-&gt;gp_lock</tt>
+	is released on line&nbsp;25, which in turn means that
+	<tt>rcu_access_pointer()</tt> suffices.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 However, all that <tt>remove_gp_cb()</tt> is doing is
@@ -1406,14 +1728,31 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx,
 so the very few places that needed something like
 <tt>synchronize_rcu()</tt> simply open-coded it.
 
-<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a>
-Earlier it was claimed that <tt>call_rcu()</tt> and
-<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
-by readers.
-But how can that be correct, given that the invocation of the callback
-and the freeing of the memory (respectively) must still wait for
-a grace period to elapse?
-<br><a href="#qq13answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Earlier it was claimed that <tt>call_rcu()</tt> and
+	<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
+	by readers.
+	But how can that be correct, given that the invocation of the callback
+	and the freeing of the memory (respectively) must still wait for
+	a grace period to elapse?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	We could define things this way, but keep in mind that this sort of
+	definition would say that updates in garbage-collected languages
+	cannot complete until the next time the garbage collector runs,
+	which does not seem at all reasonable.
+	The key point is that in most cases, an updater using either
+	<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
+	next update as soon as it has invoked <tt>call_rcu()</tt> or
+	<tt>kfree_rcu()</tt>, without having to wait for a subsequent
+	grace period.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 But what if the updater must wait for the completion of code to be
@@ -1838,11 +2177,26 @@ kthreads to be spawned.
 Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
 initialization can result in deadlock.
 
-<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a>
-So what happens with <tt>synchronize_rcu()</tt> during
-scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
-kernels?
-<br><a href="#qq14answer">Answer</a>
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	So what happens with <tt>synchronize_rcu()</tt> during
+	scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
+	kernels?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
+	maps directly to <tt>synchronize_sched()</tt>.
+	Therefore, <tt>synchronize_rcu()</tt> works normally throughout
+	boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
+	However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
+	so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
+	during scheduler initialization.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <p>
 I learned of these boot-time requirements as a result of a series of
@@ -2171,6 +2525,14 @@ This real-time requirement motivated the grace-period kthread, which
 also simplified handling of a number of race conditions.
 
 <p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
+<p>
 Finally, RCU's status as a synchronization primitive means that
 any RCU failure can result in arbitrary memory corruption that can be
 extremely difficult to debug.
@@ -2223,6 +2585,8 @@ described in a separate section.
 <li>	<a href="#Sched Flavor">Sched Flavor</a>
 <li>	<a href="#Sleepable RCU">Sleepable RCU</a>
 <li>	<a href="#Tasks RCU">Tasks RCU</a>
+<li>	<a href="#Waiting for Multiple Grace Periods">
+	Waiting for Multiple Grace Periods</a>
 </ol>
 
 <h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
@@ -2472,6 +2836,94 @@ The tasks-RCU API is quite compact, consisting only of
 <tt>synchronize_rcu_tasks()</tt>, and
 <tt>rcu_barrier_tasks()</tt>.
 
+<h3><a name="Waiting for Multiple Grace Periods">
+Waiting for Multiple Grace Periods</a></h3>
+
+<p>
+Perhaps you have an RCU protected data structure that is accessed from
+RCU read-side critical sections, from softirq handlers, and from
+hardware interrupt handlers.
+That is three flavors of RCU, the normal flavor, the bottom-half flavor,
+and the sched flavor.
+How to wait for a compound grace period?
+
+<p>
+The best approach is usually to &ldquo;just say no!&rdquo; and
+insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+around each RCU read-side critical section, regardless of what
+environment it happens to be in.
+But suppose that some of the RCU read-side critical sections are
+on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
+is not a viable option, so that <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are not free.
+What then?
+
+<p>
+You <i>could</i> wait on all three grace periods in succession, as follows:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+</pre>
+</blockquote>
+
+<p>
+This works, but triples the update-side latency penalty.
+In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
+may be used to wait on all three flavors of grace period concurrently:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+</pre>
+</blockquote>
+
+<p>
+But what if it is necessary to also wait on SRCU?
+This can be done as follows:
+
+<blockquote>
+<pre>
+ 1 static void call_my_srcu(struct rcu_head *head,
+ 2        void (*func)(struct rcu_head *head))
+ 3 {
+ 4   call_srcu(&amp;my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+</pre>
+</blockquote>
+
+<p>
+If you needed to wait on multiple different flavors of SRCU
+(but why???), you would need to create a wrapper function resembling
+<tt>call_my_srcu()</tt> for each SRCU flavor.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what if I need to wait for multiple RCU flavors, but I also need
+	the grace periods to be expedited?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	If you are using expedited grace periods, there should be less penalty
+	for waiting on them in succession.
+	But if that is nevertheless a problem, you can use workqueues
+	or multiple kthreads to wait on the various expedited grace
+	periods concurrently.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+Again, it is usually better to adjust the RCU read-side critical sections
+to use a single flavor of RCU, but when this is not feasible, you can use
+<tt>synchronize_rcu_mult()</tt>.
+
 <h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
 
 <p>
@@ -2569,329 +3021,4 @@ and is provided
 under the terms of the Creative Commons Attribution-Share Alike 3.0
 United States license.
 
-<h3><a name="Answers to Quick Quizzes">
-Answers to Quick Quizzes</a></h3>
-
-<a name="qq1answer"></a>
-<p><b>Quick Quiz 1</b>:
-Wait a minute!
-You said that updaters can make useful forward progress concurrently
-with readers, but pre-existing readers will block
-<tt>synchronize_rcu()</tt>!!!
-Just who are you trying to fool???
-
-
-</p><p><b>Answer</b>:
-First, if updaters do not wish to be blocked by readers, they can use
-<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
-be discussed later.
-Second, even when using <tt>synchronize_rcu()</tt>, the other
-update-side code does run concurrently with readers, whether pre-existing
-or not.
-
-
-</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a>
-
-<a name="qq2answer"></a>
-<p><b>Quick Quiz 2</b>:
-Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
-
-
-</p><p><b>Answer</b>:
-Without that extra grace period, memory reordering could result in
-<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
-concurrently with the last bits of <tt>recovery()</tt>.
-
-
-</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a>
-
-<a name="qq3answer"></a>
-<p><b>Quick Quiz 3</b>:
-But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
-two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
-from being reordered.
-Can't that also cause problems?
-
-
-</p><p><b>Answer</b>:
-No, it cannot.
-The readers cannot see either of these two fields until
-the assignment to <tt>gp</tt>, by which time both fields are
-fully initialized.
-So reordering the assignments
-to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
-cause any problems.
-
-
-</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a>
-
-<a name="qq4answer"></a>
-<p><b>Quick Quiz 4</b>:
-Without the <tt>rcu_dereference()</tt> or the
-<tt>rcu_access_pointer()</tt>, what destructive optimizations
-might the compiler make use of?
-
-
-</p><p><b>Answer</b>:
-Let's start with what happens to <tt>do_something_gp()</tt>
-if it fails to use <tt>rcu_dereference()</tt>.
-It could reuse a value formerly fetched from this same pointer.
-It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
-manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
-mash-up of two distince pointer values.
-It might even use value-speculation optimizations, where it makes a wrong
-guess, but by the time it gets around to checking the value, an update
-has changed the pointer to match the wrong guess.
-Too bad about any dereferences that returned pre-initialization garbage
-in the meantime!
-
-<p>
-For <tt>remove_gp_synchronous()</tt>, as long as all modifications
-to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
-the above optimizations are harmless.
-However,
-with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
-<tt>sparse</tt> will complain if you
-define <tt>gp</tt> with <tt>__rcu</tt> and then
-access it without using
-either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
-
-
-</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a>
-
-<a name="qq5answer"></a>
-<p><b>Quick Quiz 5</b>:
-Given that multiple CPUs can start RCU read-side critical sections
-at any time without any ordering whatsoever, how can RCU possibly tell whether
-or not a given RCU read-side critical section starts before a
-given instance of <tt>synchronize_rcu()</tt>?
-
-
-</p><p><b>Answer</b>:
-If RCU cannot tell whether or not a given
-RCU read-side critical section starts before a
-given instance of <tt>synchronize_rcu()</tt>,
-then it must assume that the RCU read-side critical section
-started first.
-In other words, a given instance of <tt>synchronize_rcu()</tt>
-can avoid waiting on a given RCU read-side critical section only
-if it can prove that <tt>synchronize_rcu()</tt> started first.
-
-
-</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a>
-
-<a name="qq6answer"></a>
-<p><b>Quick Quiz 6</b>:
-The first and second guarantees require unbelievably strict ordering!
-Are all these memory barriers <i> really</i> required?
-
-
-</p><p><b>Answer</b>:
-Yes, they really are required.
-To see why the first guarantee is required, consider the following
-sequence of events:
-
-<ol>
-<li>	CPU 1: <tt>rcu_read_lock()</tt>
-<li>	CPU 1: <tt>q = rcu_dereference(gp);
-	/* Very likely to return p. */</tt>
-<li>	CPU 0: <tt>list_del_rcu(p);</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
-<li>	CPU 1: <tt>do_something_with(q-&gt;a);
-	/* No smp_mb(), so might happen after kfree(). */</tt>
-<li>	CPU 1: <tt>rcu_read_unlock()</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
-<li>	CPU 0: <tt>kfree(p);</tt>
-</ol>
-
-<p>
-Therefore, there absolutely must be a full memory barrier between the
-end of the RCU read-side critical section and the end of the
-grace period.
-
-<p>
-The sequence of events demonstrating the necessity of the second rule
-is roughly similar:
-
-<ol>
-<li>	CPU 0: <tt>list_del_rcu(p);</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
-<li>	CPU 1: <tt>rcu_read_lock()</tt>
-<li>	CPU 1: <tt>q = rcu_dereference(gp);
-	/* Might return p if no memory barrier. */</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
-<li>	CPU 0: <tt>kfree(p);</tt>
-<li>	CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
-<li>	CPU 1: <tt>rcu_read_unlock()</tt>
-</ol>
-
-<p>
-And similarly, without a memory barrier between the beginning of the
-grace period and the beginning of the RCU read-side critical section,
-CPU&nbsp;1 might end up accessing the freelist.
-
-<p>
-The &ldquo;as if&rdquo; rule of course applies, so that any implementation
-that acts as if the appropriate memory barriers were in place is a
-correct implementation.
-That said, it is much easier to fool yourself into believing that you have
-adhered to the as-if rule than it is to actually adhere to it!
-
-
-</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a>
-
-<a name="qq7answer"></a>
-<p><b>Quick Quiz 7</b>:
-But how does the upgrade-to-write operation exclude other readers?
-
-
-</p><p><b>Answer</b>:
-It doesn't, just like normal RCU updates, which also do not exclude
-RCU readers.
-
-
-</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a>
-
-<a name="qq8answer"></a>
-<p><b>Quick Quiz 8</b>:
-Can't the compiler also reorder this code?
-
-
-</p><p><b>Answer</b>:
-No, the volatile casts in <tt>READ_ONCE()</tt> and
-<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
-this particular case.
-
-
-</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
-
-<a name="qq9answer"></a>
-<p><b>Quick Quiz 9</b>:
-Suppose that synchronize_rcu() did wait until all readers had completed.
-Would the updater be able to rely on this?
-
-
-</p><p><b>Answer</b>:
-No.
-Even if <tt>synchronize_rcu()</tt> were to wait until
-all readers had completed, a new reader might start immediately after
-<tt>synchronize_rcu()</tt> completed.
-Therefore, the code following
-<tt>synchronize_rcu()</tt> cannot rely on there being no readers
-in any case.
-
-
-</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
-
-<a name="qq10answer"></a>
-<p><b>Quick Quiz 10</b>:
-How long a sequence of grace periods, each separated by an RCU read-side
-critical section, would be required to partition the RCU read-side
-critical sections at the beginning and end of the chain?
-
-
-</p><p><b>Answer</b>:
-In theory, an infinite number.
-In practice, an unknown number that is sensitive to both implementation
-details and timing considerations.
-Therefore, even in practice, RCU users must abide by the theoretical rather
-than the practical answer.
-
-
-</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a>
-
-<a name="qq11answer"></a>
-<p><b>Quick Quiz 11</b>:
-What about sleeping locks?
-
-
-</p><p><b>Answer</b>:
-These are forbidden within Linux-kernel RCU read-side critical sections
-because it is not legal to place a quiescent state (in this case,
-voluntary context switch) within an RCU read-side critical section.
-However, sleeping locks may be used within userspace RCU read-side critical
-sections, and also within Linux-kernel sleepable RCU
-<a href="#Sleepable RCU">(SRCU)</a>
-read-side critical sections.
-In addition, the -rt patchset turns spinlocks into a sleeping locks so
-that the corresponding critical sections can be preempted, which
-also means that these sleeplockified spinlocks (but not other sleeping locks!)
-may be acquire within -rt-Linux-kernel RCU read-side critical sections.
-
-<p>
-Note that it <i>is</i> legal for a normal RCU read-side critical section
-to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
-but only as long as it does not loop indefinitely attempting to
-conditionally acquire that sleeping locks.
-The key point is that things like <tt>mutex_trylock()</tt>
-either return with the mutex held, or return an error indication if
-the mutex was not immediately available.
-Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
-
-
-</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a>
-
-<a name="qq12answer"></a>
-<p><b>Quick Quiz 12</b>:
-Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
-After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
-structure, which would interact badly with concurrent insertions.
-Doesn't this mean that <tt>rcu_dereference()</tt> is required?
-
-
-</p><p><b>Answer</b>:
-Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
-any changes, including any insertions that <tt>rcu_dereference()</tt>
-would protect against.
-Therefore, any insertions will be delayed until after <tt>-&gt;gp_lock</tt>
-is released on line&nbsp;25, which in turn means that
-<tt>rcu_access_pointer()</tt> suffices.
-
-
-</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a>
-
-<a name="qq13answer"></a>
-<p><b>Quick Quiz 13</b>:
-Earlier it was claimed that <tt>call_rcu()</tt> and
-<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
-by readers.
-But how can that be correct, given that the invocation of the callback
-and the freeing of the memory (respectively) must still wait for
-a grace period to elapse?
-
-
-</p><p><b>Answer</b>:
-We could define things this way, but keep in mind that this sort of
-definition would say that updates in garbage-collected languages
-cannot complete until the next time the garbage collector runs,
-which does not seem at all reasonable.
-The key point is that in most cases, an updater using either
-<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
-next update as soon as it has invoked <tt>call_rcu()</tt> or
-<tt>kfree_rcu()</tt>, without having to wait for a subsequent
-grace period.
-
-
-</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a>
-
-<a name="qq14answer"></a>
-<p><b>Quick Quiz 14</b>:
-So what happens with <tt>synchronize_rcu()</tt> during
-scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
-kernels?
-
-
-</p><p><b>Answer</b>:
-In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
-maps directly to <tt>synchronize_sched()</tt>.
-Therefore, <tt>synchronize_rcu()</tt> works normally throughout
-boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
-However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
-so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
-during scheduler initialization.
-
-
-</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
-
-
 </body></html>
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
deleted file mode 100644
index 3a97ba490c42..000000000000
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ /dev/null
@@ -1,2741 +0,0 @@
-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
-        "http://www.w3.org/TR/html4/loose.dtd">
-        <html>
-        <head><title>A Tour Through RCU's Requirements [LWN.net]</title>
-        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
-
-<h1>A Tour Through RCU's Requirements</h1>
-
-<p>Copyright IBM Corporation, 2015</p>
-<p>Author: Paul E.&nbsp;McKenney</p>
-<p><i>The initial version of this document appeared in the
-<a href="https://lwn.net/">LWN</a> articles
-<a href="https://lwn.net/Articles/652156/">here</a>,
-<a href="https://lwn.net/Articles/652677/">here</a>, and
-<a href="https://lwn.net/Articles/653326/">here</a>.</i></p>
-
-<h2>Introduction</h2>
-
-<p>
-Read-copy update (RCU) is a synchronization mechanism that is often
-used as a replacement for reader-writer locking.
-RCU is unusual in that updaters do not block readers,
-which means that RCU's read-side primitives can be exceedingly fast
-and scalable.
-In addition, updaters can make useful forward progress concurrently
-with readers.
-However, all this concurrency between RCU readers and updaters does raise
-the question of exactly what RCU readers are doing, which in turn
-raises the question of exactly what RCU's requirements are.
-
-<p>
-This document therefore summarizes RCU's requirements, and can be thought
-of as an informal, high-level specification for RCU.
-It is important to understand that RCU's specification is primarily
-empirical in nature;
-in fact, I learned about many of these requirements the hard way.
-This situation might cause some consternation, however, not only
-has this learning process been a lot of fun, but it has also been
-a great privilege to work with so many people willing to apply
-technologies in interesting new ways.
-
-<p>
-All that aside, here are the categories of currently known RCU requirements:
-</p>
-
-<ol>
-<li>	<a href="#Fundamental Requirements">
-	Fundamental Requirements</a>
-<li>	<a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a>
-<li>	<a href="#Parallelism Facts of Life">
-	Parallelism Facts of Life</a>
-<li>	<a href="#Quality-of-Implementation Requirements">
-	Quality-of-Implementation Requirements</a>
-<li>	<a href="#Linux Kernel Complications">
-	Linux Kernel Complications</a>
-<li>	<a href="#Software-Engineering Requirements">
-	Software-Engineering Requirements</a>
-<li>	<a href="#Other RCU Flavors">
-	Other RCU Flavors</a>
-<li>	<a href="#Possible Future Changes">
-	Possible Future Changes</a>
-</ol>
-
-<p>
-This is followed by a <a href="#Summary">summary</a>,
-which is in turn followed by the inevitable
-<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
-
-<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
-
-<p>
-RCU's fundamental requirements are the closest thing RCU has to hard
-mathematical requirements.
-These are:
-
-<ol>
-<li>	<a href="#Grace-Period Guarantee">
-	Grace-Period Guarantee</a>
-<li>	<a href="#Publish-Subscribe Guarantee">
-	Publish-Subscribe Guarantee</a>
-<li>	<a href="#Memory-Barrier Guarantees">
-	Memory-Barrier Guarantees</a>
-<li>	<a href="#RCU Primitives Guaranteed to Execute Unconditionally">
-	RCU Primitives Guaranteed to Execute Unconditionally</a>
-<li>	<a href="#Guaranteed Read-to-Write Upgrade">
-	Guaranteed Read-to-Write Upgrade</a>
-</ol>
-
-<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3>
-
-<p>
-RCU's grace-period guarantee is unusual in being premeditated:
-Jack Slingwine and I had this guarantee firmly in mind when we started
-work on RCU (then called &ldquo;rclock&rdquo;) in the early 1990s.
-That said, the past two decades of experience with RCU have produced
-a much more detailed understanding of this guarantee.
-
-<p>
-RCU's grace-period guarantee allows updaters to wait for the completion
-of all pre-existing RCU read-side critical sections.
-An RCU read-side critical section
-begins with the marker <tt>rcu_read_lock()</tt> and ends with
-the marker <tt>rcu_read_unlock()</tt>.
-These markers may be nested, and RCU treats a nested set as one
-big RCU read-side critical section.
-Production-quality implementations of <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> are extremely lightweight, and in
-fact have exactly zero overhead in Linux kernels built for production
-use with <tt>CONFIG_PREEMPT=n</tt>.
-
-<p>
-This guarantee allows ordering to be enforced with extremely low
-overhead to readers, for example:
-
-<blockquote>
-<pre>
- 1 int x, y;
- 2
- 3 void thread0(void)
- 4 {
- 5   rcu_read_lock();
- 6   r1 = READ_ONCE(x);
- 7   r2 = READ_ONCE(y);
- 8   rcu_read_unlock();
- 9 }
-10
-11 void thread1(void)
-12 {
-13   WRITE_ONCE(x, 1);
-14   synchronize_rcu();
-15   WRITE_ONCE(y, 1);
-16 }
-</pre>
-</blockquote>
-
-<p>
-Because the <tt>synchronize_rcu()</tt> on line&nbsp;14 waits for
-all pre-existing readers, any instance of <tt>thread0()</tt> that
-loads a value of zero from <tt>x</tt> must complete before
-<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must
-also load a value of zero from <tt>y</tt>.
-Similarly, any instance of <tt>thread0()</tt> that loads a value of
-one from <tt>y</tt> must have started after the
-<tt>synchronize_rcu()</tt> started, and must therefore also load
-a value of one from <tt>x</tt>.
-Therefore, the outcome:
-<blockquote>
-<pre>
-(r1 == 0 &amp;&amp; r2 == 1)
-</pre>
-</blockquote>
-cannot happen.
-
-<p>@@QQ@@
-Wait a minute!
-You said that updaters can make useful forward progress concurrently
-with readers, but pre-existing readers will block
-<tt>synchronize_rcu()</tt>!!!
-Just who are you trying to fool???
-<p>@@QQA@@
-First, if updaters do not wish to be blocked by readers, they can use
-<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
-be discussed later.
-Second, even when using <tt>synchronize_rcu()</tt>, the other
-update-side code does run concurrently with readers, whether pre-existing
-or not.
-<p>@@QQE@@
-
-<p>
-This scenario resembles one of the first uses of RCU in
-<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>,
-which managed a distributed lock manager's transition into
-a state suitable for handling recovery from node failure,
-more or less as follows:
-
-<blockquote>
-<pre>
- 1 #define STATE_NORMAL        0
- 2 #define STATE_WANT_RECOVERY 1
- 3 #define STATE_RECOVERING    2
- 4 #define STATE_WANT_NORMAL   3
- 5
- 6 int state = STATE_NORMAL;
- 7
- 8 void do_something_dlm(void)
- 9 {
-10   int state_snap;
-11
-12   rcu_read_lock();
-13   state_snap = READ_ONCE(state);
-14   if (state_snap == STATE_NORMAL)
-15     do_something();
-16   else
-17     do_something_carefully();
-18   rcu_read_unlock();
-19 }
-20
-21 void start_recovery(void)
-22 {
-23   WRITE_ONCE(state, STATE_WANT_RECOVERY);
-24   synchronize_rcu();
-25   WRITE_ONCE(state, STATE_RECOVERING);
-26   recovery();
-27   WRITE_ONCE(state, STATE_WANT_NORMAL);
-28   synchronize_rcu();
-29   WRITE_ONCE(state, STATE_NORMAL);
-30 }
-</pre>
-</blockquote>
-
-<p>
-The RCU read-side critical section in <tt>do_something_dlm()</tt>
-works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt>
-to guarantee that <tt>do_something()</tt> never runs concurrently
-with <tt>recovery()</tt>, but with little or no synchronization
-overhead in <tt>do_something_dlm()</tt>.
-
-<p>@@QQ@@
-Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
-<p>@@QQA@@
-Without that extra grace period, memory reordering could result in
-<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
-concurrently with the last bits of <tt>recovery()</tt>.
-<p>@@QQE@@
-
-<p>
-In order to avoid fatal problems such as deadlocks,
-an RCU read-side critical section must not contain calls to
-<tt>synchronize_rcu()</tt>.
-Similarly, an RCU read-side critical section must not
-contain anything that waits, directly or indirectly, on completion of
-an invocation of <tt>synchronize_rcu()</tt>.
-
-<p>
-Although RCU's grace-period guarantee is useful in and of itself, with
-<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>,
-it would be good to be able to use RCU to coordinate read-side
-access to linked data structures.
-For this, the grace-period guarantee is not sufficient, as can
-be seen in function <tt>add_gp_buggy()</tt> below.
-We will look at the reader's code later, but in the meantime, just think of
-the reader as locklessly picking up the <tt>gp</tt> pointer,
-and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the
-<tt>-&gt;a</tt> and <tt>-&gt;b</tt> fields.
-
-<blockquote>
-<pre>
- 1 bool add_gp_buggy(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&amp;gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&amp;gp_lock);
- 9     return false;
-10   }
-11   p-&gt;a = a;
-12   p-&gt;b = a;
-13   gp = p; /* ORDERING BUG */
-14   spin_unlock(&amp;gp_lock);
-15   return true;
-16 }
-</pre>
-</blockquote>
-
-<p>
-The problem is that both the compiler and weakly ordered CPUs are within
-their rights to reorder this code as follows:
-
-<blockquote>
-<pre>
- 1 bool add_gp_buggy_optimized(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&amp;gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&amp;gp_lock);
- 9     return false;
-10   }
-<b>11   gp = p; /* ORDERING BUG */
-12   p-&gt;a = a;
-13   p-&gt;b = a;</b>
-14   spin_unlock(&amp;gp_lock);
-15   return true;
-16 }
-</pre>
-</blockquote>
-
-<p>
-If an RCU reader fetches <tt>gp</tt> just after
-<tt>add_gp_buggy_optimized</tt> executes line&nbsp;11,
-it will see garbage in the <tt>-&gt;a</tt> and <tt>-&gt;b</tt>
-fields.
-And this is but one of many ways in which compiler and hardware optimizations
-could cause trouble.
-Therefore, we clearly need some way to prevent the compiler and the CPU from
-reordering in this manner, which brings us to the publish-subscribe
-guarantee discussed in the next section.
-
-<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3>
-
-<p>
-RCU's publish-subscribe guarantee allows data to be inserted
-into a linked data structure without disrupting RCU readers.
-The updater uses <tt>rcu_assign_pointer()</tt> to insert the
-new data, and readers use <tt>rcu_dereference()</tt> to
-access data, whether new or old.
-The following shows an example of insertion:
-
-<blockquote>
-<pre>
- 1 bool add_gp(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&amp;gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&amp;gp_lock);
- 9     return false;
-10   }
-11   p-&gt;a = a;
-12   p-&gt;b = a;
-13   rcu_assign_pointer(gp, p);
-14   spin_unlock(&amp;gp_lock);
-15   return true;
-16 }
-</pre>
-</blockquote>
-
-<p>
-The <tt>rcu_assign_pointer()</tt> on line&nbsp;13 is conceptually
-equivalent to a simple assignment statement, but also guarantees
-that its assignment will
-happen after the two assignments in lines&nbsp;11 and&nbsp;12,
-similar to the C11 <tt>memory_order_release</tt> store operation.
-It also prevents any number of &ldquo;interesting&rdquo; compiler
-optimizations, for example, the use of <tt>gp</tt> as a scratch
-location immediately preceding the assignment.
-
-<p>@@QQ@@
-But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
-two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
-from being reordered.
-Can't that also cause problems?
-<p>@@QQA@@
-No, it cannot.
-The readers cannot see either of these two fields until
-the assignment to <tt>gp</tt>, by which time both fields are
-fully initialized.
-So reordering the assignments
-to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
-cause any problems.
-<p>@@QQE@@
-
-<p>
-It is tempting to assume that the reader need not do anything special
-to control its accesses to the RCU-protected data,
-as shown in <tt>do_something_gp_buggy()</tt> below:
-
-<blockquote>
-<pre>
- 1 bool do_something_gp_buggy(void)
- 2 {
- 3   rcu_read_lock();
- 4   p = gp;  /* OPTIMIZATIONS GALORE!!! */
- 5   if (p) {
- 6     do_something(p-&gt;a, p-&gt;b);
- 7     rcu_read_unlock();
- 8     return true;
- 9   }
-10   rcu_read_unlock();
-11   return false;
-12 }
-</pre>
-</blockquote>
-
-<p>
-However, this temptation must be resisted because there are a
-surprisingly large number of ways that the compiler
-(to say nothing of
-<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>)
-can trip this code up.
-For but one example, if the compiler were short of registers, it
-might choose to refetch from <tt>gp</tt> rather than keeping
-a separate copy in <tt>p</tt> as follows:
-
-<blockquote>
-<pre>
- 1 bool do_something_gp_buggy_optimized(void)
- 2 {
- 3   rcu_read_lock();
- 4   if (gp) { /* OPTIMIZATIONS GALORE!!! */
-<b> 5     do_something(gp-&gt;a, gp-&gt;b);</b>
- 6     rcu_read_unlock();
- 7     return true;
- 8   }
- 9   rcu_read_unlock();
-10   return false;
-11 }
-</pre>
-</blockquote>
-
-<p>
-If this function ran concurrently with a series of updates that
-replaced the current structure with a new one,
-the fetches of <tt>gp-&gt;a</tt>
-and <tt>gp-&gt;b</tt> might well come from two different structures,
-which could cause serious confusion.
-To prevent this (and much else besides), <tt>do_something_gp()</tt> uses
-<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>:
-
-<blockquote>
-<pre>
- 1 bool do_something_gp(void)
- 2 {
- 3   rcu_read_lock();
- 4   p = rcu_dereference(gp);
- 5   if (p) {
- 6     do_something(p-&gt;a, p-&gt;b);
- 7     rcu_read_unlock();
- 8     return true;
- 9   }
-10   rcu_read_unlock();
-11   return false;
-12 }
-</pre>
-</blockquote>
-
-<p>
-The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha)
-memory barriers in the Linux kernel.
-Should a
-<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a>
-ever appear, then <tt>rcu_dereference()</tt> could be implemented
-as a <tt>memory_order_consume</tt> load.
-Regardless of the exact implementation, a pointer fetched by
-<tt>rcu_dereference()</tt> may not be used outside of the
-outermost RCU read-side critical section containing that
-<tt>rcu_dereference()</tt>, unless protection of
-the corresponding data element has been passed from RCU to some
-other synchronization mechanism, most commonly locking or
-<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>.
-
-<p>
-In short, updaters use <tt>rcu_assign_pointer()</tt> and readers
-use <tt>rcu_dereference()</tt>, and these two RCU API elements
-work together to ensure that readers have a consistent view of
-newly added data elements.
-
-<p>
-Of course, it is also necessary to remove elements from RCU-protected
-data structures, for example, using the following process:
-
-<ol>
-<li>	Remove the data element from the enclosing structure.
-<li>	Wait for all pre-existing RCU read-side critical sections
-	to complete (because only pre-existing readers can possibly have
-	a reference to the newly removed data element).
-<li>	At this point, only the updater has a reference to the
-	newly removed data element, so it can safely reclaim
-	the data element, for example, by passing it to <tt>kfree()</tt>.
-</ol>
-
-This process is implemented by <tt>remove_gp_synchronous()</tt>:
-
-<blockquote>
-<pre>
- 1 bool remove_gp_synchronous(void)
- 2 {
- 3   struct foo *p;
- 4
- 5   spin_lock(&amp;gp_lock);
- 6   p = rcu_access_pointer(gp);
- 7   if (!p) {
- 8     spin_unlock(&amp;gp_lock);
- 9     return false;
-10   }
-11   rcu_assign_pointer(gp, NULL);
-12   spin_unlock(&amp;gp_lock);
-13   synchronize_rcu();
-14   kfree(p);
-15   return true;
-16 }
-</pre>
-</blockquote>
-
-<p>
-This function is straightforward, with line&nbsp;13 waiting for a grace
-period before line&nbsp;14 frees the old data element.
-This waiting ensures that readers will reach line&nbsp;7 of
-<tt>do_something_gp()</tt> before the data element referenced by
-<tt>p</tt> is freed.
-The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
-<tt>rcu_dereference()</tt>, except that:
-
-<ol>
-<li>	The value returned by <tt>rcu_access_pointer()</tt>
-	cannot be dereferenced.
-	If you want to access the value pointed to as well as
-	the pointer itself, use <tt>rcu_dereference()</tt>
-	instead of <tt>rcu_access_pointer()</tt>.
-<li>	The call to <tt>rcu_access_pointer()</tt> need not be
-	protected.
-	In contrast, <tt>rcu_dereference()</tt> must either be
-	within an RCU read-side critical section or in a code
-	segment where the pointer cannot change, for example, in
-	code protected by the corresponding update-side lock.
-</ol>
-
-<p>@@QQ@@
-Without the <tt>rcu_dereference()</tt> or the
-<tt>rcu_access_pointer()</tt>, what destructive optimizations
-might the compiler make use of?
-<p>@@QQA@@
-Let's start with what happens to <tt>do_something_gp()</tt>
-if it fails to use <tt>rcu_dereference()</tt>.
-It could reuse a value formerly fetched from this same pointer.
-It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
-manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
-mash-up of two distince pointer values.
-It might even use value-speculation optimizations, where it makes a wrong
-guess, but by the time it gets around to checking the value, an update
-has changed the pointer to match the wrong guess.
-Too bad about any dereferences that returned pre-initialization garbage
-in the meantime!
-
-<p>
-For <tt>remove_gp_synchronous()</tt>, as long as all modifications
-to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
-the above optimizations are harmless.
-However,
-with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
-<tt>sparse</tt> will complain if you
-define <tt>gp</tt> with <tt>__rcu</tt> and then
-access it without using
-either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
-<p>@@QQE@@
-
-<p>
-In short, RCU's publish-subscribe guarantee is provided by the combination
-of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>.
-This guarantee allows data elements to be safely added to RCU-protected
-linked data structures without disrupting RCU readers.
-This guarantee can be used in combination with the grace-period
-guarantee to also allow data elements to be removed from RCU-protected
-linked data structures, again without disrupting RCU readers.
-
-<p>
-This guarantee was only partially premeditated.
-DYNIX/ptx used an explicit memory barrier for publication, but had nothing
-resembling <tt>rcu_dereference()</tt> for subscription, nor did it
-have anything resembling the <tt>smp_read_barrier_depends()</tt>
-that was later subsumed into <tt>rcu_dereference()</tt>.
-The need for these operations made itself known quite suddenly at a
-late-1990s meeting with the DEC Alpha architects, back in the days when
-DEC was still a free-standing company.
-It took the Alpha architects a good hour to convince me that any sort
-of barrier would ever be needed, and it then took me a good <i>two</i> hours
-to convince them that their documentation did not make this point clear.
-More recent work with the C and C++ standards committees have provided
-much education on tricks and traps from the compiler.
-In short, compilers were much less tricky in the early 1990s, but in
-2015, don't even think about omitting <tt>rcu_dereference()</tt>!
-
-<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3>
-
-<p>
-The previous section's simple linked-data-structure scenario clearly
-demonstrates the need for RCU's stringent memory-ordering guarantees on
-systems with more than one CPU:
-
-<ol>
-<li>	Each CPU that has an RCU read-side critical section that
-	begins before <tt>synchronize_rcu()</tt> starts is
-	guaranteed to execute a full memory barrier between the time
-	that the RCU read-side critical section ends and the time that
-	<tt>synchronize_rcu()</tt> returns.
-	Without this guarantee, a pre-existing RCU read-side critical section
-	might hold a reference to the newly removed <tt>struct foo</tt>
-	after the <tt>kfree()</tt> on line&nbsp;14 of
-	<tt>remove_gp_synchronous()</tt>.
-<li>	Each CPU that has an RCU read-side critical section that ends
-	after <tt>synchronize_rcu()</tt> returns is guaranteed
-	to execute a full memory barrier between the time that
-	<tt>synchronize_rcu()</tt> begins and the time that the RCU
-	read-side critical section begins.
-	Without this guarantee, a later RCU read-side critical section
-	running after the <tt>kfree()</tt> on line&nbsp;14 of
-	<tt>remove_gp_synchronous()</tt> might
-	later run <tt>do_something_gp()</tt> and find the
-	newly deleted <tt>struct foo</tt>.
-<li>	If the task invoking <tt>synchronize_rcu()</tt> remains
-	on a given CPU, then that CPU is guaranteed to execute a full
-	memory barrier sometime during the execution of
-	<tt>synchronize_rcu()</tt>.
-	This guarantee ensures that the <tt>kfree()</tt> on
-	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
-	execute after the removal on line&nbsp;11.
-<li>	If the task invoking <tt>synchronize_rcu()</tt> migrates
-	among a group of CPUs during that invocation, then each of the
-	CPUs in that group is guaranteed to execute a full memory barrier
-	sometime during the execution of <tt>synchronize_rcu()</tt>.
-	This guarantee also ensures that the <tt>kfree()</tt> on
-	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
-	execute after the removal on
-	line&nbsp;11, but also in the case where the thread executing the
-	<tt>synchronize_rcu()</tt> migrates in the meantime.
-</ol>
-
-<p>@@QQ@@
-Given that multiple CPUs can start RCU read-side critical sections
-at any time without any ordering whatsoever, how can RCU possibly tell whether
-or not a given RCU read-side critical section starts before a
-given instance of <tt>synchronize_rcu()</tt>?
-<p>@@QQA@@
-If RCU cannot tell whether or not a given
-RCU read-side critical section starts before a
-given instance of <tt>synchronize_rcu()</tt>,
-then it must assume that the RCU read-side critical section
-started first.
-In other words, a given instance of <tt>synchronize_rcu()</tt>
-can avoid waiting on a given RCU read-side critical section only
-if it can prove that <tt>synchronize_rcu()</tt> started first.
-<p>@@QQE@@
-
-<p>@@QQ@@
-The first and second guarantees require unbelievably strict ordering!
-Are all these memory barriers <i> really</i> required?
-<p>@@QQA@@
-Yes, they really are required.
-To see why the first guarantee is required, consider the following
-sequence of events:
-
-<ol>
-<li>	CPU 1: <tt>rcu_read_lock()</tt>
-<li>	CPU 1: <tt>q = rcu_dereference(gp);
-	/* Very likely to return p. */</tt>
-<li>	CPU 0: <tt>list_del_rcu(p);</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
-<li>	CPU 1: <tt>do_something_with(q-&gt;a);
-	/* No smp_mb(), so might happen after kfree(). */</tt>
-<li>	CPU 1: <tt>rcu_read_unlock()</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
-<li>	CPU 0: <tt>kfree(p);</tt>
-</ol>
-
-<p>
-Therefore, there absolutely must be a full memory barrier between the
-end of the RCU read-side critical section and the end of the
-grace period.
-
-<p>
-The sequence of events demonstrating the necessity of the second rule
-is roughly similar:
-
-<ol>
-<li>	CPU 0: <tt>list_del_rcu(p);</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
-<li>	CPU 1: <tt>rcu_read_lock()</tt>
-<li>	CPU 1: <tt>q = rcu_dereference(gp);
-	/* Might return p if no memory barrier. */</tt>
-<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
-<li>	CPU 0: <tt>kfree(p);</tt>
-<li>	CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
-<li>	CPU 1: <tt>rcu_read_unlock()</tt>
-</ol>
-
-<p>
-And similarly, without a memory barrier between the beginning of the
-grace period and the beginning of the RCU read-side critical section,
-CPU&nbsp;1 might end up accessing the freelist.
-
-<p>
-The &ldquo;as if&rdquo; rule of course applies, so that any implementation
-that acts as if the appropriate memory barriers were in place is a
-correct implementation.
-That said, it is much easier to fool yourself into believing that you have
-adhered to the as-if rule than it is to actually adhere to it!
-<p>@@QQE@@
-
-<p>
-Note that these memory-barrier requirements do not replace the fundamental
-RCU requirement that a grace period wait for all pre-existing readers.
-On the contrary, the memory barriers called out in this section must operate in
-such a way as to <i>enforce</i> this fundamental requirement.
-Of course, different implementations enforce this requirement in different
-ways, but enforce it they must.
-
-<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3>
-
-<p>
-The common-case RCU primitives are unconditional.
-They are invoked, they do their job, and they return, with no possibility
-of error, and no need to retry.
-This is a key RCU design philosophy.
-
-<p>
-However, this philosophy is pragmatic rather than pigheaded.
-If someone comes up with a good justification for a particular conditional
-RCU primitive, it might well be implemented and added.
-After all, this guarantee was reverse-engineered, not premeditated.
-The unconditional nature of the RCU primitives was initially an
-accident of implementation, and later experience with synchronization
-primitives with conditional primitives caused me to elevate this
-accident to a guarantee.
-Therefore, the justification for adding a conditional primitive to
-RCU would need to be based on detailed and compelling use cases.
-
-<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3>
-
-<p>
-As far as RCU is concerned, it is always possible to carry out an
-update within an RCU read-side critical section.
-For example, that RCU read-side critical section might search for
-a given data element, and then might acquire the update-side
-spinlock in order to update that element, all while remaining
-in that RCU read-side critical section.
-Of course, it is necessary to exit the RCU read-side critical section
-before invoking <tt>synchronize_rcu()</tt>, however, this
-inconvenience can be avoided through use of the
-<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
-described later in this document.
-
-<p>@@QQ@@
-But how does the upgrade-to-write operation exclude other readers?
-<p>@@QQA@@
-It doesn't, just like normal RCU updates, which also do not exclude
-RCU readers.
-<p>@@QQE@@
-
-<p>
-This guarantee allows lookup code to be shared between read-side
-and update-side code, and was premeditated, appearing in the earliest
-DYNIX/ptx RCU documentation.
-
-<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2>
-
-<p>
-RCU provides extremely lightweight readers, and its read-side guarantees,
-though quite useful, are correspondingly lightweight.
-It is therefore all too easy to assume that RCU is guaranteeing more
-than it really is.
-Of course, the list of things that RCU does not guarantee is infinitely
-long, however, the following sections list a few non-guarantees that
-have caused confusion.
-Except where otherwise noted, these non-guarantees were premeditated.
-
-<ol>
-<li>	<a href="#Readers Impose Minimal Ordering">
-	Readers Impose Minimal Ordering</a>
-<li>	<a href="#Readers Do Not Exclude Updaters">
-	Readers Do Not Exclude Updaters</a>
-<li>	<a href="#Updaters Only Wait For Old Readers">
-	Updaters Only Wait For Old Readers</a>
-<li>	<a href="#Grace Periods Don't Partition Read-Side Critical Sections">
-	Grace Periods Don't Partition Read-Side Critical Sections</a>
-<li>	<a href="#Read-Side Critical Sections Don't Partition Grace Periods">
-	Read-Side Critical Sections Don't Partition Grace Periods</a>
-<li>	<a href="#Disabling Preemption Does Not Block Grace Periods">
-	Disabling Preemption Does Not Block Grace Periods</a>
-</ol>
-
-<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
-
-<p>
-Reader-side markers such as <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees
-except through their interaction with the grace-period APIs such as
-<tt>synchronize_rcu()</tt>.
-To see this, consider the following pair of threads:
-
-<blockquote>
-<pre>
- 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(x, 1);
- 5   rcu_read_unlock();
- 6   rcu_read_lock();
- 7   WRITE_ONCE(y, 1);
- 8   rcu_read_unlock();
- 9 }
-10
-11 void thread1(void)
-12 {
-13   rcu_read_lock();
-14   r1 = READ_ONCE(y);
-15   rcu_read_unlock();
-16   rcu_read_lock();
-17   r2 = READ_ONCE(x);
-18   rcu_read_unlock();
-19 }
-</pre>
-</blockquote>
-
-<p>
-After <tt>thread0()</tt> and <tt>thread1()</tt> execute
-concurrently, it is quite possible to have
-
-<blockquote>
-<pre>
-(r1 == 1 &amp;&amp; r2 == 0)
-</pre>
-</blockquote>
-
-(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>),
-which would not be possible if <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> had much in the way of ordering
-properties.
-But they do not, so the CPU is within its rights
-to do significant reordering.
-This is by design:  Any significant ordering constraints would slow down
-these fast-path APIs.
-
-<p>@@QQ@@
-Can't the compiler also reorder this code?
-<p>@@QQA@@
-No, the volatile casts in <tt>READ_ONCE()</tt> and
-<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
-this particular case.
-<p>@@QQE@@
-
-<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
-
-<p>
-Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt>
-exclude updates.
-All they do is to prevent grace periods from ending.
-The following example illustrates this:
-
-<blockquote>
-<pre>
- 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   r1 = READ_ONCE(y);
- 5   if (r1) {
- 6     do_something_with_nonzero_x();
- 7     r2 = READ_ONCE(x);
- 8     WARN_ON(!r2); /* BUG!!! */
- 9   }
-10   rcu_read_unlock();
-11 }
-12
-13 void thread1(void)
-14 {
-15   spin_lock(&amp;my_lock);
-16   WRITE_ONCE(x, 1);
-17   WRITE_ONCE(y, 1);
-18   spin_unlock(&amp;my_lock);
-19 }
-</pre>
-</blockquote>
-
-<p>
-If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt>
-excluded the <tt>thread1()</tt> function's update,
-the <tt>WARN_ON()</tt> could never fire.
-But the fact is that <tt>rcu_read_lock()</tt> does not exclude
-much of anything aside from subsequent grace periods, of which
-<tt>thread1()</tt> has none, so the
-<tt>WARN_ON()</tt> can and does fire.
-
-<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3>
-
-<p>
-It might be tempting to assume that after <tt>synchronize_rcu()</tt>
-completes, there are no readers executing.
-This temptation must be avoided because
-new readers can start immediately after <tt>synchronize_rcu()</tt>
-starts, and <tt>synchronize_rcu()</tt> is under no
-obligation to wait for these new readers.
-
-<p>@@QQ@@
-Suppose that synchronize_rcu() did wait until all readers had completed.
-Would the updater be able to rely on this?
-<p>@@QQA@@
-No.
-Even if <tt>synchronize_rcu()</tt> were to wait until
-all readers had completed, a new reader might start immediately after
-<tt>synchronize_rcu()</tt> completed.
-Therefore, the code following
-<tt>synchronize_rcu()</tt> cannot rely on there being no readers
-in any case.
-<p>@@QQE@@
-
-<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
-Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
-
-<p>
-It is tempting to assume that if any part of one RCU read-side critical
-section precedes a given grace period, and if any part of another RCU
-read-side critical section follows that same grace period, then all of
-the first RCU read-side critical section must precede all of the second.
-However, this just isn't the case: A single grace period does not
-partition the set of RCU read-side critical sections.
-An example of this situation can be illustrated as follows, where
-<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero:
-
-<blockquote>
-<pre>
- 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   rcu_read_lock();
-19   r2 = READ_ONCE(b);
-20   r3 = READ_ONCE(c);
-21   rcu_read_unlock();
-22 }
-</pre>
-</blockquote>
-
-<p>
-It turns out that the outcome:
-
-<blockquote>
-<pre>
-(r1 == 1 &amp;&amp; r2 == 0 &amp;&amp; r3 == 1)
-</pre>
-</blockquote>
-
-is entirely possible.
-The following figure show how this can happen, with each circled
-<tt>QS</tt> indicating the point at which RCU recorded a
-<i>quiescent state</i> for each thread, that is, a state in which
-RCU knows that the thread cannot be in the midst of an RCU read-side
-critical section that started before the current grace period:
-
-<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p>
-
-<p>
-If it is necessary to partition RCU read-side critical sections in this
-manner, it is necessary to use two grace periods, where the first
-grace period is known to end before the second grace period starts:
-
-<blockquote>
-<pre>
- 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   r2 = READ_ONCE(c);
-19   synchronize_rcu();
-20   WRITE_ONCE(d, 1);
-21 }
-22
-23 void thread3(void)
-24 {
-25   rcu_read_lock();
-26   r3 = READ_ONCE(b);
-27   r4 = READ_ONCE(d);
-28   rcu_read_unlock();
-29 }
-</pre>
-</blockquote>
-
-<p>
-Here, if <tt>(r1 == 1)</tt>, then
-<tt>thread0()</tt>'s write to <tt>b</tt> must happen
-before the end of <tt>thread1()</tt>'s grace period.
-If in addition <tt>(r4 == 1)</tt>, then
-<tt>thread3()</tt>'s read from <tt>b</tt> must happen
-after the beginning of <tt>thread2()</tt>'s grace period.
-If it is also the case that <tt>(r2 == 1)</tt>, then the
-end of <tt>thread1()</tt>'s grace period must precede the
-beginning of <tt>thread2()</tt>'s grace period.
-This mean that the two RCU read-side critical sections cannot overlap,
-guaranteeing that <tt>(r3 == 1)</tt>.
-As a result, the outcome:
-
-<blockquote>
-<pre>
-(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 0 &amp;&amp; r4 == 1)
-</pre>
-</blockquote>
-
-cannot happen.
-
-<p>
-This non-requirement was also non-premeditated, but became apparent
-when studying RCU's interaction with memory ordering.
-
-<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods">
-Read-Side Critical Sections Don't Partition Grace Periods</a></h3>
-
-<p>
-It is also tempting to assume that if an RCU read-side critical section
-happens between a pair of grace periods, then those grace periods cannot
-overlap.
-However, this temptation leads nowhere good, as can be illustrated by
-the following, with all variables initially zero:
-
-<blockquote>
-<pre>
- 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   rcu_read_lock();
-19   WRITE_ONCE(d, 1);
-20   r2 = READ_ONCE(c);
-21   rcu_read_unlock();
-22 }
-23
-24 void thread3(void)
-25 {
-26   r3 = READ_ONCE(d);
-27   synchronize_rcu();
-28   WRITE_ONCE(e, 1);
-29 }
-30
-31 void thread4(void)
-32 {
-33   rcu_read_lock();
-34   r4 = READ_ONCE(b);
-35   r5 = READ_ONCE(e);
-36   rcu_read_unlock();
-37 }
-</pre>
-</blockquote>
-
-<p>
-In this case, the outcome:
-
-<blockquote>
-<pre>
-(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 1 &amp;&amp; r4 == 0 &amp&amp; r5 == 1)
-</pre>
-</blockquote>
-
-is entirely possible, as illustrated below:
-
-<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p>
-
-<p>
-Again, an RCU read-side critical section can overlap almost all of a
-given grace period, just so long as it does not overlap the entire
-grace period.
-As a result, an RCU read-side critical section cannot partition a pair
-of RCU grace periods.
-
-<p>@@QQ@@
-How long a sequence of grace periods, each separated by an RCU read-side
-critical section, would be required to partition the RCU read-side
-critical sections at the beginning and end of the chain?
-<p>@@QQA@@
-In theory, an infinite number.
-In practice, an unknown number that is sensitive to both implementation
-details and timing considerations.
-Therefore, even in practice, RCU users must abide by the theoretical rather
-than the practical answer.
-<p>@@QQE@@
-
-<h3><a name="Disabling Preemption Does Not Block Grace Periods">
-Disabling Preemption Does Not Block Grace Periods</a></h3>
-
-<p>
-There was a time when disabling preemption on any given CPU would block
-subsequent grace periods.
-However, this was an accident of implementation and is not a requirement.
-And in the current Linux-kernel implementation, disabling preemption
-on a given CPU in fact does not block grace periods, as Oleg Nesterov
-<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
-
-<p>
-If you need a preempt-disable region to block grace periods, you need to add
-<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
-as follows:
-
-<blockquote>
-<pre>
- 1 preempt_disable();
- 2 rcu_read_lock();
- 3 do_something();
- 4 rcu_read_unlock();
- 5 preempt_enable();
- 6
- 7 /* Spinlocks implicitly disable preemption. */
- 8 spin_lock(&amp;mylock);
- 9 rcu_read_lock();
-10 do_something();
-11 rcu_read_unlock();
-12 spin_unlock(&amp;mylock);
-</pre>
-</blockquote>
-
-<p>
-In theory, you could enter the RCU read-side critical section first,
-but it is more efficient to keep the entire RCU read-side critical
-section contained in the preempt-disable region as shown above.
-Of course, RCU read-side critical sections that extend outside of
-preempt-disable regions will work correctly, but such critical sections
-can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
-more work.
-And no, this is <i>not</i> an invitation to enclose all of your RCU
-read-side critical sections within preempt-disable regions, because
-doing so would degrade real-time response.
-
-<p>
-This non-requirement appeared with preemptible RCU.
-If you need a grace period that waits on non-preemptible code regions, use
-<a href="#Sched Flavor">RCU-sched</a>.
-
-<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
-
-<p>
-These parallelism facts of life are by no means specific to RCU, but
-the RCU implementation must abide by them.
-They therefore bear repeating:
-
-<ol>
-<li>	Any CPU or task may be delayed at any time,
-	and any attempts to avoid these delays by disabling
-	preemption, interrupts, or whatever are completely futile.
-	This is most obvious in preemptible user-level
-	environments and in virtualized environments (where
-	a given guest OS's VCPUs can be preempted at any time by
-	the underlying hypervisor), but can also happen in bare-metal
-	environments due to ECC errors, NMIs, and other hardware
-	events.
-	Although a delay of more than about 20 seconds can result
-	in splats, the RCU implementation is obligated to use
-	algorithms that can tolerate extremely long delays, but where
-	&ldquo;extremely long&rdquo; is not long enough to allow
-	wrap-around when incrementing a 64-bit counter.
-<li>	Both the compiler and the CPU can reorder memory accesses.
-	Where it matters, RCU must use compiler directives and
-	memory-barrier instructions to preserve ordering.
-<li>	Conflicting writes to memory locations in any given cache line
-	will result in expensive cache misses.
-	Greater numbers of concurrent writes and more-frequent
-	concurrent writes will result in more dramatic slowdowns.
-	RCU is therefore obligated to use algorithms that have
-	sufficient locality to avoid significant performance and
-	scalability problems.
-<li>	As a rough rule of thumb, only one CPU's worth of processing
-	may be carried out under the protection of any given exclusive
-	lock.
-	RCU must therefore use scalable locking designs.
-<li>	Counters are finite, especially on 32-bit systems.
-	RCU's use of counters must therefore tolerate counter wrap,
-	or be designed such that counter wrap would take way more
-	time than a single system is likely to run.
-	An uptime of ten years is quite possible, a runtime
-	of a century much less so.
-	As an example of the latter, RCU's dyntick-idle nesting counter
-	allows 54 bits for interrupt nesting level (this counter
-	is 64 bits even on a 32-bit system).
-	Overflowing this counter requires 2<sup>54</sup>
-	half-interrupts on a given CPU without that CPU ever going idle.
-	If a half-interrupt happened every microsecond, it would take
-	570 years of runtime to overflow this counter, which is currently
-	believed to be an acceptably long time.
-<li>	Linux systems can have thousands of CPUs running a single
-	Linux kernel in a single shared-memory environment.
-	RCU must therefore pay close attention to high-end scalability.
-</ol>
-
-<p>
-This last parallelism fact of life means that RCU must pay special
-attention to the preceding facts of life.
-The idea that Linux might scale to systems with thousands of CPUs would
-have been met with some skepticism in the 1990s, but these requirements
-would have otherwise have been unsurprising, even in the early 1990s.
-
-<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2>
-
-<p>
-These sections list quality-of-implementation requirements.
-Although an RCU implementation that ignores these requirements could
-still be used, it would likely be subject to limitations that would
-make it inappropriate for industrial-strength production use.
-Classes of quality-of-implementation requirements are as follows:
-
-<ol>
-<li>	<a href="#Specialization">Specialization</a>
-<li>	<a href="#Performance and Scalability">Performance and Scalability</a>
-<li>	<a href="#Composability">Composability</a>
-<li>	<a href="#Corner Cases">Corner Cases</a>
-</ol>
-
-<p>
-These classes is covered in the following sections.
-
-<h3><a name="Specialization">Specialization</a></h3>
-
-<p>
-RCU is and always has been intended primarily for read-mostly situations, as
-illustrated by the following figure.
-This means that RCU's read-side primitives are optimized, often at the
-expense of its update-side primitives.
-
-<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
-
-<p>
-This focus on read-mostly situations means that RCU must interoperate
-with other synchronization primitives.
-For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt>
-examples discussed earlier use RCU to protect readers and locking to
-coordinate updaters.
-However, the need extends much farther, requiring that a variety of
-synchronization primitives be legal within RCU read-side critical sections,
-including spinlocks, sequence locks, atomic operations, reference
-counters, and memory barriers.
-
-<p>@@QQ@@
-What about sleeping locks?
-<p>@@QQA@@
-These are forbidden within Linux-kernel RCU read-side critical sections
-because it is not legal to place a quiescent state (in this case,
-voluntary context switch) within an RCU read-side critical section.
-However, sleeping locks may be used within userspace RCU read-side critical
-sections, and also within Linux-kernel sleepable RCU
-<a href="#Sleepable RCU">(SRCU)</a>
-read-side critical sections.
-In addition, the -rt patchset turns spinlocks into a sleeping locks so
-that the corresponding critical sections can be preempted, which
-also means that these sleeplockified spinlocks (but not other sleeping locks!)
-may be acquire within -rt-Linux-kernel RCU read-side critical sections.
-
-<p>
-Note that it <i>is</i> legal for a normal RCU read-side critical section
-to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
-but only as long as it does not loop indefinitely attempting to
-conditionally acquire that sleeping locks.
-The key point is that things like <tt>mutex_trylock()</tt>
-either return with the mutex held, or return an error indication if
-the mutex was not immediately available.
-Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
-<p>@@QQE@@
-
-<p>
-It often comes as a surprise that many algorithms do not require a
-consistent view of data, but many can function in that mode,
-with network routing being the poster child.
-Internet routing algorithms take significant time to propagate
-updates, so that by the time an update arrives at a given system,
-that system has been sending network traffic the wrong way for
-a considerable length of time.
-Having a few threads continue to send traffic the wrong way for a
-few more milliseconds is clearly not a problem:  In the worst case,
-TCP retransmissions will eventually get the data where it needs to go.
-In general, when tracking the state of the universe outside of the
-computer, some level of inconsistency must be tolerated due to
-speed-of-light delays if nothing else.
-
-<p>
-Furthermore, uncertainty about external state is inherent in many cases.
-For example, a pair of veternarians might use heartbeat to determine
-whether or not a given cat was alive.
-But how long should they wait after the last heartbeat to decide that
-the cat is in fact dead?
-Waiting less than 400 milliseconds makes no sense because this would
-mean that a relaxed cat would be considered to cycle between death
-and life more than 100 times per minute.
-Moreover, just as with human beings, a cat's heart might stop for
-some period of time, so the exact wait period is a judgment call.
-One of our pair of veternarians might wait 30 seconds before pronouncing
-the cat dead, while the other might insist on waiting a full minute.
-The two veternarians would then disagree on the state of the cat during
-the final 30 seconds of the minute following the last heartbeat, as
-fancifully illustrated below:
-
-<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
-
-<p>
-Interestingly enough, this same situation applies to hardware.
-When push comes to shove, how do we tell whether or not some
-external server has failed?
-We send messages to it periodically, and declare it failed if we
-don't receive a response within a given period of time.
-Policy decisions can usually tolerate short
-periods of inconsistency.
-The policy was decided some time ago, and is only now being put into
-effect, so a few milliseconds of delay is normally inconsequential.
-
-<p>
-However, there are algorithms that absolutely must see consistent data.
-For example, the translation between a user-level SystemV semaphore
-ID to the corresponding in-kernel data structure is protected by RCU,
-but it is absolutely forbidden to update a semaphore that has just been
-removed.
-In the Linux kernel, this need for consistency is accommodated by acquiring
-spinlocks located in the in-kernel data structure from within
-the RCU read-side critical section, and this is indicated by the
-green box in the figure above.
-Many other techniques may be used, and are in fact used within the
-Linux kernel.
-
-<p>
-In short, RCU is not required to maintain consistency, and other
-mechanisms may be used in concert with RCU when consistency is required.
-RCU's specialization allows it to do its job extremely well, and its
-ability to interoperate with other synchronization mechanisms allows
-the right mix of synchronization tools to be used for a given job.
-
-<h3><a name="Performance and Scalability">Performance and Scalability</a></h3>
-
-<p>
-Energy efficiency is a critical component of performance today,
-and Linux-kernel RCU implementations must therefore avoid unnecessarily
-awakening idle CPUs.
-I cannot claim that this requirement was premeditated.
-In fact, I learned of it during a telephone conversation in which I
-was given &ldquo;frank and open&rdquo; feedback on the importance
-of energy efficiency in battery-powered systems and on specific
-energy-efficiency shortcomings of the Linux-kernel RCU implementation.
-In my experience, the battery-powered embedded community will consider
-any unnecessary wakeups to be extremely unfriendly acts.
-So much so that mere Linux-kernel-mailing-list posts are
-insufficient to vent their ire.
-
-<p>
-Memory consumption is not particularly important for in most
-situations, and has become decreasingly
-so as memory sizes have expanded and memory
-costs have plummeted.
-However, as I learned from Matt Mackall's
-<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a>
-efforts, memory footprint is critically important on single-CPU systems with
-non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus
-<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a>
-was born.
-Josh Triplett has since taken over the small-memory banner with his
-<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a>
-project, which resulted in
-<a href="#Sleepable RCU">SRCU</a>
-becoming optional for those kernels not needing it.
-
-<p>
-The remaining performance requirements are, for the most part,
-unsurprising.
-For example, in keeping with RCU's read-side specialization,
-<tt>rcu_dereference()</tt> should have negligible overhead (for
-example, suppression of a few minor compiler optimizations).
-Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> should have exactly zero overhead.
-
-<p>
-In preemptible environments, in the case where the RCU read-side
-critical section was not preempted (as will be the case for the
-highest-priority real-time process), <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> should have minimal overhead.
-In particular, they should not contain atomic read-modify-write
-operations, memory-barrier instructions, preemption disabling,
-interrupt disabling, or backwards branches.
-However, in the case where the RCU read-side critical section was preempted,
-<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts.
-This is why it is better to nest an RCU read-side critical section
-within a preempt-disable region than vice versa, at least in cases
-where that critical section is short enough to avoid unduly degrading
-real-time latencies.
-
-<p>
-The <tt>synchronize_rcu()</tt> grace-period-wait primitive is
-optimized for throughput.
-It may therefore incur several milliseconds of latency in addition to
-the duration of the longest RCU read-side critical section.
-On the other hand, multiple concurrent invocations of
-<tt>synchronize_rcu()</tt> are required to use batching optimizations
-so that they can be satisfied by a single underlying grace-period-wait
-operation.
-For example, in the Linux kernel, it is not unusual for a single
-grace-period-wait operation to serve more than
-<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a>
-of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation
-overhead down to nearly zero.
-However, the grace-period optimization is also required to avoid
-measurable degradation of real-time scheduling and interrupt latencies.
-
-<p>
-In some cases, the multi-millisecond <tt>synchronize_rcu()</tt>
-latencies are unacceptable.
-In these cases, <tt>synchronize_rcu_expedited()</tt> may be used
-instead, reducing the grace-period latency down to a few tens of
-microseconds on small systems, at least in cases where the RCU read-side
-critical sections are short.
-There are currently no special latency requirements for
-<tt>synchronize_rcu_expedited()</tt> on large systems, but,
-consistent with the empirical nature of the RCU specification,
-that is subject to change.
-However, there most definitely are scalability requirements:
-A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096
-CPUs should at least make reasonable forward progress.
-In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt>
-is permitted to impose modest degradation of real-time latency
-on non-idle online CPUs.
-That said, it will likely be necessary to take further steps to reduce this
-degradation, hopefully to roughly that of a scheduling-clock interrupt.
-
-<p>
-There are a number of situations where even
-<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period
-latency is unacceptable.
-In these situations, the asynchronous <tt>call_rcu()</tt> can be
-used in place of <tt>synchronize_rcu()</tt> as follows:
-
-<blockquote>
-<pre>
- 1 struct foo {
- 2   int a;
- 3   int b;
- 4   struct rcu_head rh;
- 5 };
- 6
- 7 static void remove_gp_cb(struct rcu_head *rhp)
- 8 {
- 9   struct foo *p = container_of(rhp, struct foo, rh);
-10
-11   kfree(p);
-12 }
-13
-14 bool remove_gp_asynchronous(void)
-15 {
-16   struct foo *p;
-17
-18   spin_lock(&amp;gp_lock);
-19   p = rcu_dereference(gp);
-20   if (!p) {
-21     spin_unlock(&amp;gp_lock);
-22     return false;
-23   }
-24   rcu_assign_pointer(gp, NULL);
-25   call_rcu(&amp;p-&gt;rh, remove_gp_cb);
-26   spin_unlock(&amp;gp_lock);
-27   return true;
-28 }
-</pre>
-</blockquote>
-
-<p>
-A definition of <tt>struct foo</tt> is finally needed, and appears
-on lines&nbsp;1-5.
-The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt>
-on line&nbsp;25, and will be invoked after the end of a subsequent
-grace period.
-This gets the same effect as <tt>remove_gp_synchronous()</tt>,
-but without forcing the updater to wait for a grace period to elapse.
-The <tt>call_rcu()</tt> function may be used in a number of
-situations where neither <tt>synchronize_rcu()</tt> nor
-<tt>synchronize_rcu_expedited()</tt> would be legal,
-including within preempt-disable code, <tt>local_bh_disable()</tt> code,
-interrupt-disable code, and interrupt handlers.
-However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
-The callback function (<tt>remove_gp_cb()</tt> in this case) will be
-executed within softirq (software interrupt) environment within the
-Linux kernel,
-either within a real softirq handler or under the protection
-of <tt>local_bh_disable()</tt>.
-In both the Linux kernel and in userspace, it is bad practice to
-write an RCU callback function that takes too long.
-Long-running operations should be relegated to separate threads or
-(in the Linux kernel) workqueues.
-
-<p>@@QQ@@
-Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
-After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
-structure, which would interact badly with concurrent insertions.
-Doesn't this mean that <tt>rcu_dereference()</tt> is required?
-<p>@@QQA@@
-Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
-any changes, including any insertions that <tt>rcu_dereference()</tt>
-would protect against.
-Therefore, any insertions will be delayed until after <tt>-&gt;gp_lock</tt>
-is released on line&nbsp;25, which in turn means that
-<tt>rcu_access_pointer()</tt> suffices.
-<p>@@QQE@@
-
-<p>
-However, all that <tt>remove_gp_cb()</tt> is doing is
-invoking <tt>kfree()</tt> on the data element.
-This is a common idiom, and is supported by <tt>kfree_rcu()</tt>,
-which allows &ldquo;fire and forget&rdquo; operation as shown below:
-
-<blockquote>
-<pre>
- 1 struct foo {
- 2   int a;
- 3   int b;
- 4   struct rcu_head rh;
- 5 };
- 6
- 7 bool remove_gp_faf(void)
- 8 {
- 9   struct foo *p;
-10
-11   spin_lock(&amp;gp_lock);
-12   p = rcu_dereference(gp);
-13   if (!p) {
-14     spin_unlock(&amp;gp_lock);
-15     return false;
-16   }
-17   rcu_assign_pointer(gp, NULL);
-18   kfree_rcu(p, rh);
-19   spin_unlock(&amp;gp_lock);
-20   return true;
-21 }
-</pre>
-</blockquote>
-
-<p>
-Note that <tt>remove_gp_faf()</tt> simply invokes
-<tt>kfree_rcu()</tt> and proceeds, without any need to pay any
-further attention to the subsequent grace period and <tt>kfree()</tt>.
-It is permissible to invoke <tt>kfree_rcu()</tt> from the same
-environments as for <tt>call_rcu()</tt>.
-Interestingly enough, DYNIX/ptx had the equivalents of
-<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not
-<tt>synchronize_rcu()</tt>.
-This was due to the fact that RCU was not heavily used within DYNIX/ptx,
-so the very few places that needed something like
-<tt>synchronize_rcu()</tt> simply open-coded it.
-
-<p>@@QQ@@
-Earlier it was claimed that <tt>call_rcu()</tt> and
-<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
-by readers.
-But how can that be correct, given that the invocation of the callback
-and the freeing of the memory (respectively) must still wait for
-a grace period to elapse?
-<p>@@QQA@@
-We could define things this way, but keep in mind that this sort of
-definition would say that updates in garbage-collected languages
-cannot complete until the next time the garbage collector runs,
-which does not seem at all reasonable.
-The key point is that in most cases, an updater using either
-<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
-next update as soon as it has invoked <tt>call_rcu()</tt> or
-<tt>kfree_rcu()</tt>, without having to wait for a subsequent
-grace period.
-<p>@@QQE@@
-
-<p>
-But what if the updater must wait for the completion of code to be
-executed after the end of the grace period, but has other tasks
-that can be carried out in the meantime?
-The polling-style <tt>get_state_synchronize_rcu()</tt> and
-<tt>cond_synchronize_rcu()</tt> functions may be used for this
-purpose, as shown below:
-
-<blockquote>
-<pre>
- 1 bool remove_gp_poll(void)
- 2 {
- 3   struct foo *p;
- 4   unsigned long s;
- 5
- 6   spin_lock(&amp;gp_lock);
- 7   p = rcu_access_pointer(gp);
- 8   if (!p) {
- 9     spin_unlock(&amp;gp_lock);
-10     return false;
-11   }
-12   rcu_assign_pointer(gp, NULL);
-13   spin_unlock(&amp;gp_lock);
-14   s = get_state_synchronize_rcu();
-15   do_something_while_waiting();
-16   cond_synchronize_rcu(s);
-17   kfree(p);
-18   return true;
-19 }
-</pre>
-</blockquote>
-
-<p>
-On line&nbsp;14, <tt>get_state_synchronize_rcu()</tt> obtains a
-&ldquo;cookie&rdquo; from RCU,
-then line&nbsp;15 carries out other tasks,
-and finally, line&nbsp;16 returns immediately if a grace period has
-elapsed in the meantime, but otherwise waits as required.
-The need for <tt>get_state_synchronize_rcu</tt> and
-<tt>cond_synchronize_rcu()</tt> has appeared quite recently,
-so it is too early to tell whether they will stand the test of time.
-
-<p>
-RCU thus provides a range of tools to allow updaters to strike the
-required tradeoff between latency, flexibility and CPU overhead.
-
-<h3><a name="Composability">Composability</a></h3>
-
-<p>
-Composability has received much attention in recent years, perhaps in part
-due to the collision of multicore hardware with object-oriented techniques
-designed in single-threaded environments for single-threaded use.
-And in theory, RCU read-side critical sections may be composed, and in
-fact may be nested arbitrarily deeply.
-In practice, as with all real-world implementations of composable
-constructs, there are limitations.
-
-<p>
-Implementations of RCU for which <tt>rcu_read_lock()</tt>
-and <tt>rcu_read_unlock()</tt> generate no code, such as
-Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be
-nested arbitrarily deeply.
-After all, there is no overhead.
-Except that if all these instances of <tt>rcu_read_lock()</tt>
-and <tt>rcu_read_unlock()</tt> are visible to the compiler,
-compilation will eventually fail due to exhausting memory,
-mass storage, or user patience, whichever comes first.
-If the nesting is not visible to the compiler, as is the case with
-mutually recursive functions each in its own translation unit,
-stack overflow will result.
-If the nesting takes the form of loops, either the control variable
-will overflow or (in the Linux kernel) you will get an RCU CPU stall warning.
-Nevertheless, this class of RCU implementations is one
-of the most composable constructs in existence.
-
-<p>
-RCU implementations that explicitly track nesting depth
-are limited by the nesting-depth counter.
-For example, the Linux kernel's preemptible RCU limits nesting to
-<tt>INT_MAX</tt>.
-This should suffice for almost all practical purposes.
-That said, a consecutive pair of RCU read-side critical sections
-between which there is an operation that waits for a grace period
-cannot be enclosed in another RCU read-side critical section.
-This is because it is not legal to wait for a grace period within
-an RCU read-side critical section:  To do so would result either
-in deadlock or
-in RCU implicitly splitting the enclosing RCU read-side critical
-section, neither of which is conducive to a long-lived and prosperous
-kernel.
-
-<p>
-It is worth noting that RCU is not alone in limiting composability.
-For example, many transactional-memory implementations prohibit
-composing a pair of transactions separated by an irrevocable
-operation (for example, a network receive operation).
-For another example, lock-based critical sections can be composed
-surprisingly freely, but only if deadlock is avoided.
-
-<p>
-In short, although RCU read-side critical sections are highly composable,
-care is required in some situations, just as is the case for any other
-composable synchronization mechanism.
-
-<h3><a name="Corner Cases">Corner Cases</a></h3>
-
-<p>
-A given RCU workload might have an endless and intense stream of
-RCU read-side critical sections, perhaps even so intense that there
-was never a point in time during which there was not at least one
-RCU read-side critical section in flight.
-RCU cannot allow this situation to block grace periods:  As long as
-all the RCU read-side critical sections are finite, grace periods
-must also be finite.
-
-<p>
-That said, preemptible RCU implementations could potentially result
-in RCU read-side critical sections being preempted for long durations,
-which has the effect of creating a long-duration RCU read-side
-critical section.
-This situation can arise only in heavily loaded systems, but systems using
-real-time priorities are of course more vulnerable.
-Therefore, RCU priority boosting is provided to help deal with this
-case.
-That said, the exact requirements on RCU priority boosting will likely
-evolve as more experience accumulates.
-
-<p>
-Other workloads might have very high update rates.
-Although one can argue that such workloads should instead use
-something other than RCU, the fact remains that RCU must
-handle such workloads gracefully.
-This requirement is another factor driving batching of grace periods,
-but it is also the driving force behind the checks for large numbers
-of queued RCU callbacks in the <tt>call_rcu()</tt> code path.
-Finally, high update rates should not delay RCU read-side critical
-sections, although some read-side delays can occur when using
-<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use
-of <tt>try_stop_cpus()</tt>.
-(In the future, <tt>synchronize_rcu_expedited()</tt> will be
-converted to use lighter-weight inter-processor interrupts (IPIs),
-but this will still disturb readers, though to a much smaller degree.)
-
-<p>
-Although all three of these corner cases were understood in the early
-1990s, a simple user-level test consisting of <tt>close(open(path))</tt>
-in a tight loop
-in the early 2000s suddenly provided a much deeper appreciation of the
-high-update-rate corner case.
-This test also motivated addition of some RCU code to react to high update
-rates, for example, if a given CPU finds itself with more than 10,000
-RCU callbacks queued, it will cause RCU to take evasive action by
-more aggressively starting grace periods and more aggressively forcing
-completion of grace-period processing.
-This evasive action causes the grace period to complete more quickly,
-but at the cost of restricting RCU's batching optimizations, thus
-increasing the CPU overhead incurred by that grace period.
-
-<h2><a name="Software-Engineering Requirements">
-Software-Engineering Requirements</a></h2>
-
-<p>
-Between Murphy's Law and &ldquo;To err is human&rdquo;, it is necessary to
-guard against mishaps and misuse:
-
-<ol>
-<li>	It is all too easy to forget to use <tt>rcu_read_lock()</tt>
-	everywhere that it is needed, so kernels built with
-	<tt>CONFIG_PROVE_RCU=y</tt> will spat if
-	<tt>rcu_dereference()</tt> is used outside of an
-	RCU read-side critical section.
-	Update-side code can use <tt>rcu_dereference_protected()</tt>,
-	which takes a
-	<a href="https://lwn.net/Articles/371986/">lockdep expression</a>
-	to indicate what is providing the protection.
-	If the indicated protection is not provided, a lockdep splat
-	is emitted.
-
-	<p>
-	Code shared between readers and updaters can use
-	<tt>rcu_dereference_check()</tt>, which also takes a
-	lockdep expression, and emits a lockdep splat if neither
-	<tt>rcu_read_lock()</tt> nor the indicated protection
-	is in place.
-	In addition, <tt>rcu_dereference_raw()</tt> is used in those
-	(hopefully rare) cases where the required protection cannot
-	be easily described.
-	Finally, <tt>rcu_read_lock_held()</tt> is provided to
-	allow a function to verify that it has been invoked within
-	an RCU read-side critical section.
-	I was made aware of this set of requirements shortly after Thomas
-	Gleixner audited a number of RCU uses.
-<li>	A given function might wish to check for RCU-related preconditions
-	upon entry, before using any other RCU API.
-	The <tt>rcu_lockdep_assert()</tt> does this job,
-	asserting the expression in kernels having lockdep enabled
-	and doing nothing otherwise.
-<li>	It is also easy to forget to use <tt>rcu_assign_pointer()</tt>
-	and <tt>rcu_dereference()</tt>, perhaps (incorrectly)
-	substituting a simple assignment.
-	To catch this sort of error, a given RCU-protected pointer may be
-	tagged with <tt>__rcu</tt>, after which running sparse
-	with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain
-	about simple-assignment accesses to that pointer.
-	Arnd Bergmann made me aware of this requirement, and also
-	supplied the needed
-	<a href="https://lwn.net/Articles/376011/">patch series</a>.
-<li>	Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt>
-	will splat if a data element is passed to <tt>call_rcu()</tt>
-	twice in a row, without a grace period in between.
-	(This error is similar to a double free.)
-	The corresponding <tt>rcu_head</tt> structures that are
-	dynamically allocated are automatically tracked, but
-	<tt>rcu_head</tt> structures allocated on the stack
-	must be initialized with <tt>init_rcu_head_on_stack()</tt>
-	and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>.
-	Similarly, statically allocated non-stack <tt>rcu_head</tt>
-	structures must be initialized with <tt>init_rcu_head()</tt>
-	and cleaned up with <tt>destroy_rcu_head()</tt>.
-	Mathieu Desnoyers made me aware of this requirement, and also
-	supplied the needed
-	<a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>.
-<li>	An infinite loop in an RCU read-side critical section will
-	eventually trigger an RCU CPU stall warning splat, with
-	the duration of &ldquo;eventually&rdquo; being controlled by the
-	<tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or,
-	alternatively, by the
-	<tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs
-	parameter.
-	However, RCU is not obligated to produce this splat
-	unless there is a grace period waiting on that particular
-	RCU read-side critical section.
-	<p>
-	Some extreme workloads might intentionally delay
-	RCU grace periods, and systems running those workloads can
-	be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt>
-	to suppress the splats.
-	This kernel parameter may also be set via <tt>sysfs</tt>.
-	Furthermore, RCU CPU stall warnings are counter-productive
-	during sysrq dumps and during panics.
-	RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and
-	<tt>rcu_sysrq_end()</tt> API members to be called before
-	and after long sysrq dumps.
-	RCU also supplies the <tt>rcu_panic()</tt> notifier that is
-	automatically invoked at the beginning of a panic to suppress
-	further RCU CPU stall warnings.
-
-	<p>
-	This requirement made itself known in the early 1990s, pretty
-	much the first time that it was necessary to debug a CPU stall.
-	That said, the initial implementation in DYNIX/ptx was quite
-	generic in comparison with that of Linux.
-<li>	Although it would be very good to detect pointers leaking out
-	of RCU read-side critical sections, there is currently no
-	good way of doing this.
-	One complication is the need to distinguish between pointers
-	leaking and pointers that have been handed off from RCU to
-	some other synchronization mechanism, for example, reference
-	counting.
-<li>	In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related
-	information is provided via both debugfs and event tracing.
-<li>	Open-coded use of <tt>rcu_assign_pointer()</tt> and
-	<tt>rcu_dereference()</tt> to create typical linked
-	data structures can be surprisingly error-prone.
-	Therefore, RCU-protected
-	<a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a>
-	and, more recently, RCU-protected
-	<a href="https://lwn.net/Articles/612100/">hash tables</a>
-	are available.
-	Many other special-purpose RCU-protected data structures are
-	available in the Linux kernel and the userspace RCU library.
-<li>	Some linked structures are created at compile time, but still
-	require <tt>__rcu</tt> checking.
-	The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this
-	purpose.
-<li>	It is not necessary to use <tt>rcu_assign_pointer()</tt>
-	when creating linked structures that are to be published via
-	a single external pointer.
-	The <tt>RCU_INIT_POINTER()</tt> macro is provided for
-	this task and also for assigning <tt>NULL</tt> pointers
-	at runtime.
-</ol>
-
-<p>
-This not a hard-and-fast list:  RCU's diagnostic capabilities will
-continue to be guided by the number and type of usage bugs found
-in real-world RCU usage.
-
-<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2>
-
-<p>
-The Linux kernel provides an interesting environment for all kinds of
-software, including RCU.
-Some of the relevant points of interest are as follows:
-
-<ol>
-<li>	<a href="#Configuration">Configuration</a>.
-<li>	<a href="#Firmware Interface">Firmware Interface</a>.
-<li>	<a href="#Early Boot">Early Boot</a>.
-<li>	<a href="#Interrupts and NMIs">
-	Interrupts and non-maskable interrupts (NMIs)</a>.
-<li>	<a href="#Loadable Modules">Loadable Modules</a>.
-<li>	<a href="#Hotplug CPU">Hotplug CPU</a>.
-<li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
-<li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
-<li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
-<li>	<a href="#Memory Efficiency">Memory Efficiency</a>.
-<li>	<a href="#Performance, Scalability, Response Time, and Reliability">
-	Performance, Scalability, Response Time, and Reliability</a>.
-</ol>
-
-<p>
-This list is probably incomplete, but it does give a feel for the
-most notable Linux-kernel complications.
-Each of the following sections covers one of the above topics.
-
-<h3><a name="Configuration">Configuration</a></h3>
-
-<p>
-RCU's goal is automatic configuration, so that almost nobody
-needs to worry about RCU's <tt>Kconfig</tt> options.
-And for almost all users, RCU does in fact work well
-&ldquo;out of the box.&rdquo;
-
-<p>
-However, there are specialized use cases that are handled by
-kernel boot parameters and <tt>Kconfig</tt> options.
-Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users
-about new <tt>Kconfig</tt> options, which requires almost all of them
-be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option.
-
-<p>
-This all should be quite obvious, but the fact remains that
-Linus Torvalds recently had to
-<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a>
-me of this requirement.
-
-<h3><a name="Firmware Interface">Firmware Interface</a></h3>
-
-<p>
-In many cases, kernel obtains information about the system from the
-firmware, and sometimes things are lost in translation.
-Or the translation is accurate, but the original message is bogus.
-
-<p>
-For example, some systems' firmware overreports the number of CPUs,
-sometimes by a large factor.
-If RCU naively believed the firmware, as it used to do,
-it would create too many per-CPU kthreads.
-Although the resulting system will still run correctly, the extra
-kthreads needlessly consume memory and can cause confusion
-when they show up in <tt>ps</tt> listings.
-
-<p>
-RCU must therefore wait for a given CPU to actually come online before
-it can allow itself to believe that the CPU actually exists.
-The resulting &ldquo;ghost CPUs&rdquo; (which are never going to
-come online) cause a number of
-<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>.
-
-<h3><a name="Early Boot">Early Boot</a></h3>
-
-<p>
-The Linux kernel's boot sequence is an interesting process,
-and RCU is used early, even before <tt>rcu_init()</tt>
-is invoked.
-In fact, a number of RCU's primitives can be used as soon as the
-initial task's <tt>task_struct</tt> is available and the
-boot CPU's per-CPU variables are set up.
-The read-side primitives (<tt>rcu_read_lock()</tt>,
-<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>,
-and <tt>rcu_access_pointer()</tt>) will operate normally very early on,
-as will <tt>rcu_assign_pointer()</tt>.
-
-<p>
-Although <tt>call_rcu()</tt> may be invoked at any
-time during boot, callbacks are not guaranteed to be invoked until after
-the scheduler is fully up and running.
-This delay in callback invocation is due to the fact that RCU does not
-invoke callbacks until it is fully initialized, and this full initialization
-cannot occur until after the scheduler has initialized itself to the
-point where RCU can spawn and run its kthreads.
-In theory, it would be possible to invoke callbacks earlier,
-however, this is not a panacea because there would be severe restrictions
-on what operations those callbacks could invoke.
-
-<p>
-Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
-<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
-(<a href="#Bottom-Half Flavor">discussed below</a>),
-and
-<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>
-will all operate normally
-during very early boot, the reason being that there is only one CPU
-and preemption is disabled.
-This means that the call <tt>synchronize_rcu()</tt> (or friends)
-itself is a quiescent
-state and thus a grace period, so the early-boot implementation can
-be a no-op.
-
-<p>
-Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt>
-continue to operate normally through the remainder of boot, courtesy
-of the fact that preemption is disabled across their RCU read-side
-critical sections and also courtesy of the fact that there is still
-only one CPU.
-However, once the scheduler starts initializing, preemption is enabled.
-There is still only a single CPU, but the fact that preemption is enabled
-means that the no-op implementation of <tt>synchronize_rcu()</tt> no
-longer works in <tt>CONFIG_PREEMPT=y</tt> kernels.
-Therefore, as soon as the scheduler starts initializing, the early-boot
-fastpath is disabled.
-This means that <tt>synchronize_rcu()</tt> switches to its runtime
-mode of operation where it posts callbacks, which in turn means that
-any call to <tt>synchronize_rcu()</tt> will block until the corresponding
-callback is invoked.
-Unfortunately, the callback cannot be invoked until RCU's runtime
-grace-period machinery is up and running, which cannot happen until
-the scheduler has initialized itself sufficiently to allow RCU's
-kthreads to be spawned.
-Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
-initialization can result in deadlock.
-
-<p>@@QQ@@
-So what happens with <tt>synchronize_rcu()</tt> during
-scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
-kernels?
-<p>@@QQA@@
-In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
-maps directly to <tt>synchronize_sched()</tt>.
-Therefore, <tt>synchronize_rcu()</tt> works normally throughout
-boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
-However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
-so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
-during scheduler initialization.
-<p>@@QQE@@
-
-<p>
-I learned of these boot-time requirements as a result of a series of
-system hangs.
-
-<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3>
-
-<p>
-The Linux kernel has interrupts, and RCU read-side critical sections are
-legal within interrupt handlers and within interrupt-disabled regions
-of code, as are invocations of <tt>call_rcu()</tt>.
-
-<p>
-Some Linux-kernel architectures can enter an interrupt handler from
-non-idle process context, and then just never leave it, instead stealthily
-transitioning back to process context.
-This trick is sometimes used to invoke system calls from inside the kernel.
-These &ldquo;half-interrupts&rdquo; mean that RCU has to be very careful
-about how it counts interrupt nesting levels.
-I learned of this requirement the hard way during a rewrite
-of RCU's dyntick-idle code.
-
-<p>
-The Linux kernel has non-maskable interrupts (NMIs), and
-RCU read-side critical sections are legal within NMI handlers.
-Thankfully, RCU update-side primitives, including
-<tt>call_rcu()</tt>, are prohibited within NMI handlers.
-
-<p>
-The name notwithstanding, some Linux-kernel architectures
-can have nested NMIs, which RCU must handle correctly.
-Andy Lutomirski
-<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
-with this requirement;
-he also kindly surprised me with
-<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
-that meets this requirement.
-
-<h3><a name="Loadable Modules">Loadable Modules</a></h3>
-
-<p>
-The Linux kernel has loadable modules, and these modules can
-also be unloaded.
-After a given module has been unloaded, any attempt to call
-one of its functions results in a segmentation fault.
-The module-unload functions must therefore cancel any
-delayed calls to loadable-module functions, for example,
-any outstanding <tt>mod_timer()</tt> must be dealt with
-via <tt>del_timer_sync()</tt> or similar.
-
-<p>
-Unfortunately, there is no way to cancel an RCU callback;
-once you invoke <tt>call_rcu()</tt>, the callback function is
-going to eventually be invoked, unless the system goes down first.
-Because it is normally considered socially irresponsible to crash the system
-in response to a module unload request, we need some other way
-to deal with in-flight RCU callbacks.
-
-<p>
-RCU therefore provides
-<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>,
-which waits until all in-flight RCU callbacks have been invoked.
-If a module uses <tt>call_rcu()</tt>, its exit function should therefore
-prevent any future invocation of <tt>call_rcu()</tt>, then invoke
-<tt>rcu_barrier()</tt>.
-In theory, the underlying module-unload code could invoke
-<tt>rcu_barrier()</tt> unconditionally, but in practice this would
-incur unacceptable latencies.
-
-<p>
-Nikita Danilov noted this requirement for an analogous filesystem-unmount
-situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU.
-The need for <tt>rcu_barrier()</tt> for module unloading became
-apparent later.
-
-<h3><a name="Hotplug CPU">Hotplug CPU</a></h3>
-
-<p>
-The Linux kernel supports CPU hotplug, which means that CPUs
-can come and go.
-It is of course illegal to use any RCU API member from an offline CPU.
-This requirement was present from day one in DYNIX/ptx, but
-on the other hand, the Linux kernel's CPU-hotplug implementation
-is &ldquo;interesting.&rdquo;
-
-<p>
-The Linux-kernel CPU-hotplug implementation has notifiers that
-are used to allow the various kernel subsystems (including RCU)
-to respond appropriately to a given CPU-hotplug operation.
-Most RCU operations may be invoked from CPU-hotplug notifiers,
-including even normal synchronous grace-period operations
-such as <tt>synchronize_rcu()</tt>.
-However, expedited grace-period operations such as
-<tt>synchronize_rcu_expedited()</tt> are not supported,
-due to the fact that current implementations block CPU-hotplug
-operations, which could result in deadlock.
-
-<p>
-In addition, all-callback-wait operations such as
-<tt>rcu_barrier()</tt> are also not supported, due to the
-fact that there are phases of CPU-hotplug operations where
-the outgoing CPU's callbacks will not be invoked until after
-the CPU-hotplug operation ends, which could also result in deadlock.
-
-<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3>
-
-<p>
-RCU depends on the scheduler, and the scheduler uses RCU to
-protect some of its data structures.
-This means the scheduler is forbidden from acquiring
-the runqueue locks and the priority-inheritance locks
-in the middle of an outermost RCU read-side critical section unless either
-(1)&nbsp;it releases them before exiting that same
-RCU read-side critical section, or
-(2)&nbsp;interrupts are disabled across
-that entire RCU read-side critical section.
-This same prohibition also applies (recursively!) to any lock that is acquired
-while holding any lock to which this prohibition applies.
-Adhering to this rule prevents preemptible RCU from invoking
-<tt>rcu_read_unlock_special()</tt> while either runqueue or
-priority-inheritance locks are held, thus avoiding deadlock.
-
-<p>
-Prior to v4.4, it was only necessary to disable preemption across
-RCU read-side critical sections that acquired scheduler locks.
-In v4.4, expedited grace periods started using IPIs, and these
-IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
-Therefore, this expedited-grace-period change required disabling of
-interrupts, not just preemption.
-
-<p>
-For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
-implementation must be written carefully to avoid similar deadlocks.
-In particular, <tt>rcu_read_unlock()</tt> must tolerate an
-interrupt where the interrupt handler invokes both
-<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
-This possibility requires <tt>rcu_read_unlock()</tt> to use
-negative nesting levels to avoid destructive recursion via
-interrupt handler's use of RCU.
-
-<p>
-This pair of mutual scheduler-RCU requirements came as a
-<a href="https://lwn.net/Articles/453002/">complete surprise</a>.
-
-<p>
-As noted above, RCU makes use of kthreads, and it is necessary to
-avoid excessive CPU-time accumulation by these kthreads.
-This requirement was no surprise, but RCU's violation of it
-when running context-switch-heavy workloads when built with
-<tt>CONFIG_NO_HZ_FULL=y</tt>
-<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>.
-RCU has made good progress towards meeting this requirement, even
-for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads,
-but there is room for further improvement.
-
-<h3><a name="Tracing and RCU">Tracing and RCU</a></h3>
-
-<p>
-It is possible to use tracing on RCU code, but tracing itself
-uses RCU.
-For this reason, <tt>rcu_dereference_raw_notrace()</tt>
-is provided for use by tracing, which avoids the destructive
-recursion that could otherwise ensue.
-This API is also used by virtualization in some architectures,
-where RCU readers execute in environments in which tracing
-cannot be used.
-The tracing folks both located the requirement and provided the
-needed fix, so this surprise requirement was relatively painless.
-
-<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
-
-<p>
-Interrupting idle CPUs is considered socially unacceptable,
-especially by people with battery-powered embedded systems.
-RCU therefore conserves energy by detecting which CPUs are
-idle, including tracking CPUs that have been interrupted from idle.
-This is a large part of the energy-efficiency requirement,
-so I learned of this via an irate phone call.
-
-<p>
-Because RCU avoids interrupting idle CPUs, it is illegal to
-execute an RCU read-side critical section on an idle CPU.
-(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat
-if you try it.)
-The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt>
-event tracing is provided to work around this restriction.
-In addition, <tt>rcu_is_watching()</tt> may be used to
-test whether or not it is currently legal to run RCU read-side
-critical sections on this CPU.
-I learned of the need for diagnostics on the one hand
-and <tt>RCU_NONIDLE()</tt> on the other while inspecting
-idle-loop code.
-Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
-which is used quite heavily in the idle loop.
-
-<p>
-It is similarly socially unacceptable to interrupt an
-<tt>nohz_full</tt> CPU running in userspace.
-RCU must therefore track <tt>nohz_full</tt> userspace
-execution.
-And in
-<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a>
-kernels, RCU must separately track idle CPUs on the one hand and
-CPUs that are either idle or executing in userspace on the other.
-In both cases, RCU must be able to sample state at two points in
-time, and be able to determine whether or not some other CPU spent
-any time idle and/or executing in userspace.
-
-<p>
-These energy-efficiency requirements have proven quite difficult to
-understand and to meet, for example, there have been more than five
-clean-sheet rewrites of RCU's energy-efficiency code, the last of
-which was finally able to demonstrate
-<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>.
-As noted earlier,
-I learned of many of these requirements via angry phone calls:
-Flaming me on the Linux-kernel mailing list was apparently not
-sufficient to fully vent their ire at RCU's energy-efficiency bugs!
-
-<h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
-
-<p>
-Although small-memory non-realtime systems can simply use Tiny RCU,
-code size is only one aspect of memory efficiency.
-Another aspect is the size of the <tt>rcu_head</tt> structure
-used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>.
-Although this structure contains nothing more than a pair of pointers,
-it does appear in many RCU-protected data structures, including
-some that are size critical.
-The <tt>page</tt> structure is a case in point, as evidenced by
-the many occurrences of the <tt>union</tt> keyword within that structure.
-
-<p>
-This need for memory efficiency is one reason that RCU uses hand-crafted
-singly linked lists to track the <tt>rcu_head</tt> structures that
-are waiting for a grace period to elapse.
-It is also the reason why <tt>rcu_head</tt> structures do not contain
-debug information, such as fields tracking the file and line of the
-<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them.
-Although this information might appear in debug-only kernel builds at some
-point, in the meantime, the <tt>-&gt;func</tt> field will often provide
-the needed debug information.
-
-<p>
-However, in some cases, the need for memory efficiency leads to even
-more extreme measures.
-Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field
-shares storage with a great many other structures that are used at
-various points in the corresponding page's lifetime.
-In order to correctly resolve certain
-<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>,
-the Linux kernel's memory-management subsystem needs a particular bit
-to remain zero during all phases of grace-period processing,
-and that bit happens to map to the bottom bit of the
-<tt>rcu_head</tt> structure's <tt>-&gt;next</tt> field.
-RCU makes this guarantee as long as <tt>call_rcu()</tt>
-is used to post the callback, as opposed to <tt>kfree_rcu()</tt>
-or some future &ldquo;lazy&rdquo;
-variant of <tt>call_rcu()</tt> that might one day be created for
-energy-efficiency purposes.
-
-<h3><a name="Performance, Scalability, Response Time, and Reliability">
-Performance, Scalability, Response Time, and Reliability</a></h3>
-
-<p>
-Expanding on the
-<a href="#Performance and Scalability">earlier discussion</a>,
-RCU is used heavily by hot code paths in performance-critical
-portions of the Linux kernel's networking, security, virtualization,
-and scheduling code paths.
-RCU must therefore use efficient implementations, especially in its
-read-side primitives.
-To that end, it would be good if preemptible RCU's implementation
-of <tt>rcu_read_lock()</tt> could be inlined, however, doing
-this requires resolving <tt>#include</tt> issues with the
-<tt>task_struct</tt> structure.
-
-<p>
-The Linux kernel supports hardware configurations with up to
-4096 CPUs, which means that RCU must be extremely scalable.
-Algorithms that involve frequent acquisitions of global locks or
-frequent atomic operations on global variables simply cannot be
-tolerated within the RCU implementation.
-RCU therefore makes heavy use of a combining tree based on the
-<tt>rcu_node</tt> structure.
-RCU is required to tolerate all CPUs continuously invoking any
-combination of RCU's runtime primitives with minimal per-operation
-overhead.
-In fact, in many cases, increasing load must <i>decrease</i> the
-per-operation overhead, witness the batching optimizations for
-<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>,
-<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>.
-As a general rule, RCU must cheerfully accept whatever the
-rest of the Linux kernel decides to throw at it.
-
-<p>
-The Linux kernel is used for real-time workloads, especially
-in conjunction with the
-<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>.
-The real-time-latency response requirements are such that the
-traditional approach of disabling preemption across RCU
-read-side critical sections is inappropriate.
-Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore
-use an RCU implementation that allows RCU read-side critical
-sections to be preempted.
-This requirement made its presence known after users made it
-clear that an earlier
-<a href="https://lwn.net/Articles/107930/">real-time patch</a>
-did not meet their needs, in conjunction with some
-<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a>
-encountered by a very early version of the -rt patchset.
-
-<p>
-In addition, RCU must make do with a sub-100-microsecond real-time latency
-budget.
-In fact, on smaller systems with the -rt patchset, the Linux kernel
-provides sub-20-microsecond real-time latencies for the whole kernel,
-including RCU.
-RCU's scalability and latency must therefore be sufficient for
-these sorts of configurations.
-To my surprise, the sub-100-microsecond real-time latency budget
-<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf">
-applies to even the largest systems [PDF]</a>,
-up to and including systems with 4096 CPUs.
-This real-time requirement motivated the grace-period kthread, which
-also simplified handling of a number of race conditions.
-
-<p>
-Finally, RCU's status as a synchronization primitive means that
-any RCU failure can result in arbitrary memory corruption that can be
-extremely difficult to debug.
-This means that RCU must be extremely reliable, which in
-practice also means that RCU must have an aggressive stress-test
-suite.
-This stress-test suite is called <tt>rcutorture</tt>.
-
-<p>
-Although the need for <tt>rcutorture</tt> was no surprise,
-the current immense popularity of the Linux kernel is posing
-interesting&mdash;and perhaps unprecedented&mdash;validation
-challenges.
-To see this, keep in mind that there are well over one billion
-instances of the Linux kernel running today, given Android
-smartphones, Linux-powered televisions, and servers.
-This number can be expected to increase sharply with the advent of
-the celebrated Internet of Things.
-
-<p>
-Suppose that RCU contains a race condition that manifests on average
-once per million years of runtime.
-This bug will be occurring about three times per <i>day</i> across
-the installed base.
-RCU could simply hide behind hardware error rates, given that no one
-should really expect their smartphone to last for a million years.
-However, anyone taking too much comfort from this thought should
-consider the fact that in most jurisdictions, a successful multi-year
-test of a given mechanism, which might include a Linux kernel,
-suffices for a number of types of safety-critical certifications.
-In fact, rumor has it that the Linux kernel is already being used
-in production for safety-critical applications.
-I don't know about you, but I would feel quite bad if a bug in RCU
-killed someone.
-Which might explain my recent focus on validation and verification.
-
-<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2>
-
-<p>
-One of the more surprising things about RCU is that there are now
-no fewer than five <i>flavors</i>, or API families.
-In addition, the primary flavor that has been the sole focus up to
-this point has two different implementations, non-preemptible and
-preemptible.
-The other four flavors are listed below, with requirements for each
-described in a separate section.
-
-<ol>
-<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
-<li>	<a href="#Sched Flavor">Sched Flavor</a>
-<li>	<a href="#Sleepable RCU">Sleepable RCU</a>
-<li>	<a href="#Tasks RCU">Tasks RCU</a>
-</ol>
-
-<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
-
-<p>
-The softirq-disable (AKA &ldquo;bottom-half&rdquo;,
-hence the &ldquo;_bh&rdquo; abbreviations)
-flavor of RCU, or <i>RCU-bh</i>, was developed by
-Dipankar Sarma to provide a flavor of RCU that could withstand the
-network-based denial-of-service attacks researched by Robert
-Olsson.
-These attacks placed so much networking load on the system
-that some of the CPUs never exited softirq execution,
-which in turn prevented those CPUs from ever executing a context switch,
-which, in the RCU implementation of that time, prevented grace periods
-from ever ending.
-The result was an out-of-memory condition and a system hang.
-
-<p>
-The solution was the creation of RCU-bh, which does
-<tt>local_bh_disable()</tt>
-across its read-side critical sections, and which uses the transition
-from one type of softirq processing to another as a quiescent state
-in addition to context switch, idle, user mode, and offline.
-This means that RCU-bh grace periods can complete even when some of
-the CPUs execute in softirq indefinitely, thus allowing algorithms
-based on RCU-bh to withstand network-based denial-of-service attacks.
-
-<p>
-Because
-<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt>
-disable and re-enable softirq handlers, any attempt to start a softirq
-handlers during the
-RCU-bh read-side critical section will be deferred.
-In this case, <tt>rcu_read_unlock_bh()</tt>
-will invoke softirq processing, which can take considerable time.
-One can of course argue that this softirq overhead should be associated
-with the code following the RCU-bh read-side critical section rather
-than <tt>rcu_read_unlock_bh()</tt>, but the fact
-is that most profiling tools cannot be expected to make this sort
-of fine distinction.
-For example, suppose that a three-millisecond-long RCU-bh read-side
-critical section executes during a time of heavy networking load.
-There will very likely be an attempt to invoke at least one softirq
-handler during that three milliseconds, but any such invocation will
-be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>.
-This can of course make it appear at first glance as if
-<tt>rcu_read_unlock_bh()</tt> was executing very slowly.
-
-<p>
-The
-<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a>
-includes
-<tt>rcu_read_lock_bh()</tt>,
-<tt>rcu_read_unlock_bh()</tt>,
-<tt>rcu_dereference_bh()</tt>,
-<tt>rcu_dereference_bh_check()</tt>,
-<tt>synchronize_rcu_bh()</tt>,
-<tt>synchronize_rcu_bh_expedited()</tt>,
-<tt>call_rcu_bh()</tt>,
-<tt>rcu_barrier_bh()</tt>, and
-<tt>rcu_read_lock_bh_held()</tt>.
-
-<h3><a name="Sched Flavor">Sched Flavor</a></h3>
-
-<p>
-Before preemptible RCU, waiting for an RCU grace period had the
-side effect of also waiting for all pre-existing interrupt
-and NMI handlers.
-However, there are legitimate preemptible-RCU implementations that
-do not have this property, given that any point in the code outside
-of an RCU read-side critical section can be a quiescent state.
-Therefore, <i>RCU-sched</i> was created, which follows &ldquo;classic&rdquo;
-RCU in that an RCU-sched grace period waits for for pre-existing
-interrupt and NMI handlers.
-In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched
-APIs have identical implementations, while kernels built with
-<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each.
-
-<p>
-Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels,
-<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
-disable and re-enable preemption, respectively.
-This means that if there was a preemption attempt during the
-RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt>
-will enter the scheduler, with all the latency and overhead entailed.
-Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look
-as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly.
-However, the highest-priority task won't be preempted, so that task
-will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations.
-
-<p>
-The
-<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a>
-includes
-<tt>rcu_read_lock_sched()</tt>,
-<tt>rcu_read_unlock_sched()</tt>,
-<tt>rcu_read_lock_sched_notrace()</tt>,
-<tt>rcu_read_unlock_sched_notrace()</tt>,
-<tt>rcu_dereference_sched()</tt>,
-<tt>rcu_dereference_sched_check()</tt>,
-<tt>synchronize_sched()</tt>,
-<tt>synchronize_rcu_sched_expedited()</tt>,
-<tt>call_rcu_sched()</tt>,
-<tt>rcu_barrier_sched()</tt>, and
-<tt>rcu_read_lock_sched_held()</tt>.
-However, anything that disables preemption also marks an RCU-sched
-read-side critical section, including
-<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>,
-<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>,
-and so on.
-
-<h3><a name="Sleepable RCU">Sleepable RCU</a></h3>
-
-<p>
-For well over a decade, someone saying &ldquo;I need to block within
-an RCU read-side critical section&rdquo; was a reliable indication
-that this someone did not understand RCU.
-After all, if you are always blocking in an RCU read-side critical
-section, you can probably afford to use a higher-overhead synchronization
-mechanism.
-However, that changed with the advent of the Linux kernel's notifiers,
-whose RCU read-side critical
-sections almost never sleep, but sometimes need to.
-This resulted in the introduction of
-<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>,
-or <i>SRCU</i>.
-
-<p>
-SRCU allows different domains to be defined, with each such domain
-defined by an instance of an <tt>srcu_struct</tt> structure.
-A pointer to this structure must be passed in to each SRCU function,
-for example, <tt>synchronize_srcu(&amp;ss)</tt>, where
-<tt>ss</tt> is the <tt>srcu_struct</tt> structure.
-The key benefit of these domains is that a slow SRCU reader in one
-domain does not delay an SRCU grace period in some other domain.
-That said, one consequence of these domains is that read-side code
-must pass a &ldquo;cookie&rdquo; from <tt>srcu_read_lock()</tt>
-to <tt>srcu_read_unlock()</tt>, for example, as follows:
-
-<blockquote>
-<pre>
- 1 int idx;
- 2
- 3 idx = srcu_read_lock(&amp;ss);
- 4 do_something();
- 5 srcu_read_unlock(&amp;ss, idx);
-</pre>
-</blockquote>
-
-<p>
-As noted above, it is legal to block within SRCU read-side critical sections,
-however, with great power comes great responsibility.
-If you block forever in one of a given domain's SRCU read-side critical
-sections, then that domain's grace periods will also be blocked forever.
-Of course, one good way to block forever is to deadlock, which can
-happen if any operation in a given domain's SRCU read-side critical
-section can block waiting, either directly or indirectly, for that domain's
-grace period to elapse.
-For example, this results in a self-deadlock:
-
-<blockquote>
-<pre>
- 1 int idx;
- 2
- 3 idx = srcu_read_lock(&amp;ss);
- 4 do_something();
- 5 synchronize_srcu(&amp;ss);
- 6 srcu_read_unlock(&amp;ss, idx);
-</pre>
-</blockquote>
-
-<p>
-However, if line&nbsp;5 acquired a mutex that was held across
-a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>,
-deadlock would still be possible.
-Furthermore, if line&nbsp;5 acquired a mutex that was held across
-a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>,
-and if an <tt>ss1</tt>-domain SRCU read-side critical section
-acquired another mutex that was held across as <tt>ss</tt>-domain
-<tt>synchronize_srcu()</tt>,
-deadlock would again be possible.
-Such a deadlock cycle could extend across an arbitrarily large number
-of different SRCU domains.
-Again, with great power comes great responsibility.
-
-<p>
-Unlike the other RCU flavors, SRCU read-side critical sections can
-run on idle and even offline CPUs.
-This ability requires that <tt>srcu_read_lock()</tt> and
-<tt>srcu_read_unlock()</tt> contain memory barriers, which means
-that SRCU readers will run a bit slower than would RCU readers.
-It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt>
-API, which, in combination with <tt>srcu_read_unlock()</tt>,
-guarantees a full memory barrier.
-
-<p>
-The
-<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a>
-includes
-<tt>srcu_read_lock()</tt>,
-<tt>srcu_read_unlock()</tt>,
-<tt>srcu_dereference()</tt>,
-<tt>srcu_dereference_check()</tt>,
-<tt>synchronize_srcu()</tt>,
-<tt>synchronize_srcu_expedited()</tt>,
-<tt>call_srcu()</tt>,
-<tt>srcu_barrier()</tt>, and
-<tt>srcu_read_lock_held()</tt>.
-It also includes
-<tt>DEFINE_SRCU()</tt>,
-<tt>DEFINE_STATIC_SRCU()</tt>, and
-<tt>init_srcu_struct()</tt>
-APIs for defining and initializing <tt>srcu_struct</tt> structures.
-
-<h3><a name="Tasks RCU">Tasks RCU</a></h3>
-
-<p>
-Some forms of tracing use &ldquo;tramopolines&rdquo; to handle the
-binary rewriting required to install different types of probes.
-It would be good to be able to free old trampolines, which sounds
-like a job for some form of RCU.
-However, because it is necessary to be able to install a trace
-anywhere in the code, it is not possible to use read-side markers
-such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
-In addition, it does not work to have these markers in the trampoline
-itself, because there would need to be instructions following
-<tt>rcu_read_unlock()</tt>.
-Although <tt>synchronize_rcu()</tt> would guarantee that execution
-reached the <tt>rcu_read_unlock()</tt>, it would not be able to
-guarantee that execution had completely left the trampoline.
-
-<p>
-The solution, in the form of
-<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>,
-is to have implicit
-read-side critical sections that are delimited by voluntary context
-switches, that is, calls to <tt>schedule()</tt>,
-<tt>cond_resched_rcu_qs()</tt>, and
-<tt>synchronize_rcu_tasks()</tt>.
-In addition, transitions to and from userspace execution also delimit
-tasks-RCU read-side critical sections.
-
-<p>
-The tasks-RCU API is quite compact, consisting only of
-<tt>call_rcu_tasks()</tt>,
-<tt>synchronize_rcu_tasks()</tt>, and
-<tt>rcu_barrier_tasks()</tt>.
-
-<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
-
-<p>
-One of the tricks that RCU uses to attain update-side scalability is
-to increase grace-period latency with increasing numbers of CPUs.
-If this becomes a serious problem, it will be necessary to rework the
-grace-period state machine so as to avoid the need for the additional
-latency.
-
-<p>
-Expedited grace periods scan the CPUs, so their latency and overhead
-increases with increasing numbers of CPUs.
-If this becomes a serious problem on large systems, it will be necessary
-to do some redesign to avoid this scalability problem.
-
-<p>
-RCU disables CPU hotplug in a few places, perhaps most notably in the
-expedited grace-period and <tt>rcu_barrier()</tt> operations.
-If there is a strong reason to use expedited grace periods in CPU-hotplug
-notifiers, it will be necessary to avoid disabling CPU hotplug.
-This would introduce some complexity, so there had better be a <i>very</i>
-good reason.
-
-<p>
-The tradeoff between grace-period latency on the one hand and interruptions
-of other CPUs on the other hand may need to be re-examined.
-The desire is of course for zero grace-period latency as well as zero
-interprocessor interrupts undertaken during an expedited grace period
-operation.
-While this ideal is unlikely to be achievable, it is quite possible that
-further improvements can be made.
-
-<p>
-The multiprocessor implementations of RCU use a combining tree that
-groups CPUs so as to reduce lock contention and increase cache locality.
-However, this combining tree does not spread its memory across NUMA
-nodes nor does it align the CPU groups with hardware features such
-as sockets or cores.
-Such spreading and alignment is currently believed to be unnecessary
-because the hotpath read-side primitives do not access the combining
-tree, nor does <tt>call_rcu()</tt> in the common case.
-If you believe that your architecture needs such spreading and alignment,
-then your architecture should also benefit from the
-<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set
-to the number of CPUs in a socket, NUMA node, or whatever.
-If the number of CPUs is too large, use a fraction of the number of
-CPUs.
-If the number of CPUs is a large prime number, well, that certainly
-is an &ldquo;interesting&rdquo; architectural choice!
-More flexible arrangements might be considered, but only if
-<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only
-if the inadequacy has been demonstrated by a carefully run and
-realistic system-level workload.
-
-<p>
-Please note that arrangements that require RCU to remap CPU numbers will
-require extremely good demonstration of need and full exploration of
-alternatives.
-
-<p>
-There is an embarrassingly large number of flavors of RCU, and this
-number has been increasing over time.
-Perhaps it will be possible to combine some at some future date.
-
-<p>
-RCU's various kthreads are reasonably recent additions.
-It is quite likely that adjustments will be required to more gracefully
-handle extreme loads.
-It might also be necessary to be able to relate CPU utilization by
-RCU's kthreads and softirq handlers to the code that instigated this
-CPU utilization.
-For example, RCU callback overhead might be charged back to the
-originating <tt>call_rcu()</tt> instance, though probably not
-in production kernels.
-
-<h2><a name="Summary">Summary</a></h2>
-
-<p>
-This document has presented more than two decade's worth of RCU
-requirements.
-Given that the requirements keep changing, this will not be the last
-word on this subject, but at least it serves to get an important
-subset of the requirements set forth.
-
-<h2><a name="Acknowledgments">Acknowledgments</a></h2>
-
-I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar,
-Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and
-Andy Lutomirski for their help in rendering
-this article human readable, and to Michelle Rankin for her support
-of this effort.
-Other contributions are acknowledged in the Linux kernel's git archive.
-The cartoon is copyright (c) 2013 by Melissa Broussard,
-and is provided
-under the terms of the Creative Commons Attribution-Share Alike 3.0
-United States license.
-
-<p>@@QQAL@@
-
-</body></html>
diff --git a/Documentation/RCU/Design/htmlqqz.sh b/Documentation/RCU/Design/htmlqqz.sh
deleted file mode 100755
index d354f069559b..000000000000
--- a/Documentation/RCU/Design/htmlqqz.sh
+++ /dev/null
@@ -1,108 +0,0 @@
-#!/bin/sh
-#
-# Usage: sh htmlqqz.sh file
-#
-# Extracts and converts quick quizzes in a proto-HTML document file.htmlx.
-# Commands, all of which must be on a line by themselves:
-#
-#	"<p>@@QQ@@": Start of a quick quiz.
-#	"<p>@@QQA@@": Start of a quick-quiz answer.
-#	"<p>@@QQE@@": End of a quick-quiz answer, and thus of the quick quiz.
-#	"<p>@@QQAL@@": Place to put quick-quiz answer list.
-#
-# Places the result in file.html.
-#
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation; either version 2 of the License, or
-# (at your option) any later version.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program; if not, you can access it online at
-# http://www.gnu.org/licenses/gpl-2.0.html.
-#
-# Copyright (c) 2013 Paul E. McKenney, IBM Corporation.
-
-fn=$1
-if test ! -r $fn.htmlx
-then
-	echo "Error: $fn.htmlx unreadable."
-	exit 1
-fi
-
-echo "<!-- DO NOT HAND EDIT. -->" > $fn.html
-echo "<!-- Instead, edit $fn.htmlx and run 'sh htmlqqz.sh $fn' -->" >> $fn.html
-awk < $fn.htmlx >> $fn.html '
-
-state == "" && $1 != "<p>@@QQ@@" && $1 != "<p>@@QQAL@@" {
-	print $0;
-	if ($0 ~ /^<p>@@QQ/)
-		print "Bad Quick Quiz command: " NR " (expected <p>@@QQ@@ or <p>@@QQAL@@)." > "/dev/stderr"
-	next;
-}
-
-state == "" && $1 == "<p>@@QQ@@" {
-	qqn++;
-	qqlineno = NR;
-	haveqq = 1;
-	state = "qq";
-	print "<p><a name=\"Quick Quiz " qqn "\"><b>Quick Quiz " qqn "</b>:</a>"
-	next;
-}
-
-state == "qq" && $1 != "<p>@@QQA@@" {
-	qq[qqn] = qq[qqn] $0 "\n";
-	print $0
-	if ($0 ~ /^<p>@@QQ/)
-		print "Bad Quick Quiz command: " NR ". (expected <p>@@QQA@@)" > "/dev/stderr"
-	next;
-}
-
-state == "qq" && $1 == "<p>@@QQA@@" {
-	state = "qqa";
-	print "<br><a href=\"#qq" qqn "answer\">Answer</a>"
-	next;
-}
-
-state == "qqa" && $1 != "<p>@@QQE@@" {
-	qqa[qqn] = qqa[qqn] $0 "\n";
-	if ($0 ~ /^<p>@@QQ/)
-		print "Bad Quick Quiz command: " NR " (expected <p>@@QQE@@)." > "/dev/stderr"
-	next;
-}
-
-state == "qqa" && $1 == "<p>@@QQE@@" {
-	state = "";
-	next;
-}
-
-state == "" && $1 == "<p>@@QQAL@@" {
-	haveqq = "";
-	print "<h3><a name=\"Answers to Quick Quizzes\">"
-	print "Answers to Quick Quizzes</a></h3>"
-	print "";
-	for (i = 1; i <= qqn; i++) {
-		print "<a name=\"qq" i "answer\"></a>"
-		print "<p><b>Quick Quiz " i "</b>:"
-		print qq[i];
-		print "";
-		print "</p><p><b>Answer</b>:"
-		print qqa[i];
-		print "";
-		print "</p><p><a href=\"#Quick%20Quiz%20" i "\"><b>Back to Quick Quiz " i "</b>.</a>"
-		print "";
-	}
-	next;
-}
-
-END {
-	if (state != "")
-		print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr"
-	else if (haveqq)
-		print "Missing \"<p>@@QQAL@@\", no Quick Quiz." > "/dev/stderr"
-}'
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index ec6998b1b6d0..00a3a38b375a 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -237,17 +237,17 @@ o	"ktl" is the low-order 16 bits (in hexadecimal) of the count of
 
 The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
 
-s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
+s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
 
 These fields are as follows:
 
 o	"s" is the sequence number, with an odd number indicating that
 	an expedited grace period is in progress.
 
-o	"wd0", "wd1", "wd2", and "wd3" are the number of times that an
-	attempt to start an expedited grace period found that someone
-	else had completed an expedited grace period that satisfies the
-	attempted request.  "Our work is done."
+o	"wd1", "wd2", and "wd3" are the number of times that an attempt
+	to start an expedited grace period found that someone else had
+	completed an expedited grace period that satisfies the attempted
+	request.  "Our work is done."
 
 o	"n" is number of times that a concurrent CPU-hotplug operation
 	forced a fallback to a normal grace period.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index dc49c6712b17..111770ffa10e 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -681,22 +681,30 @@ Although RCU can be used in many different ways, a very common use of
 RCU is analogous to reader-writer locking.  The following unified
 diff shows how closely related RCU and reader-writer locking can be.
 
+	@@ -5,5 +5,5 @@ struct el {
+	 	int data;
+	 	/* Other data fields */
+	 };
+	-rwlock_t listmutex;
+	+spinlock_t listmutex;
+	 struct el head;
+
 	@@ -13,15 +14,15 @@
 		struct list_head *lp;
 		struct el *p;
 
-	-	read_lock();
+	-	read_lock(&listmutex);
 	-	list_for_each_entry(p, head, lp) {
 	+	rcu_read_lock();
 	+	list_for_each_entry_rcu(p, head, lp) {
 			if (p->key == key) {
 				*result = p->data;
-	-			read_unlock();
+	-			read_unlock(&listmutex);
 	+			rcu_read_unlock();
 				return 1;
 			}
 		}
-	-	read_unlock();
+	-	read_unlock(&listmutex);
 	+	rcu_read_unlock();
 		return 0;
 	 }
@@ -732,7 +740,7 @@ Or, for those who prefer a side-by-side listing:
  5   int data;                          5   int data;
  6   /* Other data fields */            6   /* Other data fields */
  7 };                                   7 };
- 8 spinlock_t listmutex;                8 spinlock_t listmutex;
+ 8 rwlock_t listmutex;                  8 spinlock_t listmutex;
  9 struct el head;                      9 struct el head;
 
  1 int search(long key, int *result)    1 int search(long key, int *result)
@@ -740,15 +748,15 @@ Or, for those who prefer a side-by-side listing:
  3   struct list_head *lp;              3   struct list_head *lp;
  4   struct el *p;                      4   struct el *p;
  5                                      5
- 6   read_lock();                       6   rcu_read_lock();
+ 6   read_lock(&listmutex);             6   rcu_read_lock();
  7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
  8     if (p->key == key) {             8     if (p->key == key) {
  9       *result = p->data;             9       *result = p->data;
-10       read_unlock();                10       rcu_read_unlock();
+10       read_unlock(&listmutex);      10       rcu_read_unlock();
 11       return 1;                     11       return 1;
 12     }                               12     }
 13   }                                 13   }
-14   read_unlock();                    14   rcu_read_unlock();
+14   read_unlock(&listmutex);          14   rcu_read_unlock();
 15   return 0;                         15   return 0;
 16 }                                   16 }
 
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 0b3de80ec8f6..49673bd30b87 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3284,6 +3284,44 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Lazy RCU callbacks are those which RCU can
 			prove do nothing more than free memory.
 
+	rcuperf.gp_exp= [KNL]
+			Measure performance of expedited synchronous
+			grace-period primitives.
+
+	rcuperf.holdoff= [KNL]
+			Set test-start holdoff period.  The purpose of
+			this parameter is to delay the start of the
+			test until boot completes in order to avoid
+			interference.
+
+	rcuperf.nreaders= [KNL]
+			Set number of RCU readers.  The value -1 selects
+			N, where N is the number of CPUs.  A value
+			"n" less than -1 selects N-n+1, where N is again
+			the number of CPUs.  For example, -2 selects N
+			(the number of CPUs), -3 selects N+1, and so on.
+			A value of "n" less than or equal to -N selects
+			a single reader.
+
+	rcuperf.nwriters= [KNL]
+			Set number of RCU writers.  The values operate
+			the same as for rcuperf.nreaders.
+			N, where N is the number of CPUs
+
+	rcuperf.perf_runnable= [BOOT]
+			Start rcuperf running at boot time.
+
+	rcuperf.shutdown= [KNL]
+			Shut the system down after performance tests
+			complete.  This is useful for hands-off automated
+			testing.
+
+	rcuperf.perf_type= [KNL]
+			Specify the RCU implementation to test.
+
+	rcuperf.verbose= [KNL]
+			Enable additional printk() statements.
+
 	rcutorture.cbflood_inter_holdoff= [KNL]
 			Set holdoff time (jiffies) between successive
 			callback-flood tests.
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2657aff2725b..5f1533e3d032 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -508,14 +508,7 @@ int rcu_read_lock_bh_held(void);
  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
  * critical section unless it can prove otherwise.
  */
-#ifdef CONFIG_PREEMPT_COUNT
 int rcu_read_lock_sched_held(void);
-#else /* #ifdef CONFIG_PREEMPT_COUNT */
-static inline int rcu_read_lock_sched_held(void)
-{
-	return 1;
-}
-#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
 
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
@@ -532,18 +525,10 @@ static inline int rcu_read_lock_bh_held(void)
 	return 1;
 }
 
-#ifdef CONFIG_PREEMPT_COUNT
 static inline int rcu_read_lock_sched_held(void)
 {
-	return preempt_count() != 0 || irqs_disabled();
-}
-#else /* #ifdef CONFIG_PREEMPT_COUNT */
-static inline int rcu_read_lock_sched_held(void)
-{
-	return 1;
+	return !preemptible();
 }
-#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
-
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 #ifdef CONFIG_PROVE_RCU
@@ -1144,4 +1129,17 @@ static inline void rcu_sysidle_force_exit(void)
 #endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 
 
+/*
+ * Dump the ftrace buffer, but only one time per callsite per boot.
+ */
+#define rcu_ftrace_dump(oops_dump_mode) \
+do { \
+	static atomic_t ___rfd_beenhere = ATOMIC_INIT(0); \
+	\
+	if (!atomic_read(&___rfd_beenhere) && \
+	    !atomic_xchg(&___rfd_beenhere, 1)) \
+		ftrace_dump(oops_dump_mode); \
+} while (0)
+
+
 #endif /* __LINUX_RCUPDATE_H */
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 64809aea661c..93aea75029fb 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -149,6 +149,22 @@ static inline unsigned long rcu_batches_completed_sched(void)
 	return 0;
 }
 
+/*
+ * Return the number of expedited grace periods completed.
+ */
+static inline unsigned long rcu_exp_batches_completed(void)
+{
+	return 0;
+}
+
+/*
+ * Return the number of expedited sched grace periods completed.
+ */
+static inline unsigned long rcu_exp_batches_completed_sched(void)
+{
+	return 0;
+}
+
 static inline void rcu_force_quiescent_state(void)
 {
 }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index ad1eda9fa4da..5043cb823fb2 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -87,6 +87,8 @@ unsigned long rcu_batches_started_sched(void);
 unsigned long rcu_batches_completed(void);
 unsigned long rcu_batches_completed_bh(void);
 unsigned long rcu_batches_completed_sched(void);
+unsigned long rcu_exp_batches_completed(void);
+unsigned long rcu_exp_batches_completed_sched(void);
 void show_rcu_gp_kthreads(void);
 
 void rcu_force_quiescent_state(void);
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index ef72c4aada56..d3e756539d44 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -172,6 +172,77 @@ TRACE_EVENT(rcu_grace_period_init,
 );
 
 /*
+ * Tracepoint for expedited grace-period events.  Takes a string identifying
+ * the RCU flavor, the expedited grace-period sequence number, and a string
+ * identifying the grace-period-related event as follows:
+ *
+ *	"snap": Captured snapshot of expedited grace period sequence number.
+ *	"start": Started a real expedited grace period.
+ *	"end": Ended a real expedited grace period.
+ *	"endwake": Woke piggybackers up.
+ *	"done": Someone else did the expedited grace period for us.
+ */
+TRACE_EVENT(rcu_exp_grace_period,
+
+	TP_PROTO(const char *rcuname, unsigned long gpseq, const char *gpevent),
+
+	TP_ARGS(rcuname, gpseq, gpevent),
+
+	TP_STRUCT__entry(
+		__field(const char *, rcuname)
+		__field(unsigned long, gpseq)
+		__field(const char *, gpevent)
+	),
+
+	TP_fast_assign(
+		__entry->rcuname = rcuname;
+		__entry->gpseq = gpseq;
+		__entry->gpevent = gpevent;
+	),
+
+	TP_printk("%s %lu %s",
+		  __entry->rcuname, __entry->gpseq, __entry->gpevent)
+);
+
+/*
+ * Tracepoint for expedited grace-period funnel-locking events.  Takes a
+ * string identifying the RCU flavor, an integer identifying the rcu_node
+ * combining-tree level, another pair of integers identifying the lowest-
+ * and highest-numbered CPU associated with the current rcu_node structure,
+ * and a string.  identifying the grace-period-related event as follows:
+ *
+ *	"nxtlvl": Advance to next level of rcu_node funnel
+ *	"wait": Wait for someone else to do expedited GP
+ */
+TRACE_EVENT(rcu_exp_funnel_lock,
+
+	TP_PROTO(const char *rcuname, u8 level, int grplo, int grphi,
+		 const char *gpevent),
+
+	TP_ARGS(rcuname, level, grplo, grphi, gpevent),
+
+	TP_STRUCT__entry(
+		__field(const char *, rcuname)
+		__field(u8, level)
+		__field(int, grplo)
+		__field(int, grphi)
+		__field(const char *, gpevent)
+	),
+
+	TP_fast_assign(
+		__entry->rcuname = rcuname;
+		__entry->level = level;
+		__entry->grplo = grplo;
+		__entry->grphi = grphi;
+		__entry->gpevent = gpevent;
+	),
+
+	TP_printk("%s %d %d %d %s",
+		  __entry->rcuname, __entry->level, __entry->grplo,
+		  __entry->grphi, __entry->gpevent)
+);
+
+/*
  * Tracepoint for RCU no-CBs CPU callback handoffs.  This event is intended
  * to assist debugging of these handoffs.
  *
@@ -704,11 +775,15 @@ TRACE_EVENT(rcu_barrier,
 #else /* #ifdef CONFIG_RCU_TRACE */
 
 #define trace_rcu_grace_period(rcuname, gpnum, gpevent) do { } while (0)
-#define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
-				    qsmask) do { } while (0)
 #define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
+#define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
+				    qsmask) do { } while (0)
+#define trace_rcu_exp_grace_period(rcuname, gqseq, gpevent) \
+	do { } while (0)
+#define trace_rcu_exp_funnel_lock(rcuname, level, grplo, grphi, gpevent) \
+	do { } while (0)
 #define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
 #define trace_rcu_preempt_task(rcuname, pid, gpnum) do { } while (0)
 #define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)
diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
index 032b2c015beb..18dfc485225c 100644
--- a/kernel/rcu/Makefile
+++ b/kernel/rcu/Makefile
@@ -5,6 +5,7 @@ KCOV_INSTRUMENT := n
 obj-y += update.o sync.o
 obj-$(CONFIG_SRCU) += srcu.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
+obj-$(CONFIG_RCU_PERF_TEST) += rcuperf.o
 obj-$(CONFIG_TREE_RCU) += tree.o
 obj-$(CONFIG_PREEMPT_RCU) += tree.o
 obj-$(CONFIG_TREE_RCU_TRACE) += tree_trace.o
diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
new file mode 100644
index 000000000000..3cee0d8393ed
--- /dev/null
+++ b/kernel/rcu/rcuperf.c
@@ -0,0 +1,655 @@
+/*
+ * Read-Copy Update module-based performance-test facility
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, you can access it online at
+ * http://www.gnu.org/licenses/gpl-2.0.html.
+ *
+ * Copyright (C) IBM Corporation, 2015
+ *
+ * Authors: Paul E. McKenney <paulmck@us.ibm.com>
+ */
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/kthread.h>
+#include <linux/err.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/rcupdate.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <linux/atomic.h>
+#include <linux/bitops.h>
+#include <linux/completion.h>
+#include <linux/moduleparam.h>
+#include <linux/percpu.h>
+#include <linux/notifier.h>
+#include <linux/reboot.h>
+#include <linux/freezer.h>
+#include <linux/cpu.h>
+#include <linux/delay.h>
+#include <linux/stat.h>
+#include <linux/srcu.h>
+#include <linux/slab.h>
+#include <asm/byteorder.h>
+#include <linux/torture.h>
+#include <linux/vmalloc.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.vnet.ibm.com>");
+
+#define PERF_FLAG "-perf:"
+#define PERFOUT_STRING(s) \
+	pr_alert("%s" PERF_FLAG s "\n", perf_type)
+#define VERBOSE_PERFOUT_STRING(s) \
+	do { if (verbose) pr_alert("%s" PERF_FLAG " %s\n", perf_type, s); } while (0)
+#define VERBOSE_PERFOUT_ERRSTRING(s) \
+	do { if (verbose) pr_alert("%s" PERF_FLAG "!!! %s\n", perf_type, s); } while (0)
+
+torture_param(bool, gp_exp, true, "Use expedited GP wait primitives");
+torture_param(int, holdoff, 10, "Holdoff time before test start (s)");
+torture_param(int, nreaders, -1, "Number of RCU reader threads");
+torture_param(int, nwriters, -1, "Number of RCU updater threads");
+torture_param(bool, shutdown, false, "Shutdown at end of performance tests.");
+torture_param(bool, verbose, true, "Enable verbose debugging printk()s");
+
+static char *perf_type = "rcu";
+module_param(perf_type, charp, 0444);
+MODULE_PARM_DESC(perf_type, "Type of RCU to performance-test (rcu, rcu_bh, ...)");
+
+static int nrealreaders;
+static int nrealwriters;
+static struct task_struct **writer_tasks;
+static struct task_struct **reader_tasks;
+static struct task_struct *shutdown_task;
+
+static u64 **writer_durations;
+static int *writer_n_durations;
+static atomic_t n_rcu_perf_reader_started;
+static atomic_t n_rcu_perf_writer_started;
+static atomic_t n_rcu_perf_writer_finished;
+static wait_queue_head_t shutdown_wq;
+static u64 t_rcu_perf_writer_started;
+static u64 t_rcu_perf_writer_finished;
+static unsigned long b_rcu_perf_writer_started;
+static unsigned long b_rcu_perf_writer_finished;
+
+static int rcu_perf_writer_state;
+#define RTWS_INIT		0
+#define RTWS_EXP_SYNC		1
+#define RTWS_SYNC		2
+#define RTWS_IDLE		2
+#define RTWS_STOPPING		3
+
+#define MAX_MEAS 10000
+#define MIN_MEAS 100
+
+#if defined(MODULE) || defined(CONFIG_RCU_PERF_TEST_RUNNABLE)
+#define RCUPERF_RUNNABLE_INIT 1
+#else
+#define RCUPERF_RUNNABLE_INIT 0
+#endif
+static int perf_runnable = RCUPERF_RUNNABLE_INIT;
+module_param(perf_runnable, int, 0444);
+MODULE_PARM_DESC(perf_runnable, "Start rcuperf at boot");
+
+/*
+ * Operations vector for selecting different types of tests.
+ */
+
+struct rcu_perf_ops {
+	int ptype;
+	void (*init)(void);
+	void (*cleanup)(void);
+	int (*readlock)(void);
+	void (*readunlock)(int idx);
+	unsigned long (*started)(void);
+	unsigned long (*completed)(void);
+	unsigned long (*exp_completed)(void);
+	void (*sync)(void);
+	void (*exp_sync)(void);
+	const char *name;
+};
+
+static struct rcu_perf_ops *cur_ops;
+
+/*
+ * Definitions for rcu perf testing.
+ */
+
+static int rcu_perf_read_lock(void) __acquires(RCU)
+{
+	rcu_read_lock();
+	return 0;
+}
+
+static void rcu_perf_read_unlock(int idx) __releases(RCU)
+{
+	rcu_read_unlock();
+}
+
+static unsigned long __maybe_unused rcu_no_completed(void)
+{
+	return 0;
+}
+
+static void rcu_sync_perf_init(void)
+{
+}
+
+static struct rcu_perf_ops rcu_ops = {
+	.ptype		= RCU_FLAVOR,
+	.init		= rcu_sync_perf_init,
+	.readlock	= rcu_perf_read_lock,
+	.readunlock	= rcu_perf_read_unlock,
+	.started	= rcu_batches_started,
+	.completed	= rcu_batches_completed,
+	.exp_completed	= rcu_exp_batches_completed,
+	.sync		= synchronize_rcu,
+	.exp_sync	= synchronize_rcu_expedited,
+	.name		= "rcu"
+};
+
+/*
+ * Definitions for rcu_bh perf testing.
+ */
+
+static int rcu_bh_perf_read_lock(void) __acquires(RCU_BH)
+{
+	rcu_read_lock_bh();
+	return 0;
+}
+
+static void rcu_bh_perf_read_unlock(int idx) __releases(RCU_BH)
+{
+	rcu_read_unlock_bh();
+}
+
+static struct rcu_perf_ops rcu_bh_ops = {
+	.ptype		= RCU_BH_FLAVOR,
+	.init		= rcu_sync_perf_init,
+	.readlock	= rcu_bh_perf_read_lock,
+	.readunlock	= rcu_bh_perf_read_unlock,
+	.started	= rcu_batches_started_bh,
+	.completed	= rcu_batches_completed_bh,
+	.exp_completed	= rcu_exp_batches_completed_sched,
+	.sync		= synchronize_rcu_bh,
+	.exp_sync	= synchronize_rcu_bh_expedited,
+	.name		= "rcu_bh"
+};
+
+/*
+ * Definitions for srcu perf testing.
+ */
+
+DEFINE_STATIC_SRCU(srcu_ctl_perf);
+static struct srcu_struct *srcu_ctlp = &srcu_ctl_perf;
+
+static int srcu_perf_read_lock(void) __acquires(srcu_ctlp)
+{
+	return srcu_read_lock(srcu_ctlp);
+}
+
+static void srcu_perf_read_unlock(int idx) __releases(srcu_ctlp)
+{
+	srcu_read_unlock(srcu_ctlp, idx);
+}
+
+static unsigned long srcu_perf_completed(void)
+{
+	return srcu_batches_completed(srcu_ctlp);
+}
+
+static void srcu_perf_synchronize(void)
+{
+	synchronize_srcu(srcu_ctlp);
+}
+
+static void srcu_perf_synchronize_expedited(void)
+{
+	synchronize_srcu_expedited(srcu_ctlp);
+}
+
+static struct rcu_perf_ops srcu_ops = {
+	.ptype		= SRCU_FLAVOR,
+	.init		= rcu_sync_perf_init,
+	.readlock	= srcu_perf_read_lock,
+	.readunlock	= srcu_perf_read_unlock,
+	.started	= NULL,
+	.completed	= srcu_perf_completed,
+	.exp_completed	= srcu_perf_completed,
+	.sync		= srcu_perf_synchronize,
+	.exp_sync	= srcu_perf_synchronize_expedited,
+	.name		= "srcu"
+};
+
+/*
+ * Definitions for sched perf testing.
+ */
+
+static int sched_perf_read_lock(void)
+{
+	preempt_disable();
+	return 0;
+}
+
+static void sched_perf_read_unlock(int idx)
+{
+	preempt_enable();
+}
+
+static struct rcu_perf_ops sched_ops = {
+	.ptype		= RCU_SCHED_FLAVOR,
+	.init		= rcu_sync_perf_init,
+	.readlock	= sched_perf_read_lock,
+	.readunlock	= sched_perf_read_unlock,
+	.started	= rcu_batches_started_sched,
+	.completed	= rcu_batches_completed_sched,
+	.exp_completed	= rcu_exp_batches_completed_sched,
+	.sync		= synchronize_sched,
+	.exp_sync	= synchronize_sched_expedited,
+	.name		= "sched"
+};
+
+#ifdef CONFIG_TASKS_RCU
+
+/*
+ * Definitions for RCU-tasks perf testing.
+ */
+
+static int tasks_perf_read_lock(void)
+{
+	return 0;
+}
+
+static void tasks_perf_read_unlock(int idx)
+{
+}
+
+static struct rcu_perf_ops tasks_ops = {
+	.ptype		= RCU_TASKS_FLAVOR,
+	.init		= rcu_sync_perf_init,
+	.readlock	= tasks_perf_read_lock,
+	.readunlock	= tasks_perf_read_unlock,
+	.started	= rcu_no_completed,
+	.completed	= rcu_no_completed,
+	.sync		= synchronize_rcu_tasks,
+	.exp_sync	= synchronize_rcu_tasks,
+	.name		= "tasks"
+};
+
+#define RCUPERF_TASKS_OPS &tasks_ops,
+
+static bool __maybe_unused torturing_tasks(void)
+{
+	return cur_ops == &tasks_ops;
+}
+
+#else /* #ifdef CONFIG_TASKS_RCU */
+
+#define RCUPERF_TASKS_OPS
+
+static bool __maybe_unused torturing_tasks(void)
+{
+	return false;
+}
+
+#endif /* #else #ifdef CONFIG_TASKS_RCU */
+
+/*
+ * If performance tests complete, wait for shutdown to commence.
+ */
+static void rcu_perf_wait_shutdown(void)
+{
+	cond_resched_rcu_qs();
+	if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters)
+		return;
+	while (!torture_must_stop())
+		schedule_timeout_uninterruptible(1);
+}
+
+/*
+ * RCU perf reader kthread.  Repeatedly does empty RCU read-side
+ * critical section, minimizing update-side interference.
+ */
+static int
+rcu_perf_reader(void *arg)
+{
+	unsigned long flags;
+	int idx;
+	long me = (long)arg;
+
+	VERBOSE_PERFOUT_STRING("rcu_perf_reader task started");
+	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
+	set_user_nice(current, MAX_NICE);
+	atomic_inc(&n_rcu_perf_reader_started);
+
+	do {
+		local_irq_save(flags);
+		idx = cur_ops->readlock();
+		cur_ops->readunlock(idx);
+		local_irq_restore(flags);
+		rcu_perf_wait_shutdown();
+	} while (!torture_must_stop());
+	torture_kthread_stopping("rcu_perf_reader");
+	return 0;
+}
+
+/*
+ * RCU perf writer kthread.  Repeatedly does a grace period.
+ */
+static int
+rcu_perf_writer(void *arg)
+{
+	int i = 0;
+	int i_max;
+	long me = (long)arg;
+	struct sched_param sp;
+	bool started = false, done = false, alldone = false;
+	u64 t;
+	u64 *wdp;
+	u64 *wdpp = writer_durations[me];
+
+	VERBOSE_PERFOUT_STRING("rcu_perf_writer task started");
+	WARN_ON(rcu_gp_is_expedited() && !rcu_gp_is_normal() && !gp_exp);
+	WARN_ON(rcu_gp_is_normal() && gp_exp);
+	WARN_ON(!wdpp);
+	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
+	sp.sched_priority = 1;
+	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
+
+	if (holdoff)
+		schedule_timeout_uninterruptible(holdoff * HZ);
+
+	t = ktime_get_mono_fast_ns();
+	if (atomic_inc_return(&n_rcu_perf_writer_started) >= nrealwriters) {
+		t_rcu_perf_writer_started = t;
+		if (gp_exp) {
+			b_rcu_perf_writer_started =
+				cur_ops->exp_completed() / 2;
+		} else {
+			b_rcu_perf_writer_started =
+				cur_ops->completed();
+		}
+	}
+
+	do {
+		wdp = &wdpp[i];
+		*wdp = ktime_get_mono_fast_ns();
+		if (gp_exp) {
+			rcu_perf_writer_state = RTWS_EXP_SYNC;
+			cur_ops->exp_sync();
+		} else {
+			rcu_perf_writer_state = RTWS_SYNC;
+			cur_ops->sync();
+		}
+		rcu_perf_writer_state = RTWS_IDLE;
+		t = ktime_get_mono_fast_ns();
+		*wdp = t - *wdp;
+		i_max = i;
+		if (!started &&
+		    atomic_read(&n_rcu_perf_writer_started) >= nrealwriters)
+			started = true;
+		if (!done && i >= MIN_MEAS) {
+			done = true;
+			sp.sched_priority = 0;
+			sched_setscheduler_nocheck(current,
+						   SCHED_NORMAL, &sp);
+			pr_alert("%s" PERF_FLAG
+				 "rcu_perf_writer %ld has %d measurements\n",
+				 perf_type, me, MIN_MEAS);
+			if (atomic_inc_return(&n_rcu_perf_writer_finished) >=
+			    nrealwriters) {
+				schedule_timeout_interruptible(10);
+				rcu_ftrace_dump(DUMP_ALL);
+				PERFOUT_STRING("Test complete");
+				t_rcu_perf_writer_finished = t;
+				if (gp_exp) {
+					b_rcu_perf_writer_finished =
+						cur_ops->exp_completed() / 2;
+				} else {
+					b_rcu_perf_writer_finished =
+						cur_ops->completed();
+				}
+				if (shutdown) {
+					smp_mb(); /* Assign before wake. */
+					wake_up(&shutdown_wq);
+				}
+			}
+		}
+		if (done && !alldone &&
+		    atomic_read(&n_rcu_perf_writer_finished) >= nrealwriters)
+			alldone = true;
+		if (started && !alldone && i < MAX_MEAS - 1)
+			i++;
+		rcu_perf_wait_shutdown();
+	} while (!torture_must_stop());
+	rcu_perf_writer_state = RTWS_STOPPING;
+	writer_n_durations[me] = i_max;
+	torture_kthread_stopping("rcu_perf_writer");
+	return 0;
+}
+
+static inline void
+rcu_perf_print_module_parms(struct rcu_perf_ops *cur_ops, const char *tag)
+{
+	pr_alert("%s" PERF_FLAG
+		 "--- %s: nreaders=%d nwriters=%d verbose=%d shutdown=%d\n",
+		 perf_type, tag, nrealreaders, nrealwriters, verbose, shutdown);
+}
+
+static void
+rcu_perf_cleanup(void)
+{
+	int i;
+	int j;
+	int ngps = 0;
+	u64 *wdp;
+	u64 *wdpp;
+
+	if (torture_cleanup_begin())
+		return;
+
+	if (reader_tasks) {
+		for (i = 0; i < nrealreaders; i++)
+			torture_stop_kthread(rcu_perf_reader,
+					     reader_tasks[i]);
+		kfree(reader_tasks);
+	}
+
+	if (writer_tasks) {
+		for (i = 0; i < nrealwriters; i++) {
+			torture_stop_kthread(rcu_perf_writer,
+					     writer_tasks[i]);
+			if (!writer_n_durations)
+				continue;
+			j = writer_n_durations[i];
+			pr_alert("%s%s writer %d gps: %d\n",
+				 perf_type, PERF_FLAG, i, j);
+			ngps += j;
+		}
+		pr_alert("%s%s start: %llu end: %llu duration: %llu gps: %d batches: %ld\n",
+			 perf_type, PERF_FLAG,
+			 t_rcu_perf_writer_started, t_rcu_perf_writer_finished,
+			 t_rcu_perf_writer_finished -
+			 t_rcu_perf_writer_started,
+			 ngps,
+			 b_rcu_perf_writer_finished -
+			 b_rcu_perf_writer_started);
+		for (i = 0; i < nrealwriters; i++) {
+			if (!writer_durations)
+				break;
+			if (!writer_n_durations)
+				continue;
+			wdpp = writer_durations[i];
+			if (!wdpp)
+				continue;
+			for (j = 0; j <= writer_n_durations[i]; j++) {
+				wdp = &wdpp[j];
+				pr_alert("%s%s %4d writer-duration: %5d %llu\n",
+					perf_type, PERF_FLAG,
+					i, j, *wdp);
+				if (j % 100 == 0)
+					schedule_timeout_uninterruptible(1);
+			}
+			kfree(writer_durations[i]);
+		}
+		kfree(writer_tasks);
+		kfree(writer_durations);
+		kfree(writer_n_durations);
+	}
+
+	/* Do flavor-specific cleanup operations.  */
+	if (cur_ops->cleanup != NULL)
+		cur_ops->cleanup();
+
+	torture_cleanup_end();
+}
+
+/*
+ * Return the number if non-negative.  If -1, the number of CPUs.
+ * If less than -1, that much less than the number of CPUs, but
+ * at least one.
+ */
+static int compute_real(int n)
+{
+	int nr;
+
+	if (n >= 0) {
+		nr = n;
+	} else {
+		nr = num_online_cpus() + 1 + n;
+		if (nr <= 0)
+			nr = 1;
+	}
+	return nr;
+}
+
+/*
+ * RCU perf shutdown kthread.  Just waits to be awakened, then shuts
+ * down system.
+ */
+static int
+rcu_perf_shutdown(void *arg)
+{
+	do {
+		wait_event(shutdown_wq,
+			   atomic_read(&n_rcu_perf_writer_finished) >=
+			   nrealwriters);
+	} while (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters);
+	smp_mb(); /* Wake before output. */
+	rcu_perf_cleanup();
+	kernel_power_off();
+	return -EINVAL;
+}
+
+static int __init
+rcu_perf_init(void)
+{
+	long i;
+	int firsterr = 0;
+	static struct rcu_perf_ops *perf_ops[] = {
+		&rcu_ops, &rcu_bh_ops, &srcu_ops, &sched_ops,
+		RCUPERF_TASKS_OPS
+	};
+
+	if (!torture_init_begin(perf_type, verbose, &perf_runnable))
+		return -EBUSY;
+
+	/* Process args and tell the world that the perf'er is on the job. */
+	for (i = 0; i < ARRAY_SIZE(perf_ops); i++) {
+		cur_ops = perf_ops[i];
+		if (strcmp(perf_type, cur_ops->name) == 0)
+			break;
+	}
+	if (i == ARRAY_SIZE(perf_ops)) {
+		pr_alert("rcu-perf: invalid perf type: \"%s\"\n",
+			 perf_type);
+		pr_alert("rcu-perf types:");
+		for (i = 0; i < ARRAY_SIZE(perf_ops); i++)
+			pr_alert(" %s", perf_ops[i]->name);
+		pr_alert("\n");
+		firsterr = -EINVAL;
+		goto unwind;
+	}
+	if (cur_ops->init)
+		cur_ops->init();
+
+	nrealwriters = compute_real(nwriters);
+	nrealreaders = compute_real(nreaders);
+	atomic_set(&n_rcu_perf_reader_started, 0);
+	atomic_set(&n_rcu_perf_writer_started, 0);
+	atomic_set(&n_rcu_perf_writer_finished, 0);
+	rcu_perf_print_module_parms(cur_ops, "Start of test");
+
+	/* Start up the kthreads. */
+
+	if (shutdown) {
+		init_waitqueue_head(&shutdown_wq);
+		firsterr = torture_create_kthread(rcu_perf_shutdown, NULL,
+						  shutdown_task);
+		if (firsterr)
+			goto unwind;
+		schedule_timeout_uninterruptible(1);
+	}
+	reader_tasks = kcalloc(nrealreaders, sizeof(reader_tasks[0]),
+			       GFP_KERNEL);
+	if (reader_tasks == NULL) {
+		VERBOSE_PERFOUT_ERRSTRING("out of memory");
+		firsterr = -ENOMEM;
+		goto unwind;
+	}
+	for (i = 0; i < nrealreaders; i++) {
+		firsterr = torture_create_kthread(rcu_perf_reader, (void *)i,
+						  reader_tasks[i]);
+		if (firsterr)
+			goto unwind;
+	}
+	while (atomic_read(&n_rcu_perf_reader_started) < nrealreaders)
+		schedule_timeout_uninterruptible(1);
+	writer_tasks = kcalloc(nrealwriters, sizeof(reader_tasks[0]),
+			       GFP_KERNEL);
+	writer_durations = kcalloc(nrealwriters, sizeof(*writer_durations),
+				   GFP_KERNEL);
+	writer_n_durations =
+		kcalloc(nrealwriters, sizeof(*writer_n_durations),
+			GFP_KERNEL);
+	if (!writer_tasks || !writer_durations || !writer_n_durations) {
+		VERBOSE_PERFOUT_ERRSTRING("out of memory");
+		firsterr = -ENOMEM;
+		goto unwind;
+	}
+	for (i = 0; i < nrealwriters; i++) {
+		writer_durations[i] =
+			kcalloc(MAX_MEAS, sizeof(*writer_durations[i]),
+				GFP_KERNEL);
+		if (!writer_durations[i])
+			goto unwind;
+		firsterr = torture_create_kthread(rcu_perf_writer, (void *)i,
+						  writer_tasks[i]);
+		if (firsterr)
+			goto unwind;
+	}
+	torture_init_end();
+	return 0;
+
+unwind:
+	torture_init_end();
+	rcu_perf_cleanup();
+	return firsterr;
+}
+
+module_init(rcu_perf_init);
+module_exit(rcu_perf_cleanup);
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 250ea67c1615..084a28a732eb 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -130,8 +130,8 @@ static struct rcu_torture __rcu *rcu_torture_current;
 static unsigned long rcu_torture_current_version;
 static struct rcu_torture rcu_tortures[10 * RCU_TORTURE_PIPE_LEN];
 static DEFINE_SPINLOCK(rcu_torture_lock);
-static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_count) = { 0 };
-static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_batch) = { 0 };
+static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_count);
+static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_batch);
 static atomic_t rcu_torture_wcount[RCU_TORTURE_PIPE_LEN + 1];
 static atomic_t n_rcu_torture_alloc;
 static atomic_t n_rcu_torture_alloc_fail;
@@ -916,7 +916,7 @@ rcu_torture_fqs(void *arg)
 static int
 rcu_torture_writer(void *arg)
 {
-	bool can_expedite = !rcu_gp_is_expedited();
+	bool can_expedite = !rcu_gp_is_expedited() && !rcu_gp_is_normal();
 	int expediting = 0;
 	unsigned long gp_snap;
 	bool gp_cond1 = gp_cond, gp_exp1 = gp_exp, gp_normal1 = gp_normal;
@@ -932,7 +932,7 @@ rcu_torture_writer(void *arg)
 	VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
 	if (!can_expedite) {
 		pr_alert("%s" TORTURE_FLAG
-			 " Grace periods expedited from boot/sysfs for %s,\n",
+			 " GP expediting controlled from boot/sysfs for %s,\n",
 			 torture_type, cur_ops->name);
 		pr_alert("%s" TORTURE_FLAG
 			 " Disabled dynamic grace-period expediting.\n",
@@ -1082,17 +1082,6 @@ rcu_torture_fakewriter(void *arg)
 	return 0;
 }
 
-static void rcutorture_trace_dump(void)
-{
-	static atomic_t beenhere = ATOMIC_INIT(0);
-
-	if (atomic_read(&beenhere))
-		return;
-	if (atomic_xchg(&beenhere, 1) != 0)
-		return;
-	ftrace_dump(DUMP_ALL);
-}
-
 /*
  * RCU torture reader from timer handler.  Dereferences rcu_torture_current,
  * incrementing the corresponding element of the pipeline array.  The
@@ -1142,7 +1131,7 @@ static void rcu_torture_timer(unsigned long unused)
 	if (pipe_count > 1) {
 		do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu, ts,
 					  started, completed);
-		rcutorture_trace_dump();
+		rcu_ftrace_dump(DUMP_ALL);
 	}
 	__this_cpu_inc(rcu_torture_count[pipe_count]);
 	completed = completed - started;
@@ -1215,7 +1204,7 @@ rcu_torture_reader(void *arg)
 		if (pipe_count > 1) {
 			do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu,
 						  ts, started, completed);
-			rcutorture_trace_dump();
+			rcu_ftrace_dump(DUMP_ALL);
 		}
 		__this_cpu_inc(rcu_torture_count[pipe_count]);
 		completed = completed - started;
@@ -1333,7 +1322,7 @@ rcu_torture_stats_print(void)
 			 rcu_torture_writer_state,
 			 gpnum, completed, flags);
 		show_rcu_gp_kthreads();
-		rcutorture_trace_dump();
+		rcu_ftrace_dump(DUMP_ALL);
 	}
 	rtcv_snap = rcu_torture_current_version;
 }
@@ -1489,7 +1478,9 @@ static int rcu_torture_barrier_cbs(void *arg)
 		 * The above smp_load_acquire() ensures barrier_phase load
 		 * is ordered before the folloiwng ->call().
 		 */
+		local_irq_disable(); /* Just to test no-irq call_rcu(). */
 		cur_ops->call(&rcu, rcu_torture_barrier_cbf);
+		local_irq_enable();
 		if (atomic_dec_and_test(&barrier_cbs_count))
 			wake_up(&barrier_wq);
 	} while (!torture_must_stop());
@@ -1596,7 +1587,7 @@ static int rcutorture_cpu_notify(struct notifier_block *self,
 {
 	long cpu = (long)hcpu;
 
-	switch (action) {
+	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_ONLINE:
 	case CPU_DOWN_FAILED:
 		(void)rcutorture_booster_init(cpu);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9a535a86e732..c7f1bc4f817c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -102,6 +102,8 @@ struct rcu_state sname##_state = { \
 	.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
 	.name = RCU_STATE_NAME(sname), \
 	.abbr = sabbr, \
+	.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
+	.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
 }
 
 RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
@@ -370,6 +372,21 @@ void rcu_all_qs(void)
 		rcu_momentary_dyntick_idle();
 		local_irq_restore(flags);
 	}
+	if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))) {
+		/*
+		 * Yes, we just checked a per-CPU variable with preemption
+		 * enabled, so we might be migrated to some other CPU at
+		 * this point.  That is OK because in that case, the
+		 * migration will supply the needed quiescent state.
+		 * We might end up needlessly disabling preemption and
+		 * invoking rcu_sched_qs() on the destination CPU, but
+		 * the probability and cost are both quite low, so this
+		 * should not be a problem in practice.
+		 */
+		preempt_disable();
+		rcu_sched_qs();
+		preempt_enable();
+	}
 	this_cpu_inc(rcu_qs_ctr);
 	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }
@@ -385,9 +402,11 @@ module_param(qlowmark, long, 0444);
 
 static ulong jiffies_till_first_fqs = ULONG_MAX;
 static ulong jiffies_till_next_fqs = ULONG_MAX;
+static bool rcu_kick_kthreads;
 
 module_param(jiffies_till_first_fqs, ulong, 0644);
 module_param(jiffies_till_next_fqs, ulong, 0644);
+module_param(rcu_kick_kthreads, bool, 0644);
 
 /*
  * How long the grace period must be before we start recruiting
@@ -460,6 +479,28 @@ unsigned long rcu_batches_completed_bh(void)
 EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
 
 /*
+ * Return the number of RCU expedited batches completed thus far for
+ * debug & stats.  Odd numbers mean that a batch is in progress, even
+ * numbers mean idle.  The value returned will thus be roughly double
+ * the cumulative batches since boot.
+ */
+unsigned long rcu_exp_batches_completed(void)
+{
+	return rcu_state_p->expedited_sequence;
+}
+EXPORT_SYMBOL_GPL(rcu_exp_batches_completed);
+
+/*
+ * Return the number of RCU-sched expedited batches completed thus far
+ * for debug & stats.  Similar to rcu_exp_batches_completed().
+ */
+unsigned long rcu_exp_batches_completed_sched(void)
+{
+	return rcu_sched_state.expedited_sequence;
+}
+EXPORT_SYMBOL_GPL(rcu_exp_batches_completed_sched);
+
+/*
  * Force a quiescent state.
  */
 void rcu_force_quiescent_state(void)
@@ -637,7 +678,7 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
 			idle_task(smp_processor_id());
 
 		trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
-		ftrace_dump(DUMP_ORIG);
+		rcu_ftrace_dump(DUMP_ORIG);
 		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
 			  current->pid, current->comm,
 			  idle->pid, idle->comm); /* must be idle task! */
@@ -799,7 +840,7 @@ static void rcu_eqs_exit_common(long long oldval, int user)
 
 		trace_rcu_dyntick(TPS("Error on exit: not idle task"),
 				  oldval, rdtp->dynticks_nesting);
-		ftrace_dump(DUMP_ORIG);
+		rcu_ftrace_dump(DUMP_ORIG);
 		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
 			  current->pid, current->comm,
 			  idle->pid, idle->comm); /* must be idle task! */
@@ -1224,8 +1265,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
 		       rsp->gp_flags,
 		       gp_state_getname(rsp->gp_state), rsp->gp_state,
 		       rsp->gp_kthread ? rsp->gp_kthread->state : ~0);
-		if (rsp->gp_kthread)
+		if (rsp->gp_kthread) {
 			sched_show_task(rsp->gp_kthread);
+			wake_up_process(rsp->gp_kthread);
+		}
 	}
 }
 
@@ -1249,6 +1292,25 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 	}
 }
 
+/*
+ * If too much time has passed in the current grace period, and if
+ * so configured, go kick the relevant kthreads.
+ */
+static void rcu_stall_kick_kthreads(struct rcu_state *rsp)
+{
+	unsigned long j;
+
+	if (!rcu_kick_kthreads)
+		return;
+	j = READ_ONCE(rsp->jiffies_kick_kthreads);
+	if (time_after(jiffies, j) && rsp->gp_kthread) {
+		WARN_ONCE(1, "Kicking %s grace-period kthread\n", rsp->name);
+		rcu_ftrace_dump(DUMP_ALL);
+		wake_up_process(rsp->gp_kthread);
+		WRITE_ONCE(rsp->jiffies_kick_kthreads, j + HZ);
+	}
+}
+
 static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 {
 	int cpu;
@@ -1260,6 +1322,11 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 	long totqlen = 0;
 
+	/* Kick and suppress, if so configured. */
+	rcu_stall_kick_kthreads(rsp);
+	if (rcu_cpu_stall_suppress)
+		return;
+
 	/* Only let one CPU complain about others per time interval. */
 
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
@@ -1333,6 +1400,11 @@ static void print_cpu_stall(struct rcu_state *rsp)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 	long totqlen = 0;
 
+	/* Kick and suppress, if so configured. */
+	rcu_stall_kick_kthreads(rsp);
+	if (rcu_cpu_stall_suppress)
+		return;
+
 	/*
 	 * OK, time to rat on ourselves...
 	 * See Documentation/RCU/stallwarn.txt for info on how to debug
@@ -1377,8 +1449,10 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
 	unsigned long js;
 	struct rcu_node *rnp;
 
-	if (rcu_cpu_stall_suppress || !rcu_gp_in_progress(rsp))
+	if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
+	    !rcu_gp_in_progress(rsp))
 		return;
+	rcu_stall_kick_kthreads(rsp);
 	j = jiffies;
 
 	/*
@@ -2117,8 +2191,11 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		}
 		ret = 0;
 		for (;;) {
-			if (!ret)
+			if (!ret) {
 				rsp->jiffies_force_qs = jiffies + j;
+				WRITE_ONCE(rsp->jiffies_kick_kthreads,
+					   jiffies + 3 * j);
+			}
 			trace_rcu_grace_period(rsp->name,
 					       READ_ONCE(rsp->gpnum),
 					       TPS("fqswait"));
@@ -2144,6 +2221,15 @@ static int __noreturn rcu_gp_kthread(void *arg)
 						       TPS("fqsend"));
 				cond_resched_rcu_qs();
 				WRITE_ONCE(rsp->gp_activity, jiffies);
+				ret = 0; /* Force full wait till next FQS. */
+				j = jiffies_till_next_fqs;
+				if (j > HZ) {
+					j = HZ;
+					jiffies_till_next_fqs = HZ;
+				} else if (j < 1) {
+					j = 1;
+					jiffies_till_next_fqs = 1;
+				}
 			} else {
 				/* Deal with stray signal. */
 				cond_resched_rcu_qs();
@@ -2152,14 +2238,12 @@ static int __noreturn rcu_gp_kthread(void *arg)
 				trace_rcu_grace_period(rsp->name,
 						       READ_ONCE(rsp->gpnum),
 						       TPS("fqswaitsig"));
-			}
-			j = jiffies_till_next_fqs;
-			if (j > HZ) {
-				j = HZ;
-				jiffies_till_next_fqs = HZ;
-			} else if (j < 1) {
-				j = 1;
-				jiffies_till_next_fqs = 1;
+				ret = 1; /* Keep old FQS timing. */
+				j = jiffies;
+				if (time_after(jiffies, rsp->jiffies_force_qs))
+					j = 1;
+				else
+					j = rsp->jiffies_force_qs - j;
 			}
 		}
 
@@ -3376,8 +3460,12 @@ static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
 }
 static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
 {
+	unsigned long s;
+
 	smp_mb(); /* Caller's modifications seen first by other CPUs. */
-	return rcu_seq_snap(&rsp->expedited_sequence);
+	s = rcu_seq_snap(&rsp->expedited_sequence);
+	trace_rcu_exp_grace_period(rsp->name, s, TPS("snap"));
+	return s;
 }
 static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
 {
@@ -3469,7 +3557,7 @@ static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
  * for the current expedited grace period.  Works only for preemptible
  * RCU -- other RCU implementation use other means.
  *
- * Caller must hold the root rcu_node's exp_funnel_mutex.
+ * Caller must hold the rcu_state's exp_mutex.
  */
 static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
 {
@@ -3485,8 +3573,8 @@ static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
  * recursively up the tree.  (Calm down, calm down, we do the recursion
  * iteratively!)
  *
- * Caller must hold the root rcu_node's exp_funnel_mutex and the
- * specified rcu_node structure's ->lock.
+ * Caller must hold the rcu_state's exp_mutex and the specified rcu_node
+ * structure's ->lock.
  */
 static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
 				 bool wake, unsigned long flags)
@@ -3523,7 +3611,7 @@ static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
  * Report expedited quiescent state for specified node.  This is a
  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
  *
- * Caller must hold the root rcu_node's exp_funnel_mutex.
+ * Caller must hold the rcu_state's exp_mutex.
  */
 static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
 					      struct rcu_node *rnp, bool wake)
@@ -3536,8 +3624,8 @@ static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
 
 /*
  * Report expedited quiescent state for multiple CPUs, all covered by the
- * specified leaf rcu_node structure.  Caller must hold the root
- * rcu_node's exp_funnel_mutex.
+ * specified leaf rcu_node structure.  Caller must hold the rcu_state's
+ * exp_mutex.
  */
 static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
 				    unsigned long mask, bool wake)
@@ -3555,7 +3643,6 @@ static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
 
 /*
  * Report expedited quiescent state for specified rcu_data (CPU).
- * Caller must hold the root rcu_node's exp_funnel_mutex.
  */
 static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
 			       bool wake)
@@ -3564,15 +3651,11 @@ static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
 }
 
 /* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
-static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
-			       struct rcu_data *rdp,
-			       atomic_long_t *stat, unsigned long s)
+static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
+			       unsigned long s)
 {
 	if (rcu_exp_gp_seq_done(rsp, s)) {
-		if (rnp)
-			mutex_unlock(&rnp->exp_funnel_mutex);
-		else if (rdp)
-			mutex_unlock(&rdp->exp_funnel_mutex);
+		trace_rcu_exp_grace_period(rsp->name, s, TPS("done"));
 		/* Ensure test happens before caller kfree(). */
 		smp_mb__before_atomic(); /* ^^^ */
 		atomic_long_inc(stat);
@@ -3582,59 +3665,65 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
 }
 
 /*
- * Funnel-lock acquisition for expedited grace periods.  Returns a
- * pointer to the root rcu_node structure, or NULL if some other
- * task did the expedited grace period for us.
+ * Funnel-lock acquisition for expedited grace periods.  Returns true
+ * if some other task completed an expedited grace period that this task
+ * can piggy-back on, and with no mutex held.  Otherwise, returns false
+ * with the mutex held, indicating that the caller must actually do the
+ * expedited grace period.
  */
-static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
+static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 {
 	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
-	struct rcu_node *rnp0;
-	struct rcu_node *rnp1 = NULL;
+	struct rcu_node *rnp = rdp->mynode;
+	struct rcu_node *rnp_root = rcu_get_root(rsp);
+
+	/* Low-contention fastpath. */
+	if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
+	    (rnp == rnp_root ||
+	     ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
+	    !mutex_is_locked(&rsp->exp_mutex) &&
+	    mutex_trylock(&rsp->exp_mutex))
+		goto fastpath;
 
 	/*
-	 * First try directly acquiring the root lock in order to reduce
-	 * latency in the common case where expedited grace periods are
-	 * rare.  We check mutex_is_locked() to avoid pathological levels of
-	 * memory contention on ->exp_funnel_mutex in the heavy-load case.
+	 * Each pass through the following loop works its way up
+	 * the rcu_node tree, returning if others have done the work or
+	 * otherwise falls through to acquire rsp->exp_mutex.  The mapping
+	 * from CPU to rcu_node structure can be inexact, as it is just
+	 * promoting locality and is not strictly needed for correctness.
 	 */
-	rnp0 = rcu_get_root(rsp);
-	if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
-		if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
-			if (sync_exp_work_done(rsp, rnp0, NULL,
-					       &rdp->expedited_workdone0, s))
-				return NULL;
-			return rnp0;
+	for (; rnp != NULL; rnp = rnp->parent) {
+		if (sync_exp_work_done(rsp, &rdp->exp_workdone1, s))
+			return true;
+
+		/* Work not done, either wait here or go up. */
+		spin_lock(&rnp->exp_lock);
+		if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
+
+			/* Someone else doing GP, so wait for them. */
+			spin_unlock(&rnp->exp_lock);
+			trace_rcu_exp_funnel_lock(rsp->name, rnp->level,
+						  rnp->grplo, rnp->grphi,
+						  TPS("wait"));
+			wait_event(rnp->exp_wq[(s >> 1) & 0x3],
+				   sync_exp_work_done(rsp,
+						      &rdp->exp_workdone2, s));
+			return true;
 		}
+		rnp->exp_seq_rq = s; /* Followers can wait on us. */
+		spin_unlock(&rnp->exp_lock);
+		trace_rcu_exp_funnel_lock(rsp->name, rnp->level, rnp->grplo,
+					  rnp->grphi, TPS("nxtlvl"));
 	}
-
-	/*
-	 * Each pass through the following loop works its way
-	 * up the rcu_node tree, returning if others have done the
-	 * work or otherwise falls through holding the root rnp's
-	 * ->exp_funnel_mutex.  The mapping from CPU to rcu_node structure
-	 * can be inexact, as it is just promoting locality and is not
-	 * strictly needed for correctness.
-	 */
-	if (sync_exp_work_done(rsp, NULL, NULL, &rdp->expedited_workdone1, s))
-		return NULL;
-	mutex_lock(&rdp->exp_funnel_mutex);
-	rnp0 = rdp->mynode;
-	for (; rnp0 != NULL; rnp0 = rnp0->parent) {
-		if (sync_exp_work_done(rsp, rnp1, rdp,
-				       &rdp->expedited_workdone2, s))
-			return NULL;
-		mutex_lock(&rnp0->exp_funnel_mutex);
-		if (rnp1)
-			mutex_unlock(&rnp1->exp_funnel_mutex);
-		else
-			mutex_unlock(&rdp->exp_funnel_mutex);
-		rnp1 = rnp0;
+	mutex_lock(&rsp->exp_mutex);
+fastpath:
+	if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) {
+		mutex_unlock(&rsp->exp_mutex);
+		return true;
 	}
-	if (sync_exp_work_done(rsp, rnp1, rdp,
-			       &rdp->expedited_workdone3, s))
-		return NULL;
-	return rnp1;
+	rcu_exp_gp_seq_start(rsp);
+	trace_rcu_exp_grace_period(rsp->name, s, TPS("start"));
+	return false;
 }
 
 /* Invoked on each online non-idle CPU for expedited quiescent state. */
@@ -3649,6 +3738,11 @@ static void sync_sched_exp_handler(void *data)
 	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
 	    __this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
 		return;
+	if (rcu_is_cpu_rrupt_from_idle()) {
+		rcu_report_exp_rdp(&rcu_sched_state,
+				   this_cpu_ptr(&rcu_sched_data), true);
+		return;
+	}
 	__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
 	resched_cpu(smp_processor_id());
 }
@@ -3773,7 +3867,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 		       rsp->name);
 		ndetected = 0;
 		rcu_for_each_leaf_node(rsp, rnp) {
-			ndetected = rcu_print_task_exp_stall(rnp);
+			ndetected += rcu_print_task_exp_stall(rnp);
 			mask = 1;
 			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
 				struct rcu_data *rdp;
@@ -3783,7 +3877,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 				ndetected++;
 				rdp = per_cpu_ptr(rsp->rda, cpu);
 				pr_cont(" %d-%c%c%c", cpu,
-					"O."[cpu_online(cpu)],
+					"O."[!!cpu_online(cpu)],
 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
 					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
 			}
@@ -3792,7 +3886,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 			jiffies - jiffies_start, rsp->expedited_sequence,
 			rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
-		if (!ndetected) {
+		if (ndetected) {
 			pr_err("blocking rcu_node structures:");
 			rcu_for_each_node_breadth_first(rsp, rnp) {
 				if (rnp == rnp_root)
@@ -3818,6 +3912,41 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 	}
 }
 
+/*
+ * Wait for the current expedited grace period to complete, and then
+ * wake up everyone who piggybacked on the just-completed expedited
+ * grace period.  Also update all the ->exp_seq_rq counters as needed
+ * in order to avoid counter-wrap problems.
+ */
+static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
+{
+	struct rcu_node *rnp;
+
+	synchronize_sched_expedited_wait(rsp);
+	rcu_exp_gp_seq_end(rsp);
+	trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
+
+	/*
+	 * Switch over to wakeup mode, allowing the next GP, but -only- the
+	 * next GP, to proceed.
+	 */
+	mutex_lock(&rsp->exp_wake_mutex);
+	mutex_unlock(&rsp->exp_mutex);
+
+	rcu_for_each_node_breadth_first(rsp, rnp) {
+		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
+			spin_lock(&rnp->exp_lock);
+			/* Recheck, avoid hang in case someone just arrived. */
+			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
+				rnp->exp_seq_rq = s;
+			spin_unlock(&rnp->exp_lock);
+		}
+		wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]);
+	}
+	trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake"));
+	mutex_unlock(&rsp->exp_wake_mutex);
+}
+
 /**
  * synchronize_sched_expedited - Brute-force RCU-sched grace period
  *
@@ -3837,7 +3966,6 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 void synchronize_sched_expedited(void)
 {
 	unsigned long s;
-	struct rcu_node *rnp;
 	struct rcu_state *rsp = &rcu_sched_state;
 
 	/* If only one CPU, this is automatically a grace period. */
@@ -3852,17 +3980,14 @@ void synchronize_sched_expedited(void)
 
 	/* Take a snapshot of the sequence number.  */
 	s = rcu_exp_gp_seq_snap(rsp);
-
-	rnp = exp_funnel_lock(rsp, s);
-	if (rnp == NULL)
+	if (exp_funnel_lock(rsp, s))
 		return;  /* Someone else did our work for us. */
 
-	rcu_exp_gp_seq_start(rsp);
+	/* Initialize the rcu_node tree in preparation for the wait. */
 	sync_rcu_exp_select_cpus(rsp, sync_sched_exp_handler);
-	synchronize_sched_expedited_wait(rsp);
 
-	rcu_exp_gp_seq_end(rsp);
-	mutex_unlock(&rnp->exp_funnel_mutex);
+	/* Wait and clean up, including waking everyone. */
+	rcu_exp_wait_wake(rsp, s);
 }
 EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
 
@@ -4162,7 +4287,6 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
 	WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
 	rdp->cpu = cpu;
 	rdp->rsp = rsp;
-	mutex_init(&rdp->exp_funnel_mutex);
 	rcu_boot_init_nocb_percpu_data(rdp);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
@@ -4420,10 +4544,8 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 {
 	static const char * const buf[] = RCU_NODE_NAME_INIT;
 	static const char * const fqs[] = RCU_FQS_NAME_INIT;
-	static const char * const exp[] = RCU_EXP_NAME_INIT;
 	static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
 	static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
-	static struct lock_class_key rcu_exp_class[RCU_NUM_LVLS];
 	static u8 fl_mask = 0x1;
 
 	int levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
@@ -4482,9 +4604,11 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 			rnp->level = i;
 			INIT_LIST_HEAD(&rnp->blkd_tasks);
 			rcu_init_one_nocb(rnp);
-			mutex_init(&rnp->exp_funnel_mutex);
-			lockdep_set_class_and_name(&rnp->exp_funnel_mutex,
-						   &rcu_exp_class[i], exp[i]);
+			init_waitqueue_head(&rnp->exp_wq[0]);
+			init_waitqueue_head(&rnp->exp_wq[1]);
+			init_waitqueue_head(&rnp->exp_wq[2]);
+			init_waitqueue_head(&rnp->exp_wq[3]);
+			spin_lock_init(&rnp->exp_lock);
 		}
 	}
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index df668c0f9e64..e3959f5e6ddf 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -70,7 +70,6 @@
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
-#  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
 #elif NR_CPUS <= RCU_FANOUT_2
 #  define RCU_NUM_LVLS	      2
 #  define NUM_RCU_LVL_0	      1
@@ -79,7 +78,6 @@
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
-#  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
 #elif NR_CPUS <= RCU_FANOUT_3
 #  define RCU_NUM_LVLS	      3
 #  define NUM_RCU_LVL_0	      1
@@ -89,7 +87,6 @@
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
-#  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
 #elif NR_CPUS <= RCU_FANOUT_4
 #  define RCU_NUM_LVLS	      4
 #  define NUM_RCU_LVL_0	      1
@@ -100,7 +97,6 @@
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
-#  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
 #else
 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
 #endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */
@@ -252,7 +248,9 @@ struct rcu_node {
 				/* Counts of upcoming no-CB GP requests. */
 	raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 
-	struct mutex exp_funnel_mutex ____cacheline_internodealigned_in_smp;
+	spinlock_t exp_lock ____cacheline_internodealigned_in_smp;
+	unsigned long exp_seq_rq;
+	wait_queue_head_t exp_wq[4];
 } ____cacheline_internodealigned_in_smp;
 
 /*
@@ -387,11 +385,9 @@ struct rcu_data {
 #ifdef CONFIG_RCU_FAST_NO_HZ
 	struct rcu_head oom_head;
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
-	struct mutex exp_funnel_mutex;
-	atomic_long_t expedited_workdone0;	/* # done by others #0. */
-	atomic_long_t expedited_workdone1;	/* # done by others #1. */
-	atomic_long_t expedited_workdone2;	/* # done by others #2. */
-	atomic_long_t expedited_workdone3;	/* # done by others #3. */
+	atomic_long_t exp_workdone1;	/* # done by others #1. */
+	atomic_long_t exp_workdone2;	/* # done by others #2. */
+	atomic_long_t exp_workdone3;	/* # done by others #3. */
 
 	/* 7) Callback offloading. */
 #ifdef CONFIG_RCU_NOCB_CPU
@@ -505,6 +501,8 @@ struct rcu_state {
 						/*  _rcu_barrier(). */
 	/* End of fields guarded by barrier_mutex. */
 
+	struct mutex exp_mutex;			/* Serialize expedited GP. */
+	struct mutex exp_wake_mutex;		/* Serialize wakeup. */
 	unsigned long expedited_sequence;	/* Take a ticket. */
 	atomic_long_t expedited_normal;		/* # fallbacks to normal. */
 	atomic_t expedited_need_qs;		/* # CPUs left to check in. */
@@ -513,6 +511,8 @@ struct rcu_state {
 
 	unsigned long jiffies_force_qs;		/* Time at which to invoke */
 						/*  force_quiescent_state(). */
+	unsigned long jiffies_kick_kthreads;	/* Time at which to kick */
+						/*  kthreads, if configured. */
 	unsigned long n_force_qs;		/* Number of calls to */
 						/*  force_quiescent_state(). */
 	unsigned long n_force_qs_lh;		/* ~Number of calls leaving */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index efdf7b61ce12..ff1cd4e1188d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -722,18 +722,22 @@ static void sync_rcu_exp_handler(void *info)
  * synchronize_rcu_expedited - Brute-force RCU grace period
  *
  * Wait for an RCU-preempt grace period, but expedite it.  The basic
- * idea is to invoke synchronize_sched_expedited() to push all the tasks to
- * the ->blkd_tasks lists and wait for this list to drain.  This consumes
- * significant time on all CPUs and is unfriendly to real-time workloads,
- * so is thus not recommended for any sort of common-case code.
- * In fact, if you are using synchronize_rcu_expedited() in a loop,
- * please restructure your code to batch your updates, and then Use a
- * single synchronize_rcu() instead.
+ * idea is to IPI all non-idle non-nohz online CPUs.  The IPI handler
+ * checks whether the CPU is in an RCU-preempt critical section, and
+ * if so, it sets a flag that causes the outermost rcu_read_unlock()
+ * to report the quiescent state.  On the other hand, if the CPU is
+ * not in an RCU read-side critical section, the IPI handler reports
+ * the quiescent state immediately.
+ *
+ * Although this is a greate improvement over previous expedited
+ * implementations, it is still unfriendly to real-time workloads, so is
+ * thus not recommended for any sort of common-case code.  In fact, if
+ * you are using synchronize_rcu_expedited() in a loop, please restructure
+ * your code to batch your updates, and then Use a single synchronize_rcu()
+ * instead.
  */
 void synchronize_rcu_expedited(void)
 {
-	struct rcu_node *rnp;
-	struct rcu_node *rnp_unlock;
 	struct rcu_state *rsp = rcu_state_p;
 	unsigned long s;
 
@@ -744,23 +748,14 @@ void synchronize_rcu_expedited(void)
 	}
 
 	s = rcu_exp_gp_seq_snap(rsp);
-
-	rnp_unlock = exp_funnel_lock(rsp, s);
-	if (rnp_unlock == NULL)
+	if (exp_funnel_lock(rsp, s))
 		return;  /* Someone else did our work for us. */
 
-	rcu_exp_gp_seq_start(rsp);
-
 	/* Initialize the rcu_node tree in preparation for the wait. */
 	sync_rcu_exp_select_cpus(rsp, sync_rcu_exp_handler);
 
-	/* Wait for snapshotted ->blkd_tasks lists to drain. */
-	rnp = rcu_get_root(rsp);
-	synchronize_sched_expedited_wait(rsp);
-
-	/* Clean up and exit. */
-	rcu_exp_gp_seq_end(rsp);
-	mutex_unlock(&rnp_unlock->exp_funnel_mutex);
+	/* Wait for ->blkd_tasks lists to drain, then wake everyone up. */
+	rcu_exp_wait_wake(rsp, s);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 1088e64f01ad..86782f9a4604 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -185,17 +185,16 @@ static int show_rcuexp(struct seq_file *m, void *v)
 	int cpu;
 	struct rcu_state *rsp = (struct rcu_state *)m->private;
 	struct rcu_data *rdp;
-	unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0;
+	unsigned long s1 = 0, s2 = 0, s3 = 0;
 
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(rsp->rda, cpu);
-		s0 += atomic_long_read(&rdp->expedited_workdone0);
-		s1 += atomic_long_read(&rdp->expedited_workdone1);
-		s2 += atomic_long_read(&rdp->expedited_workdone2);
-		s3 += atomic_long_read(&rdp->expedited_workdone3);
+		s1 += atomic_long_read(&rdp->exp_workdone1);
+		s2 += atomic_long_read(&rdp->exp_workdone2);
+		s3 += atomic_long_read(&rdp->exp_workdone3);
 	}
-	seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
-		   rsp->expedited_sequence, s0, s1, s2, s3,
+	seq_printf(m, "s=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
+		   rsp->expedited_sequence, s1, s2, s3,
 		   atomic_long_read(&rsp->expedited_normal),
 		   atomic_read(&rsp->expedited_need_qs),
 		   rsp->expedited_sequence / 2);
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ca828b41c938..3ccdc8eebc5a 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -67,7 +67,7 @@ static int rcu_normal_after_boot;
 module_param(rcu_normal_after_boot, int, 0);
 #endif /* #ifndef CONFIG_TINY_RCU */
 
-#if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
 /**
  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
  *
@@ -111,7 +111,7 @@ int rcu_read_lock_sched_held(void)
 		return 0;
 	if (debug_locks)
 		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
-	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
+	return lockdep_opinion || !preemptible();
 }
 EXPORT_SYMBOL(rcu_read_lock_sched_held);
 #endif
diff --git a/kernel/torture.c b/kernel/torture.c
index 44aa462d033f..fa0bdeee17ac 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -451,6 +451,7 @@ static int torture_shutdown(void *arg)
 		torture_shutdown_hook();
 	else
 		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
+	ftrace_dump(DUMP_ALL);
 	kernel_power_off();	/* Shut down the system. */
 	return 0;
 }
@@ -602,8 +603,9 @@ bool torture_init_begin(char *ttype, bool v, int *runnable)
 {
 	mutex_lock(&fullstop_mutex);
 	if (torture_type != NULL) {
-		pr_alert("torture_init_begin: refusing %s init: %s running",
+		pr_alert("torture_init_begin: Refusing %s init: %s running.\n",
 			 ttype, torture_type);
+		pr_alert("torture_init_begin: One torture test at a time!\n");
 		mutex_unlock(&fullstop_mutex);
 		return false;
 	}
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1e9a607534ca..f4b797a690ba 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1289,6 +1289,39 @@ config TORTURE_TEST
 	tristate
 	default n
 
+config RCU_PERF_TEST
+	tristate "performance tests for RCU"
+	depends on DEBUG_KERNEL
+	select TORTURE_TEST
+	select SRCU
+	select TASKS_RCU
+	default n
+	help
+	  This option provides a kernel module that runs performance
+	  tests on the RCU infrastructure.  The kernel module may be built
+	  after the fact on the running kernel to be tested, if desired.
+
+	  Say Y here if you want RCU performance tests to be built into
+	  the kernel.
+	  Say M if you want the RCU performance tests to build as a module.
+	  Say N if you are unsure.
+
+config RCU_PERF_TEST_RUNNABLE
+	bool "performance tests for RCU runnable by default"
+	depends on RCU_PERF_TEST = y
+	default n
+	help
+	  This option provides a way to build the RCU performance tests
+	  directly into the kernel without them starting up at boot time.
+	  You can use /sys/module to manually override this setting.
+	  This /proc file is available only when the RCU performance
+	  tests have been built into the kernel.
+
+	  Say Y here if you want the RCU performance tests to start during
+	  boot (you probably don't).
+	  Say N here if you want the RCU performance tests to start only
+	  after being manually enabled via /sys/module.
+
 config RCU_TORTURE_TEST
 	tristate "torture tests for RCU"
 	depends on DEBUG_KERNEL
diff --git a/tools/testing/selftests/rcutorture/bin/jitter.sh b/tools/testing/selftests/rcutorture/bin/jitter.sh
new file mode 100755
index 000000000000..3633828375e3
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/bin/jitter.sh
@@ -0,0 +1,90 @@
+#!/bin/bash
+#
+# Alternate sleeping and spinning on randomly selected CPUs.  The purpose
+# of this script is to inflict random OS jitter on a concurrently running
+# test.
+#
+# Usage: jitter.sh me duration [ sleepmax [ spinmax ] ]
+#
+# me: Random-number-generator seed salt.
+# duration: Time to run in seconds.
+# sleepmax: Maximum microseconds to sleep, defaults to one second.
+# spinmax: Maximum microseconds to spin, defaults to one millisecond.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, you can access it online at
+# http://www.gnu.org/licenses/gpl-2.0.html.
+#
+# Copyright (C) IBM Corporation, 2016
+#
+# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+
+me=$(($1 * 1000))
+duration=$2
+sleepmax=${3-1000000}
+spinmax=${4-1000}
+
+n=1
+
+starttime=`awk 'BEGIN { print systime(); }' < /dev/null`
+
+while :
+do
+	# Check for done.
+	t=`awk -v s=$starttime 'BEGIN { print systime() - s; }' < /dev/null`
+	if test "$t" -gt "$duration"
+	then
+		exit 0;
+	fi
+
+	# Set affinity to randomly selected CPU
+	cpus=`ls /sys/devices/system/cpu/*/online |
+		sed -e 's,/[^/]*$,,' -e 's/^[^0-9]*//' |
+		grep -v '^0*$'`
+	cpumask=`awk -v cpus="$cpus" -v me=$me -v n=$n 'BEGIN {
+		srand(n + me + systime());
+		ncpus = split(cpus, ca);
+		curcpu = ca[int(rand() * ncpus + 1)];
+		mask = lshift(1, curcpu);
+		if (mask + 0 <= 0)
+			mask = 1;
+		printf("%#x\n", mask);
+	}' < /dev/null`
+	n=$(($n+1))
+	if ! taskset -p $cpumask $$ > /dev/null 2>&1
+	then
+		echo taskset failure: '"taskset -p ' $cpumask $$ '"'
+		exit 1
+	fi
+
+	# Sleep a random duration
+	sleeptime=`awk -v me=$me -v n=$n -v sleepmax=$sleepmax 'BEGIN {
+		srand(n + me + systime());
+		printf("%06d", int(rand() * sleepmax));
+	}' < /dev/null`
+	n=$(($n+1))
+	sleep .$sleeptime
+
+	# Spin a random duration
+	limit=`awk -v me=$me -v n=$n -v spinmax=$spinmax 'BEGIN {
+		srand(n + me + systime());
+		printf("%06d", int(rand() * spinmax));
+	}' < /dev/null`
+	n=$(($n+1))
+	for i in {1..$limit}
+	do
+		echo > /dev/null
+	done
+done
+
+exit 1
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf-ftrace.sh b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf-ftrace.sh
new file mode 100755
index 000000000000..f79b0e9e84fc
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf-ftrace.sh
@@ -0,0 +1,121 @@
+#!/bin/bash
+#
+# Analyze a given results directory for rcuperf performance measurements,
+# looking for ftrace data.  Exits with 0 if data was found, analyzed, and
+# printed.  Intended to be invoked from kvm-recheck-rcuperf.sh after
+# argument checking.
+#
+# Usage: kvm-recheck-rcuperf-ftrace.sh resdir
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, you can access it online at
+# http://www.gnu.org/licenses/gpl-2.0.html.
+#
+# Copyright (C) IBM Corporation, 2016
+#
+# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+
+i="$1"
+. tools/testing/selftests/rcutorture/bin/functions.sh
+
+if test "`grep -c 'rcu_exp_grace_period.*start' < $i/console.log`" -lt 100
+then
+	exit 10
+fi
+
+sed -e 's/^\[[^]]*]//' < $i/console.log |
+grep 'us : rcu_exp_grace_period' |
+sed -e 's/us : / : /' |
+tr -d '\015' |
+awk '
+$8 == "start" {
+	if (starttask != "")
+		nlost++;
+	starttask = $1;
+	starttime = $3;
+	startseq = $7;
+}
+
+$8 == "end" {
+	if (starttask == $1 && startseq == $7) {
+		curgpdur = $3 - starttime;
+		gptimes[++n] = curgpdur;
+		gptaskcnt[starttask]++;
+		sum += curgpdur;
+		if (curgpdur > 1000)
+			print "Long GP " starttime "us to " $3 "us (" curgpdur "us)";
+		starttask = "";
+	} else {
+		# Lost a message or some such, reset.
+		starttask = "";
+		nlost++;
+	}
+}
+
+$8 == "done" {
+	piggybackcnt[$1]++;
+}
+
+END {
+	newNR = asort(gptimes);
+	if (newNR <= 0) {
+		print "No ftrace records found???"
+		exit 10;
+	}
+	pct50 = int(newNR * 50 / 100);
+	if (pct50 < 1)
+		pct50 = 1;
+	pct90 = int(newNR * 90 / 100);
+	if (pct90 < 1)
+		pct90 = 1;
+	pct99 = int(newNR * 99 / 100);
+	if (pct99 < 1)
+		pct99 = 1;
+	div = 10 ** int(log(gptimes[pct90]) / log(10) + .5) / 100;
+	print "Histogram bucket size: " div;
+	last = gptimes[1] - 10;
+	count = 0;
+	for (i = 1; i <= newNR; i++) {
+		current = div * int(gptimes[i] / div);
+		if (last == current) {
+			count++;
+		} else {
+			if (count > 0)
+				print last, count;
+			count = 1;
+			last = current;
+		}
+	}
+	if (count > 0)
+		print last, count;
+	print "Distribution of grace periods across tasks:";
+	for (i in gptaskcnt) {
+		print "\t" i, gptaskcnt[i];
+		nbatches += gptaskcnt[i];
+	}
+	ngps = nbatches;
+	print "Distribution of piggybacking across tasks:";
+	for (i in piggybackcnt) {
+		print "\t" i, piggybackcnt[i];
+		ngps += piggybackcnt[i];
+	}
+	print "Average grace-period duration: " sum / newNR " microseconds";
+	print "Minimum grace-period duration: " gptimes[1];
+	print "50th percentile grace-period duration: " gptimes[pct50];
+	print "90th percentile grace-period duration: " gptimes[pct90];
+	print "99th percentile grace-period duration: " gptimes[pct99];
+	print "Maximum grace-period duration: " gptimes[newNR];
+	print "Grace periods: " ngps + 0 " Batches: " nbatches + 0 " Ratio: " ngps / nbatches " Lost: " nlost + 0;
+	print "Computed from ftrace data.";
+}'
+exit 0
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh
new file mode 100755
index 000000000000..8f3121afc716
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh
@@ -0,0 +1,96 @@
+#!/bin/bash
+#
+# Analyze a given results directory for rcuperf performance measurements.
+#
+# Usage: kvm-recheck-rcuperf.sh resdir
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, you can access it online at
+# http://www.gnu.org/licenses/gpl-2.0.html.
+#
+# Copyright (C) IBM Corporation, 2016
+#
+# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+
+i="$1"
+if test -d $i
+then
+	:
+else
+	echo Unreadable results directory: $i
+	exit 1
+fi
+PATH=`pwd`/tools/testing/selftests/rcutorture/bin:$PATH; export PATH
+. tools/testing/selftests/rcutorture/bin/functions.sh
+
+if kvm-recheck-rcuperf-ftrace.sh $i
+then
+	# ftrace data was successfully analyzed, call it good!
+	exit 0
+fi
+
+configfile=`echo $i | sed -e 's/^.*\///'`
+
+sed -e 's/^\[[^]]*]//' < $i/console.log |
+awk '
+/-perf: .* gps: .* batches:/ {
+	ngps = $9;
+	nbatches = $11;
+}
+
+/-perf: .*writer-duration/ {
+	gptimes[++n] = $5 / 1000.;
+	sum += $5 / 1000.;
+}
+
+END {
+	newNR = asort(gptimes);
+	if (newNR <= 0) {
+		print "No rcuperf records found???"
+		exit;
+	}
+	pct50 = int(newNR * 50 / 100);
+	if (pct50 < 1)
+		pct50 = 1;
+	pct90 = int(newNR * 90 / 100);
+	if (pct90 < 1)
+		pct90 = 1;
+	pct99 = int(newNR * 99 / 100);
+	if (pct99 < 1)
+		pct99 = 1;
+	div = 10 ** int(log(gptimes[pct90]) / log(10) + .5) / 100;
+	print "Histogram bucket size: " div;
+	last = gptimes[1] - 10;
+	count = 0;
+	for (i = 1; i <= newNR; i++) {
+		current = div * int(gptimes[i] / div);
+		if (last == current) {
+			count++;
+		} else {
+			if (count > 0)
+				print last, count;
+			count = 1;
+			last = current;
+		}
+	}
+	if (count > 0)
+		print last, count;
+	print "Average grace-period duration: " sum / newNR " microseconds";
+	print "Minimum grace-period duration: " gptimes[1];
+	print "50th percentile grace-period duration: " gptimes[pct50];
+	print "90th percentile grace-period duration: " gptimes[pct90];
+	print "99th percentile grace-period duration: " gptimes[pct99];
+	print "Maximum grace-period duration: " gptimes[newNR];
+	print "Grace periods: " ngps + 0 " Batches: " nbatches + 0 " Ratio: " ngps / nbatches;
+	print "Computed from rcuperf printk output.";
+}'
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-recheck.sh b/tools/testing/selftests/rcutorture/bin/kvm-recheck.sh
index d86bdd6b6cc2..f659346d3358 100755
--- a/tools/testing/selftests/rcutorture/bin/kvm-recheck.sh
+++ b/tools/testing/selftests/rcutorture/bin/kvm-recheck.sh
@@ -48,7 +48,10 @@ do
 				cat $i/Make.oldconfig.err
 			fi
 			parse-build.sh $i/Make.out $configfile
-			parse-torture.sh $i/console.log $configfile
+			if test "$TORTURE_SUITE" != rcuperf
+			then
+				parse-torture.sh $i/console.log $configfile
+			fi
 			parse-console.sh $i/console.log $configfile
 			if test -r $i/Warnings
 			then
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh b/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
index 0f80eefb0bfd..4109f306d855 100755
--- a/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
+++ b/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
@@ -6,7 +6,7 @@
 # Execute this in the source tree.  Do not run it as a background task
 # because qemu does not seem to like that much.
 #
-# Usage: kvm-test-1-run.sh config builddir resdir minutes qemu-args boot_args
+# Usage: kvm-test-1-run.sh config builddir resdir seconds qemu-args boot_args
 #
 # qemu-args defaults to "-enable-kvm -soundhw pcspk -nographic", along with
 #			arguments specifying the number of CPUs and other
@@ -91,25 +91,33 @@ fi
 # CONFIG_PCMCIA=n
 # CONFIG_CARDBUS=n
 # CONFIG_YENTA=n
-if kvm-build.sh $config_template $builddir $T
+base_resdir=`echo $resdir | sed -e 's/\.[0-9]\+$//'`
+if test "$base_resdir" != "$resdir" -a -f $base_resdir/bzImage -a -f $base_resdir/vmlinux
 then
+	# Rerunning previous test, so use that test's kernel.
+	QEMU="`identify_qemu $base_resdir/vmlinux`"
+	KERNEL=$base_resdir/bzImage
+	ln -s $base_resdir/Make*.out $resdir  # for kvm-recheck.sh
+	ln -s $base_resdir/.config $resdir  # for kvm-recheck.sh
+elif kvm-build.sh $config_template $builddir $T
+then
+	# Had to build a kernel for this test.
 	QEMU="`identify_qemu $builddir/vmlinux`"
 	BOOT_IMAGE="`identify_boot_image $QEMU`"
 	cp $builddir/Make*.out $resdir
+	cp $builddir/vmlinux $resdir
 	cp $builddir/.config $resdir
 	if test -n "$BOOT_IMAGE"
 	then
 		cp $builddir/$BOOT_IMAGE $resdir
+		KERNEL=$resdir/bzImage
 	else
 		echo No identifiable boot image, not running KVM, see $resdir.
 		echo Do the torture scripts know about your architecture?
 	fi
 	parse-build.sh $resdir/Make.out $title
-	if test -f $builddir.wait
-	then
-		mv $builddir.wait $builddir.ready
-	fi
 else
+	# Build failed.
 	cp $builddir/Make*.out $resdir
 	cp $builddir/.config $resdir || :
 	echo Build failed, not running KVM, see $resdir.
@@ -119,12 +127,15 @@ else
 	fi
 	exit 1
 fi
+if test -f $builddir.wait
+then
+	mv $builddir.wait $builddir.ready
+fi
 while test -f $builddir.ready
 do
 	sleep 1
 done
-minutes=$4
-seconds=$(($minutes * 60))
+seconds=$4
 qemu_args=$5
 boot_args=$6
 
@@ -167,15 +178,26 @@ then
 	exit 0
 fi
 echo "NOTE: $QEMU either did not run or was interactive" > $resdir/console.log
-echo $QEMU $qemu_args -m 512 -kernel $resdir/bzImage -append \"$qemu_append $boot_args\" > $resdir/qemu-cmd
-( $QEMU $qemu_args -m 512 -kernel $resdir/bzImage -append "$qemu_append $boot_args"; echo $? > $resdir/qemu-retval ) &
-qemu_pid=$!
+echo $QEMU $qemu_args -m 512 -kernel $KERNEL -append \"$qemu_append $boot_args\" > $resdir/qemu-cmd
+( $QEMU $qemu_args -m 512 -kernel $KERNEL -append "$qemu_append $boot_args"& echo $! > $resdir/qemu_pid; wait `cat  $resdir/qemu_pid`; echo $? > $resdir/qemu-retval ) &
 commandcompleted=0
-echo Monitoring qemu job at pid $qemu_pid
+sleep 10 # Give qemu's pid a chance to reach the file
+if test -s "$resdir/qemu_pid"
+then
+	qemu_pid=`cat "$resdir/qemu_pid"`
+	echo Monitoring qemu job at pid $qemu_pid
+else
+	qemu_pid=""
+	echo Monitoring qemu job at yet-as-unknown pid
+fi
 while :
 do
+	if test -z "$qemu_pid" -a -s "$resdir/qemu_pid"
+	then
+		qemu_pid=`cat "$resdir/qemu_pid"`
+	fi
 	kruntime=`awk 'BEGIN { print systime() - '"$kstarttime"' }' < /dev/null`
-	if kill -0 $qemu_pid > /dev/null 2>&1
+	if test -z "$qemu_pid" || kill -0 "$qemu_pid" > /dev/null 2>&1
 	then
 		if test $kruntime -ge $seconds
 		then
@@ -195,12 +217,16 @@ do
 				ps -fp $killpid >> $resdir/Warnings 2>&1
 			fi
 		else
-			echo ' ---' `date`: Kernel done
+			echo ' ---' `date`: "Kernel done"
 		fi
 		break
 	fi
 done
-if test $commandcompleted -eq 0
+if test -z "$qemu_pid" -a -s "$resdir/qemu_pid"
+then
+	qemu_pid=`cat "$resdir/qemu_pid"`
+fi
+if test $commandcompleted -eq 0 -a -n "$qemu_pid"
 then
 	echo Grace period for qemu job at pid $qemu_pid
 	while :
@@ -220,6 +246,9 @@ then
 		fi
 		sleep 1
 	done
+elif test -z "$qemu_pid"
+then
+	echo Unknown PID, cannot kill qemu command
 fi
 
 parse-torture.sh $resdir/console.log $title
diff --git a/tools/testing/selftests/rcutorture/bin/kvm.sh b/tools/testing/selftests/rcutorture/bin/kvm.sh
index 4a431767f77a..0d598145873e 100755
--- a/tools/testing/selftests/rcutorture/bin/kvm.sh
+++ b/tools/testing/selftests/rcutorture/bin/kvm.sh
@@ -34,7 +34,7 @@ T=/tmp/kvm.sh.$$
 trap 'rm -rf $T' 0
 mkdir $T
 
-dur=30
+dur=$((30*60))
 dryrun=""
 KVM="`pwd`/tools/testing/selftests/rcutorture"; export KVM
 PATH=${KVM}/bin:$PATH; export PATH
@@ -48,6 +48,7 @@ resdir=""
 configs=""
 cpus=0
 ds=`date +%Y.%m.%d-%H:%M:%S`
+jitter=0
 
 . functions.sh
 
@@ -63,6 +64,7 @@ usage () {
 	echo "       --dryrun sched|script"
 	echo "       --duration minutes"
 	echo "       --interactive"
+	echo "       --jitter N [ maxsleep (us) [ maxspin (us) ] ]"
 	echo "       --kmake-arg kernel-make-arguments"
 	echo "       --mac nn:nn:nn:nn:nn:nn"
 	echo "       --no-initrd"
@@ -116,12 +118,17 @@ do
 		;;
 	--duration)
 		checkarg --duration "(minutes)" $# "$2" '^[0-9]*$' '^error'
-		dur=$2
+		dur=$(($2*60))
 		shift
 		;;
 	--interactive)
 		TORTURE_QEMU_INTERACTIVE=1; export TORTURE_QEMU_INTERACTIVE
 		;;
+	--jitter)
+		checkarg --jitter "(# threads [ sleep [ spin ] ])" $# "$2" '^-\{,1\}[0-9]\+\( \+[0-9]\+\)\{,2\} *$' '^error$'
+		jitter="$2"
+		shift
+		;;
 	--kmake-arg)
 		checkarg --kmake-arg "(kernel make arguments)" $# "$2" '.*' '^error$'
 		TORTURE_KMAKE_ARG="$2"
@@ -156,7 +163,7 @@ do
 		shift
 		;;
 	--torture)
-		checkarg --torture "(suite name)" "$#" "$2" '^\(lock\|rcu\)$' '^--'
+		checkarg --torture "(suite name)" "$#" "$2" '^\(lock\|rcu\|rcuperf\)$' '^--'
 		TORTURE_SUITE=$2
 		shift
 		;;
@@ -299,6 +306,7 @@ awk < $T/cfgcpu.pack \
 	-v CONFIGDIR="$CONFIGFRAG/" \
 	-v KVM="$KVM" \
 	-v ncpus=$cpus \
+	-v jitter="$jitter" \
 	-v rd=$resdir/$ds/ \
 	-v dur=$dur \
 	-v TORTURE_QEMU_ARG="$TORTURE_QEMU_ARG" \
@@ -359,6 +367,16 @@ function dump(first, pastlast, batchnum)
 		print "\techo ----", cfr[j], cpusr[j] ovf ": Starting kernel. `date` >> " rd "/log";
 		print "fi"
 	}
+	njitter = 0;
+	split(jitter, ja);
+	if (ja[1] == -1 && ncpus == 0)
+		njitter = 1;
+	else if (ja[1] == -1)
+		njitter = ncpus;
+	else
+		njitter = ja[1];
+	for (j = 0; j < njitter; j++)
+		print "jitter.sh " j " " dur " " ja[2] " " ja[3] "&"
 	print "wait"
 	print "if test -z \"$TORTURE_BUILDONLY\""
 	print "then"
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE04 b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
index 39a2c6d7d7ec..17cbe098b115 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE04
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
@@ -14,7 +14,7 @@ CONFIG_HOTPLUG_CPU=n
 CONFIG_SUSPEND=n
 CONFIG_HIBERNATION=n
 CONFIG_RCU_FANOUT=4
-CONFIG_RCU_FANOUT_LEAF=4
+CONFIG_RCU_FANOUT_LEAF=3
 CONFIG_RCU_NOCB_CPU=n
 CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE04.boot b/tools/testing/selftests/rcutorture/configs/rcu/TREE04.boot
index 0fc8a3428938..e34c33430447 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE04.boot
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE04.boot
@@ -1 +1 @@
-rcutorture.torture_type=rcu_bh
+rcutorture.torture_type=rcu_bh rcutree.rcu_fanout_leaf=4
diff --git a/tools/testing/selftests/rcutorture/configs/rcuperf/CFLIST b/tools/testing/selftests/rcutorture/configs/rcuperf/CFLIST
new file mode 100644
index 000000000000..c9f56cf20775
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcuperf/CFLIST
@@ -0,0 +1 @@
+TREE
diff --git a/tools/testing/selftests/rcutorture/configs/rcuperf/CFcommon b/tools/testing/selftests/rcutorture/configs/rcuperf/CFcommon
new file mode 100644
index 000000000000..a09816b8c0f3
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcuperf/CFcommon
@@ -0,0 +1,2 @@
+CONFIG_RCU_PERF_TEST=y
+CONFIG_PRINTK_TIME=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcuperf/TREE b/tools/testing/selftests/rcutorture/configs/rcuperf/TREE
new file mode 100644
index 000000000000..a312f671a29a
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcuperf/TREE
@@ -0,0 +1,20 @@
+CONFIG_SMP=y
+CONFIG_PREEMPT_NONE=n
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=y
+#CHECK#CONFIG_PREEMPT_RCU=y
+CONFIG_HZ_PERIODIC=n
+CONFIG_NO_HZ_IDLE=y
+CONFIG_NO_HZ_FULL=n
+CONFIG_RCU_FAST_NO_HZ=n
+CONFIG_RCU_TRACE=n
+CONFIG_HOTPLUG_CPU=n
+CONFIG_SUSPEND=n
+CONFIG_HIBERNATION=n
+CONFIG_RCU_NOCB_CPU=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_PROVE_LOCKING=n
+CONFIG_RCU_BOOST=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=y
+CONFIG_RCU_TRACE=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcuperf/TREE54 b/tools/testing/selftests/rcutorture/configs/rcuperf/TREE54
new file mode 100644
index 000000000000..985fb170d13c
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcuperf/TREE54
@@ -0,0 +1,23 @@
+CONFIG_SMP=y
+CONFIG_NR_CPUS=54
+CONFIG_PREEMPT_NONE=n
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=y
+#CHECK#CONFIG_PREEMPT_RCU=y
+CONFIG_HZ_PERIODIC=n
+CONFIG_NO_HZ_IDLE=y
+CONFIG_NO_HZ_FULL=n
+CONFIG_RCU_FAST_NO_HZ=n
+CONFIG_RCU_TRACE=n
+CONFIG_HOTPLUG_CPU=n
+CONFIG_SUSPEND=n
+CONFIG_HIBERNATION=n
+CONFIG_RCU_FANOUT=3
+CONFIG_RCU_FANOUT_LEAF=2
+CONFIG_RCU_NOCB_CPU=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_PROVE_LOCKING=n
+CONFIG_RCU_BOOST=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=y
+CONFIG_RCU_TRACE=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcuperf/ver_functions.sh b/tools/testing/selftests/rcutorture/configs/rcuperf/ver_functions.sh
new file mode 100644
index 000000000000..34f2a1b35ee5
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcuperf/ver_functions.sh
@@ -0,0 +1,52 @@
+#!/bin/bash
+#
+# Torture-suite-dependent shell functions for the rest of the scripts.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, you can access it online at
+# http://www.gnu.org/licenses/gpl-2.0.html.
+#
+# Copyright (C) IBM Corporation, 2015
+#
+# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+
+# rcuperf_param_nreaders bootparam-string
+#
+# Adds nreaders rcuperf module parameter if not already specified.
+rcuperf_param_nreaders () {
+	if ! echo "$1" | grep -q "rcuperf.nreaders"
+	then
+		echo rcuperf.nreaders=-1
+	fi
+}
+
+# rcuperf_param_nwriters bootparam-string
+#
+# Adds nwriters rcuperf module parameter if not already specified.
+rcuperf_param_nwriters () {
+	if ! echo "$1" | grep -q "rcuperf.nwriters"
+	then
+		echo rcuperf.nwriters=-1
+	fi
+}
+
+# per_version_boot_params bootparam-string config-file seconds
+#
+# Adds per-version torture-module parameters to kernels supporting them.
+per_version_boot_params () {
+	echo $1 `rcuperf_param_nreaders "$1"` \
+		`rcuperf_param_nwriters "$1"` \
+		rcuperf.perf_runnable=1 \
+		rcuperf.shutdown=1 \
+		rcuperf.verbose=1
+}
author	Linus Torvalds <torvalds@linux-foundation.org>	2016-05-16 12:02:08 -0700
committer	Linus Torvalds <torvalds@linux-foundation.org>	2016-05-16 12:02:08 -0700
commit	a3871bd434cfc313cdcd1edc433507298f4aa928 (patch)
tree	e47f2796fb8381219aff3cec54328589c948d683
parent	0052af4411b048eb6c0b0adb73d0fb4803ba1794 (diff)
parent	41ed943d855c3fa2bf6fecc33250b294bd568b7a (diff)
download	linux-a3871bd434cfc313cdcd1edc433507298f4aa928.tar.bz2