diff options
Diffstat (limited to 'Documentation/RCU')
| -rw-r--r-- | Documentation/RCU/UP.rst (renamed from Documentation/RCU/UP.txt) | 50 | ||||
| -rw-r--r-- | Documentation/RCU/index.rst | 19 | ||||
| -rw-r--r-- | Documentation/RCU/listRCU.rst (renamed from Documentation/RCU/listRCU.txt) | 38 | ||||
| -rw-r--r-- | Documentation/RCU/rcu.rst | 92 | ||||
| -rw-r--r-- | Documentation/RCU/rcu.txt | 89 |
5 files changed, 163 insertions, 125 deletions
diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.rst index 53bde717017b..e26dda27430c 100644 --- a/Documentation/RCU/UP.txt +++ b/Documentation/RCU/UP.rst @@ -1,17 +1,19 @@ -RCU on Uniprocessor Systems +.. _up_doc: +RCU on Uniprocessor Systems +=========================== A common misconception is that, on UP systems, the call_rcu() primitive may immediately invoke its function. The basis of this misconception is that since there is only one CPU, it should not be necessary to wait for anything else to get done, since there are no other CPUs for -anything else to be happening on. Although this approach will -sort- -of- +anything else to be happening on. Although this approach will *sort of* work a surprising amount of the time, it is a very bad idea in general. This document presents three examples that demonstrate exactly how bad an idea this is. - Example 1: softirq Suicide +-------------------------- Suppose that an RCU-based algorithm scans a linked list containing elements A, B, and C in process context, and can delete elements from @@ -28,8 +30,8 @@ your kernel. This same problem can occur if call_rcu() is invoked from a hardware interrupt handler. - Example 2: Function-Call Fatality +--------------------------------- Of course, one could avert the suicide described in the preceding example by having call_rcu() directly invoke its arguments only if it was called @@ -46,11 +48,13 @@ its arguments would cause it to fail to make the fundamental guarantee underlying RCU, namely that call_rcu() defers invoking its arguments until all RCU read-side critical sections currently executing have completed. -Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in - this case? +Quick Quiz #1: + Why is it *not* legal to invoke synchronize_rcu() in this case? +:ref:`Answers to Quick Quiz <answer_quick_quiz_up>` Example 3: Death by Deadlock +---------------------------- Suppose that call_rcu() is invoked while holding a lock, and that the callback function must acquire this same lock. In this case, if @@ -76,25 +80,30 @@ there are cases where this can be quite ugly: If call_rcu() directly invokes the callback, painful locking restrictions or API changes would be required. -Quick Quiz #2: What locking restriction must RCU callbacks respect? +Quick Quiz #2: + What locking restriction must RCU callbacks respect? +:ref:`Answers to Quick Quiz <answer_quick_quiz_up>` Summary +------- Permitting call_rcu() to immediately invoke its arguments breaks RCU, even on a UP system. So do not do it! Even on a UP system, the RCU -infrastructure -must- respect grace periods, and -must- invoke callbacks +infrastructure *must* respect grace periods, and *must* invoke callbacks from a known environment in which no locks are held. -Note that it -is- safe for synchronize_rcu() to return immediately on -UP systems, including !PREEMPT SMP builds running on UP systems. +Note that it *is* safe for synchronize_rcu() to return immediately on +UP systems, including PREEMPT SMP builds running on UP systems. -Quick Quiz #3: Why can't synchronize_rcu() return immediately on - UP systems running preemptable RCU? +Quick Quiz #3: + Why can't synchronize_rcu() return immediately on UP systems running + preemptable RCU? +.. _answer_quick_quiz_up: Answer to Quick Quiz #1: - Why is it -not- legal to invoke synchronize_rcu() in this case? + Why is it *not* legal to invoke synchronize_rcu() in this case? Because the calling function is scanning an RCU-protected linked list, and is therefore within an RCU read-side critical section. @@ -104,12 +113,13 @@ Answer to Quick Quiz #1: Answer to Quick Quiz #2: What locking restriction must RCU callbacks respect? - Any lock that is acquired within an RCU callback must be - acquired elsewhere using an _irq variant of the spinlock - primitive. For example, if "mylock" is acquired by an - RCU callback, then a process-context acquisition of this - lock must use something like spin_lock_irqsave() to - acquire the lock. + Any lock that is acquired within an RCU callback must be acquired + elsewhere using an _bh variant of the spinlock primitive. + For example, if "mylock" is acquired by an RCU callback, then + a process-context acquisition of this lock must use something + like spin_lock_bh() to acquire the lock. Please note that + it is also OK to use _irq variants of spinlocks, for example, + spin_lock_irqsave(). If the process-context code were to simply use spin_lock(), then, since RCU callbacks can be invoked from softirq context, @@ -119,7 +129,7 @@ Answer to Quick Quiz #2: This restriction might seem gratuitous, since very few RCU callbacks acquire locks directly. However, a great many RCU - callbacks do acquire locks -indirectly-, for example, via + callbacks do acquire locks *indirectly*, for example, via the kfree() primitive. Answer to Quick Quiz #3: diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst new file mode 100644 index 000000000000..340a9725676c --- /dev/null +++ b/Documentation/RCU/index.rst @@ -0,0 +1,19 @@ +.. _rcu_concepts: + +============ +RCU concepts +============ + +.. toctree:: + :maxdepth: 1 + + rcu + listRCU + UP + +.. only:: subproject and html + + Indices + ======= + + * :ref:`genindex` diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.rst index adb5a3782846..7956ff33042b 100644 --- a/Documentation/RCU/listRCU.txt +++ b/Documentation/RCU/listRCU.rst @@ -1,5 +1,7 @@ -Using RCU to Protect Read-Mostly Linked Lists +.. _list_rcu_doc: +Using RCU to Protect Read-Mostly Linked Lists +============================================= One of the best applications of RCU is to protect read-mostly linked lists ("struct list_head" in list.h). One big advantage of this approach @@ -7,8 +9,8 @@ is that all of the required memory barriers are included for you in the list macros. This document describes several applications of RCU, with the best fits first. - Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates +---------------------------------------------------------------------- The best applications are cases where, if reader-writer locking were used, the read-side lock would be dropped before taking any action @@ -24,7 +26,7 @@ added or deleted, rather than being modified in place. A straightforward example of this use of RCU may be found in the system-call auditing support. For example, a reader-writer locked -implementation of audit_filter_task() might be as follows: +implementation of audit_filter_task() might be as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -48,7 +50,7 @@ the corresponding value is returned. By the time that this value is acted on, the list may well have been modified. This makes sense, since if you are turning auditing off, it is OK to audit a few extra system calls. -This means that RCU can be easily applied to the read side, as follows: +This means that RCU can be easily applied to the read side, as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -73,7 +75,7 @@ become list_for_each_entry_rcu(). The _rcu() list-traversal primitives insert the read-side memory barriers that are required on DEC Alpha CPUs. The changes to the update side are also straightforward. A reader-writer -lock might be used as follows for deletion and insertion: +lock might be used as follows for deletion and insertion:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) @@ -106,7 +108,7 @@ lock might be used as follows for deletion and insertion: return 0; } -Following are the RCU equivalents for these two functions: +Following are the RCU equivalents for these two functions:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) @@ -154,13 +156,13 @@ otherwise cause concurrent readers to fail spectacularly. So, when readers can tolerate stale data and when entries are either added or deleted, without in-place modification, it is very easy to use RCU! - Example 2: Handling In-Place Updates +------------------------------------ The system-call auditing code does not update auditing rules in place. However, if it did, reader-writer-locked code to do so might look as follows (presumably, the field_count is only permitted to decrease, -otherwise, the added fields would need to be filled in): +otherwise, the added fields would need to be filled in):: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, @@ -187,7 +189,7 @@ otherwise, the added fields would need to be filled in): The RCU version creates a copy, updates the copy, then replaces the old entry with the newly updated entry. This sequence of actions, allowing concurrent reads while doing a copy to perform an update, is what gives -RCU ("read-copy update") its name. The RCU code is as follows: +RCU ("read-copy update") its name. The RCU code is as follows:: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, @@ -216,8 +218,8 @@ RCU ("read-copy update") its name. The RCU code is as follows: Again, this assumes that the caller holds audit_netlink_sem. Normally, the reader-writer lock would become a spinlock in this sort of code. - Example 3: Eliminating Stale Data +--------------------------------- The auditing examples above tolerate stale data, as do most algorithms that are tracking external state. Because there is a delay from the @@ -231,13 +233,16 @@ per-entry spinlock, and, if the "deleted" flag is set, pretends that the entry does not exist. For this to be helpful, the search function must return holding the per-entry spinlock, as ipc_lock() does in fact do. -Quick Quiz: Why does the search function need to return holding the - per-entry lock for this deleted-flag technique to be helpful? +Quick Quiz: + Why does the search function need to return holding the per-entry lock for + this deleted-flag technique to be helpful? + +:ref:`Answer to Quick Quiz <answer_quick_quiz_list>` If the system-call audit module were to ever need to reject stale data, one way to accomplish this would be to add a "deleted" flag and a "lock" spinlock to the audit_entry structure, and modify audit_filter_task() -as follows: +as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -268,7 +273,7 @@ audit_upd_rule() would need additional memory barriers to ensure that the list_add_rcu() was really executed before the list_del_rcu(). The audit_del_rule() function would need to set the "deleted" -flag under the spinlock as follows: +flag under the spinlock as follows:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) @@ -290,8 +295,8 @@ flag under the spinlock as follows: return -EFAULT; /* No matching rule */ } - Summary +------- Read-mostly list-based data structures that can tolerate stale data are the most amenable to use of RCU. The simplest case is where entries are @@ -302,8 +307,9 @@ If stale data cannot be tolerated, then a "deleted" flag may be used in conjunction with a per-entry spinlock in order to allow the search function to reject newly deleted data. +.. _answer_quick_quiz_list: -Answer to Quick Quiz +Answer to Quick Quiz: Why does the search function need to return holding the per-entry lock for this deleted-flag technique to be helpful? diff --git a/Documentation/RCU/rcu.rst b/Documentation/RCU/rcu.rst new file mode 100644 index 000000000000..8dfb437dacc3 --- /dev/null +++ b/Documentation/RCU/rcu.rst @@ -0,0 +1,92 @@ +.. _rcu_doc: + +RCU Concepts +============ + +The basic idea behind RCU (read-copy update) is to split destructive +operations into two parts, one that prevents anyone from seeing the data +item being destroyed, and one that actually carries out the destruction. +A "grace period" must elapse between the two parts, and this grace period +must be long enough that any readers accessing the item being deleted have +since dropped their references. For example, an RCU-protected deletion +from a linked list would first remove the item from the list, wait for +a grace period to elapse, then free the element. See the +Documentation/RCU/listRCU.rst file for more information on using RCU with +linked lists. + +Frequently Asked Questions +-------------------------- + +- Why would anyone want to use RCU? + + The advantage of RCU's two-part approach is that RCU readers need + not acquire any locks, perform any atomic instructions, write to + shared memory, or (on CPUs other than Alpha) execute any memory + barriers. The fact that these operations are quite expensive + on modern CPUs is what gives RCU its performance advantages + in read-mostly situations. The fact that RCU readers need not + acquire locks can also greatly simplify deadlock-avoidance code. + +- How can the updater tell when a grace period has completed + if the RCU readers give no indication when they are done? + + Just as with spinlocks, RCU readers are not permitted to + block, switch to user-mode execution, or enter the idle loop. + Therefore, as soon as a CPU is seen passing through any of these + three states, we know that that CPU has exited any previous RCU + read-side critical sections. So, if we remove an item from a + linked list, and then wait until all CPUs have switched context, + executed in user mode, or executed in the idle loop, we can + safely free up that item. + + Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the + same effect, but require that the readers manipulate CPU-local + counters. These counters allow limited types of blocking within + RCU read-side critical sections. SRCU also uses CPU-local + counters, and permits general blocking within RCU read-side + critical sections. These variants of RCU detect grace periods + by sampling these counters. + +- If I am running on a uniprocessor kernel, which can only do one + thing at a time, why should I wait for a grace period? + + See the Documentation/RCU/UP.rst file for more information. + +- How can I see where RCU is currently used in the Linux kernel? + + Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu", + "rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock", + "srcu_read_unlock", "synchronize_rcu", "synchronize_net", + "synchronize_srcu", and the other RCU primitives. Or grab one + of the cscope databases from: + + (http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html). + +- What guidelines should I follow when writing code that uses RCU? + + See the checklist.txt file in this directory. + +- Why the name "RCU"? + + "RCU" stands for "read-copy update". The file Documentation/RCU/listRCU.rst + has more information on where this name came from, search for + "read-copy update" to find it. + +- I hear that RCU is patented? What is with that? + + Yes, it is. There are several known patents related to RCU, + search for the string "Patent" in RTFP.txt to find them. + Of these, one was allowed to lapse by the assignee, and the + others have been contributed to the Linux kernel under GPL. + There are now also LGPL implementations of user-level RCU + available (http://liburcu.org/). + +- I hear that RCU needs work in order to support realtime kernels? + + Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU + kernel configuration parameter. + +- Where can I find more information on RCU? + + See the RTFP.txt file in this directory. + Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/). diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt deleted file mode 100644 index c818cf65c5a9..000000000000 --- a/Documentation/RCU/rcu.txt +++ /dev/null @@ -1,89 +0,0 @@ -RCU Concepts - - -The basic idea behind RCU (read-copy update) is to split destructive -operations into two parts, one that prevents anyone from seeing the data -item being destroyed, and one that actually carries out the destruction. -A "grace period" must elapse between the two parts, and this grace period -must be long enough that any readers accessing the item being deleted have -since dropped their references. For example, an RCU-protected deletion -from a linked list would first remove the item from the list, wait for -a grace period to elapse, then free the element. See the listRCU.txt -file for more information on using RCU with linked lists. - - -Frequently Asked Questions - -o Why would anyone want to use RCU? - - The advantage of RCU's two-part approach is that RCU readers need - not acquire any locks, perform any atomic instructions, write to - shared memory, or (on CPUs other than Alpha) execute any memory - barriers. The fact that these operations are quite expensive - on modern CPUs is what gives RCU its performance advantages - in read-mostly situations. The fact that RCU readers need not - acquire locks can also greatly simplify deadlock-avoidance code. - -o How can the updater tell when a grace period has completed - if the RCU readers give no indication when they are done? - - Just as with spinlocks, RCU readers are not permitted to - block, switch to user-mode execution, or enter the idle loop. - Therefore, as soon as a CPU is seen passing through any of these - three states, we know that that CPU has exited any previous RCU - read-side critical sections. So, if we remove an item from a - linked list, and then wait until all CPUs have switched context, - executed in user mode, or executed in the idle loop, we can - safely free up that item. - - Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the - same effect, but require that the readers manipulate CPU-local - counters. These counters allow limited types of blocking within - RCU read-side critical sections. SRCU also uses CPU-local - counters, and permits general blocking within RCU read-side - critical sections. These variants of RCU detect grace periods - by sampling these counters. - -o If I am running on a uniprocessor kernel, which can only do one - thing at a time, why should I wait for a grace period? - - See the UP.txt file in this directory. - -o How can I see where RCU is currently used in the Linux kernel? - - Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu", - "rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock", - "srcu_read_unlock", "synchronize_rcu", "synchronize_net", - "synchronize_srcu", and the other RCU primitives. Or grab one - of the cscope databases from: - - http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html - -o What guidelines should I follow when writing code that uses RCU? - - See the checklist.txt file in this directory. - -o Why the name "RCU"? - - "RCU" stands for "read-copy update". The file listRCU.txt has - more information on where this name came from, search for - "read-copy update" to find it. - -o I hear that RCU is patented? What is with that? - - Yes, it is. There are several known patents related to RCU, - search for the string "Patent" in RTFP.txt to find them. - Of these, one was allowed to lapse by the assignee, and the - others have been contributed to the Linux kernel under GPL. - There are now also LGPL implementations of user-level RCU - available (http://liburcu.org/). - -o I hear that RCU needs work in order to support realtime kernels? - - Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU - kernel configuration parameter. - -o Where can I find more information on RCU? - - See the RTFP.txt file in this directory. - Or point your browser at http://www.rdrop.com/users/paulmck/RCU/. |