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author | Phong Tran <tranmanphong@gmail.com> | 2019-11-06 20:09:50 +0700 |
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committer | Paul E. McKenney <paulmck@kernel.org> | 2019-12-10 18:51:53 -0800 |
commit | 5e1bc932818f74082e8331b59aa550101ead08e0 (patch) | |
tree | 243c5dd7a81e45c5a1ba579c7d80ae819c7fb4fd /Documentation/RCU | |
parent | 6705cae433cffc37b183ded6ca9fe5c6d8ae8a9d (diff) | |
download | linux-5e1bc932818f74082e8331b59aa550101ead08e0.tar.bz2 |
doc: Convert whatisRCU.txt to .rst
This commit updates whatisRCU.txt to the new .rst format.
This change includes:
- Formatting bullet lists
- Adding literal blocks
- Links from table of contents to corresponding sections
- Links to external documents
- Reformat quick quizzes
Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Tested-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
[ tranmanphong: Apply Amol Grover feedback. ]
Reviewed-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'Documentation/RCU')
-rw-r--r-- | Documentation/RCU/index.rst | 1 | ||||
-rw-r--r-- | Documentation/RCU/whatisRCU.rst (renamed from Documentation/RCU/whatisRCU.txt) | 284 |
2 files changed, 178 insertions, 107 deletions
diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst index 627128c230dc..b9b11481c727 100644 --- a/Documentation/RCU/index.rst +++ b/Documentation/RCU/index.rst @@ -8,6 +8,7 @@ RCU concepts :maxdepth: 3 arrayRCU + whatisRCU rcu listRCU NMI-RCU diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.rst index 58ba05c4d97f..2f6f6ebbc8b0 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.rst @@ -1,15 +1,18 @@ +.. _whatisrcu_doc: + What is RCU? -- "Read, Copy, Update" +====================================== Please note that the "What is RCU?" LWN series is an excellent place to start learning about RCU: -1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ -2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ -3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ -4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ - 2010 Big API Table http://lwn.net/Articles/419086/ -5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/ - 2014 Big API Table http://lwn.net/Articles/609973/ +| 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ +| 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ +| 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ +| 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ +| 2010 Big API Table http://lwn.net/Articles/419086/ +| 5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/ +| 2014 Big API Table http://lwn.net/Articles/609973/ What is RCU? @@ -24,14 +27,21 @@ the experience has been that different people must take different paths to arrive at an understanding of RCU. This document provides several different paths, as follows: -1. RCU OVERVIEW -2. WHAT IS RCU'S CORE API? -3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? -4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? -5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? -6. ANALOGY WITH READER-WRITER LOCKING -7. FULL LIST OF RCU APIs -8. ANSWERS TO QUICK QUIZZES +:ref:`1. RCU OVERVIEW <1_whatisRCU>` + +:ref:`2. WHAT IS RCU'S CORE API? <2_whatisRCU>` + +:ref:`3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>` + +:ref:`4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>` + +:ref:`5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>` + +:ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>` + +:ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>` + +:ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>` People who prefer starting with a conceptual overview should focus on Section 1, though most readers will profit by reading this section at @@ -49,8 +59,10 @@ everything, feel free to read the whole thing -- but if you are really that type of person, you have perused the source code and will therefore never need this document anyway. ;-) +.. _1_whatisRCU: 1. RCU OVERVIEW +---------------- The basic idea behind RCU is to split updates into "removal" and "reclamation" phases. The removal phase removes references to data items @@ -116,8 +128,10 @@ So how the heck can a reclaimer tell when a reader is done, given that readers are not doing any sort of synchronization operations??? Read on to learn about how RCU's API makes this easy. +.. _2_whatisRCU: 2. WHAT IS RCU'S CORE API? +--------------------------- The core RCU API is quite small: @@ -136,7 +150,7 @@ later. See the kernel docbook documentation for more info, or look directly at the function header comments. rcu_read_lock() - +^^^^^^^^^^^^^^^ void rcu_read_lock(void); Used by a reader to inform the reclaimer that the reader is @@ -150,7 +164,7 @@ rcu_read_lock() longer-term references to data structures. rcu_read_unlock() - +^^^^^^^^^^^^^^^^^ void rcu_read_unlock(void); Used by a reader to inform the reclaimer that the reader is @@ -158,15 +172,15 @@ rcu_read_unlock() read-side critical sections may be nested and/or overlapping. synchronize_rcu() - +^^^^^^^^^^^^^^^^^ void synchronize_rcu(void); Marks the end of updater code and the beginning of reclaimer code. It does this by blocking until all pre-existing RCU read-side critical sections on all CPUs have completed. - Note that synchronize_rcu() will -not- necessarily wait for + Note that synchronize_rcu() will **not** necessarily wait for any subsequent RCU read-side critical sections to complete. - For example, consider the following sequence of events: + For example, consider the following sequence of events:: CPU 0 CPU 1 CPU 2 ----------------- ------------------------- --------------- @@ -182,7 +196,7 @@ synchronize_rcu() any that begin after synchronize_rcu() is invoked. Of course, synchronize_rcu() does not necessarily return - -immediately- after the last pre-existing RCU read-side critical + **immediately** after the last pre-existing RCU read-side critical section completes. For one thing, there might well be scheduling delays. For another thing, many RCU implementations process requests in batches in order to improve efficiencies, which can @@ -211,10 +225,10 @@ synchronize_rcu() checklist.txt for some approaches to limiting the update rate. rcu_assign_pointer() - +^^^^^^^^^^^^^^^^^^^^ void rcu_assign_pointer(p, typeof(p) v); - Yes, rcu_assign_pointer() -is- implemented as a macro, though it + Yes, rcu_assign_pointer() **is** implemented as a macro, though it would be cool to be able to declare a function in this manner. (Compiler experts will no doubt disagree.) @@ -231,7 +245,7 @@ rcu_assign_pointer() the _rcu list-manipulation primitives such as list_add_rcu(). rcu_dereference() - +^^^^^^^^^^^^^^^^^ typeof(p) rcu_dereference(p); Like rcu_assign_pointer(), rcu_dereference() must be implemented @@ -248,13 +262,13 @@ rcu_dereference() Common coding practice uses rcu_dereference() to copy an RCU-protected pointer to a local variable, then dereferences - this local variable, for example as follows: + this local variable, for example as follows:: p = rcu_dereference(head.next); return p->data; However, in this case, one could just as easily combine these - into one statement: + into one statement:: return rcu_dereference(head.next)->data; @@ -266,8 +280,8 @@ rcu_dereference() unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid - only within the enclosing RCU read-side critical section [1]. - For example, the following is -not- legal: + only within the enclosing RCU read-side critical section [1]_. + For example, the following is **not** legal:: rcu_read_lock(); p = rcu_dereference(head.next); @@ -290,9 +304,9 @@ rcu_dereference() at any time, including immediately after the rcu_dereference(). And, again like rcu_assign_pointer(), rcu_dereference() is typically used indirectly, via the _rcu list-manipulation - primitives, such as list_for_each_entry_rcu() [2]. + primitives, such as list_for_each_entry_rcu() [2]_. - [1] The variant rcu_dereference_protected() can be used outside +.. [1] The variant rcu_dereference_protected() can be used outside of an RCU read-side critical section as long as the usage is protected by locks acquired by the update-side code. This variant avoids the lockdep warning that would happen when using (for @@ -305,7 +319,7 @@ rcu_dereference() a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst and the API's code comments for more details and example usage. - [2] If the list_for_each_entry_rcu() instance might be used by +.. [2] If the list_for_each_entry_rcu() instance might be used by update-side code as well as by RCU readers, then an additional lockdep expression can be added to its list of arguments. For example, given an additional "lock_is_held(&mylock)" argument, @@ -315,6 +329,7 @@ rcu_dereference() The following diagram shows how each API communicates among the reader, updater, and reclaimer. +:: rcu_assign_pointer() @@ -375,12 +390,16 @@ c. RCU applied to scheduler and interrupt/NMI-handler tasks. Again, most uses will be of (a). The (b) and (c) cases are important for specialized uses, but are relatively uncommon. +.. _3_whatisRCU: 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? +----------------------------------------------- This section shows a simple use of the core RCU API to protect a global pointer to a dynamically allocated structure. More-typical -uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. +uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`, +:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`. +:: struct foo { int a; @@ -440,40 +459,43 @@ uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. So, to sum up: -o Use rcu_read_lock() and rcu_read_unlock() to guard RCU +- Use rcu_read_lock() and rcu_read_unlock() to guard RCU read-side critical sections. -o Within an RCU read-side critical section, use rcu_dereference() +- Within an RCU read-side critical section, use rcu_dereference() to dereference RCU-protected pointers. -o Use some solid scheme (such as locks or semaphores) to +- Use some solid scheme (such as locks or semaphores) to keep concurrent updates from interfering with each other. -o Use rcu_assign_pointer() to update an RCU-protected pointer. +- Use rcu_assign_pointer() to update an RCU-protected pointer. This primitive protects concurrent readers from the updater, - -not- concurrent updates from each other! You therefore still + **not** concurrent updates from each other! You therefore still need to use locking (or something similar) to keep concurrent rcu_assign_pointer() primitives from interfering with each other. -o Use synchronize_rcu() -after- removing a data element from an - RCU-protected data structure, but -before- reclaiming/freeing +- Use synchronize_rcu() **after** removing a data element from an + RCU-protected data structure, but **before** reclaiming/freeing the data element, in order to wait for the completion of all RCU read-side critical sections that might be referencing that data item. See checklist.txt for additional rules to follow when using RCU. -And again, more-typical uses of RCU may be found in listRCU.txt, -arrayRCU.txt, and NMI-RCU.txt. +And again, more-typical uses of RCU may be found in :ref:`listRCU.rst +<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst +<NMI_rcu_doc>`. +.. _4_whatisRCU: 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? +-------------------------------------------- In the example above, foo_update_a() blocks until a grace period elapses. This is quite simple, but in some cases one cannot afford to wait so long -- there might be other high-priority work to be done. In such cases, one uses call_rcu() rather than synchronize_rcu(). -The call_rcu() API is as follows: +The call_rcu() API is as follows:: void call_rcu(struct rcu_head * head, void (*func)(struct rcu_head *head)); @@ -481,7 +503,7 @@ The call_rcu() API is as follows: This function invokes func(head) after a grace period has elapsed. This invocation might happen from either softirq or process context, so the function is not permitted to block. The foo struct needs to -have an rcu_head structure added, perhaps as follows: +have an rcu_head structure added, perhaps as follows:: struct foo { int a; @@ -490,7 +512,7 @@ have an rcu_head structure added, perhaps as follows: struct rcu_head rcu; }; -The foo_update_a() function might then be written as follows: +The foo_update_a() function might then be written as follows:: /* * Create a new struct foo that is the same as the one currently @@ -520,7 +542,7 @@ The foo_update_a() function might then be written as follows: call_rcu(&old_fp->rcu, foo_reclaim); } -The foo_reclaim() function might appear as follows: +The foo_reclaim() function might appear as follows:: void foo_reclaim(struct rcu_head *rp) { @@ -544,7 +566,7 @@ namely foo_reclaim(). The summary of advice is the same as for the previous section, except that we are now using call_rcu() rather than synchronize_rcu(): -o Use call_rcu() -after- removing a data element from an +- Use call_rcu() **after** removing a data element from an RCU-protected data structure in order to register a callback function that will be invoked after the completion of all RCU read-side critical sections that might be referencing that @@ -552,14 +574,16 @@ o Use call_rcu() -after- removing a data element from an If the callback for call_rcu() is not doing anything more than calling kfree() on the structure, you can use kfree_rcu() instead of call_rcu() -to avoid having to write your own callback: +to avoid having to write your own callback:: kfree_rcu(old_fp, rcu); Again, see checklist.txt for additional rules governing the use of RCU. +.. _5_whatisRCU: 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? +------------------------------------------------ One of the nice things about RCU is that it has extremely simple "toy" implementations that are a good first step towards understanding the @@ -579,7 +603,7 @@ more details on the current implementation as of early 2004. 5A. "TOY" IMPLEMENTATION #1: LOCKING - +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section presents a "toy" RCU implementation that is based on familiar locking primitives. Its overhead makes it a non-starter for real-life use, as does its lack of scalability. It is also unsuitable @@ -591,7 +615,7 @@ you allow nested rcu_read_lock() calls, you can deadlock. However, it is probably the easiest implementation to relate to, so is a good starting point. -It is extremely simple: +It is extremely simple:: static DEFINE_RWLOCK(rcu_gp_mutex); @@ -614,7 +638,7 @@ It is extremely simple: [You can ignore rcu_assign_pointer() and rcu_dereference() without missing much. But here are simplified versions anyway. And whatever you do, -don't forget about them when submitting patches making use of RCU!] +don't forget about them when submitting patches making use of RCU!]:: #define rcu_assign_pointer(p, v) \ ({ \ @@ -647,18 +671,23 @@ that the only thing that can block rcu_read_lock() is a synchronize_rcu(). But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex, so there can be no deadlock cycle. -Quick Quiz #1: Why is this argument naive? How could a deadlock +.. _quiz_1: + +Quick Quiz #1: + Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? How could this deadlock be avoided? +:ref:`Answers to Quick Quiz <8_whatisRCU>` 5B. "TOY" EXAMPLE #2: CLASSIC RCU - +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. +:: void rcu_read_lock(void) { } @@ -683,14 +712,14 @@ CPU in turn. The run_on() primitive can be implemented straightforwardly in terms of the sched_setaffinity() primitive. Of course, a somewhat less "toy" implementation would restore the affinity upon completion rather than just leaving all tasks running on the last CPU, but when I said -"toy", I meant -toy-! +"toy", I meant **toy**! So how the heck is this supposed to work??? Remember that it is illegal to block while in an RCU read-side critical section. Therefore, if a given CPU executes a context switch, we know that it must have completed all preceding RCU read-side critical sections. -Once -all- CPUs have executed a context switch, then -all- preceding +Once **all** CPUs have executed a context switch, then **all** preceding RCU read-side critical sections will have completed. So, suppose that we remove a data item from its structure and then invoke @@ -698,19 +727,32 @@ synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed that there are no RCU read-side critical sections holding a reference to that data item, so we can safely reclaim it. -Quick Quiz #2: Give an example where Classic RCU's read-side - overhead is -negative-. +.. _quiz_2: + +Quick Quiz #2: + Give an example where Classic RCU's read-side + overhead is **negative**. + +:ref:`Answers to Quick Quiz <8_whatisRCU>` -Quick Quiz #3: If it is illegal to block in an RCU read-side +.. _quiz_3: + +Quick Quiz #3: + If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? +:ref:`Answers to Quick Quiz <8_whatisRCU>` + +.. _6_whatisRCU: 6. ANALOGY WITH READER-WRITER LOCKING +-------------------------------------- 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; @@ -762,7 +804,7 @@ diff shows how closely related RCU and reader-writer locking can be. return 0; } -Or, for those who prefer a side-by-side listing: +Or, for those who prefer a side-by-side listing:: 1 struct el { 1 struct el { 2 struct list_head list; 2 struct list_head list; @@ -774,40 +816,44 @@ Or, for those who prefer a side-by-side listing: 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) - 2 { 2 { - 3 struct list_head *lp; 3 struct list_head *lp; - 4 struct el *p; 4 struct el *p; - 5 5 - 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(&listmutex); 10 rcu_read_unlock(); -11 return 1; 11 return 1; -12 } 12 } -13 } 13 } -14 read_unlock(&listmutex); 14 rcu_read_unlock(); -15 return 0; 15 return 0; -16 } 16 } - - 1 int delete(long key) 1 int delete(long key) - 2 { 2 { - 3 struct el *p; 3 struct el *p; - 4 4 - 5 write_lock(&listmutex); 5 spin_lock(&listmutex); - 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { - 7 if (p->key == key) { 7 if (p->key == key) { - 8 list_del(&p->list); 8 list_del_rcu(&p->list); - 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); - 10 synchronize_rcu(); -10 kfree(p); 11 kfree(p); -11 return 1; 12 return 1; -12 } 13 } -13 } 14 } -14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); -15 return 0; 16 return 0; -16 } 17 } +:: + + 1 int search(long key, int *result) 1 int search(long key, int *result) + 2 { 2 { + 3 struct list_head *lp; 3 struct list_head *lp; + 4 struct el *p; 4 struct el *p; + 5 5 + 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(&listmutex); 10 rcu_read_unlock(); + 11 return 1; 11 return 1; + 12 } 12 } + 13 } 13 } + 14 read_unlock(&listmutex); 14 rcu_read_unlock(); + 15 return 0; 15 return 0; + 16 } 16 } + +:: + + 1 int delete(long key) 1 int delete(long key) + 2 { 2 { + 3 struct el *p; 3 struct el *p; + 4 4 + 5 write_lock(&listmutex); 5 spin_lock(&listmutex); + 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { + 7 if (p->key == key) { 7 if (p->key == key) { + 8 list_del(&p->list); 8 list_del_rcu(&p->list); + 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); + 10 synchronize_rcu(); + 10 kfree(p); 11 kfree(p); + 11 return 1; 12 return 1; + 12 } 13 } + 13 } 14 } + 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); + 15 return 0; 16 return 0; + 16 } 17 } Either way, the differences are quite small. Read-side locking moves to rcu_read_lock() and rcu_read_unlock, update-side locking moves from @@ -825,15 +871,17 @@ delete() can now block. If this is a problem, there is a callback-based mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can be used in place of synchronize_rcu(). +.. _7_whatisRCU: 7. FULL LIST OF RCU APIs +------------------------- The RCU APIs are documented in docbook-format header comments in the Linux-kernel source code, but it helps to have a full list of the APIs, since there does not appear to be a way to categorize them in docbook. Here is the list, by category. -RCU list traversal: +RCU list traversal:: list_entry_rcu list_first_entry_rcu @@ -854,7 +902,7 @@ RCU list traversal: hlist_bl_first_rcu hlist_bl_for_each_entry_rcu -RCU pointer/list update: +RCU pointer/list udate:: rcu_assign_pointer list_add_rcu @@ -876,7 +924,9 @@ RCU pointer/list update: hlist_bl_del_rcu hlist_bl_set_first_rcu -RCU: Critical sections Grace period Barrier +RCU:: + + Critical sections Grace period Barrier rcu_read_lock synchronize_net rcu_barrier rcu_read_unlock synchronize_rcu @@ -885,7 +935,9 @@ RCU: Critical sections Grace period Barrier rcu_dereference_check kfree_rcu rcu_dereference_protected -bh: Critical sections Grace period Barrier +bh:: + + Critical sections Grace period Barrier rcu_read_lock_bh call_rcu rcu_barrier rcu_read_unlock_bh synchronize_rcu @@ -896,7 +948,9 @@ bh: Critical sections Grace period Barrier rcu_dereference_bh_protected rcu_read_lock_bh_held -sched: Critical sections Grace period Barrier +sched:: + + Critical sections Grace period Barrier rcu_read_lock_sched call_rcu rcu_barrier rcu_read_unlock_sched synchronize_rcu @@ -910,7 +964,9 @@ sched: Critical sections Grace period Barrier rcu_read_lock_sched_held -SRCU: Critical sections Grace period Barrier +SRCU:: + + Critical sections Grace period Barrier srcu_read_lock call_srcu srcu_barrier srcu_read_unlock synchronize_srcu @@ -918,13 +974,14 @@ SRCU: Critical sections Grace period Barrier srcu_dereference_check srcu_read_lock_held -SRCU: Initialization/cleanup +SRCU: Initialization/cleanup:: + DEFINE_SRCU DEFINE_STATIC_SRCU init_srcu_struct cleanup_srcu_struct -All: lockdep-checked RCU-protected pointer access +All: lockdep-checked RCU-protected pointer access:: rcu_access_pointer rcu_dereference_raw @@ -974,15 +1031,19 @@ g. Otherwise, use RCU. Of course, this all assumes that you have determined that RCU is in fact the right tool for your job. +.. _8_whatisRCU: 8. ANSWERS TO QUICK QUIZZES +---------------------------- -Quick Quiz #1: Why is this argument naive? How could a deadlock +Quick Quiz #1: + Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? [Referring to the lock-based "toy" RCU algorithm.] -Answer: Consider the following sequence of events: +Answer: + Consider the following sequence of events: 1. CPU 0 acquires some unrelated lock, call it "problematic_lock", disabling irq via @@ -1021,10 +1082,14 @@ Answer: Consider the following sequence of events: approach where tasks in RCU read-side critical sections cannot be blocked by tasks executing synchronize_rcu(). -Quick Quiz #2: Give an example where Classic RCU's read-side - overhead is -negative-. +:ref:`Back to Quick Quiz #1 <quiz_1>` + +Quick Quiz #2: + Give an example where Classic RCU's read-side + overhead is **negative**. -Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT +Answer: + Imagine a single-CPU system with a non-CONFIG_PREEMPT kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling @@ -1046,11 +1111,15 @@ Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT even the theoretical possibility of negative overhead for a synchronization primitive is a bit unexpected. ;-) -Quick Quiz #3: If it is illegal to block in an RCU read-side +:ref:`Back to Quick Quiz #2 <quiz_2>` + +Quick Quiz #3: + If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? -Answer: Just as PREEMPT_RT permits preemption of spinlock +Answer: + Just as PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical @@ -1069,6 +1138,7 @@ Answer: Just as PREEMPT_RT permits preemption of spinlock Besides, how does the computer know what pizza parlor the human being went to??? +:ref:`Back to Quick Quiz #3 <quiz_3>` ACKNOWLEDGEMENTS |