1 Please note that the "What is RCU?" LWN series is an excellent place
2 to start learning about RCU:
4 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
5 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
6 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
7 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
12 RCU is a synchronization mechanism that was added to the Linux kernel
13 during the 2.5 development effort that is optimized for read-mostly
14 situations. Although RCU is actually quite simple once you understand it,
15 getting there can sometimes be a challenge. Part of the problem is that
16 most of the past descriptions of RCU have been written with the mistaken
17 assumption that there is "one true way" to describe RCU. Instead,
18 the experience has been that different people must take different paths
19 to arrive at an understanding of RCU. This document provides several
20 different paths, as follows:
23 2. WHAT IS RCU'S CORE API?
24 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
25 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
26 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
27 6. ANALOGY WITH READER-WRITER LOCKING
28 7. FULL LIST OF RCU APIs
29 8. ANSWERS TO QUICK QUIZZES
31 People who prefer starting with a conceptual overview should focus on
32 Section 1, though most readers will profit by reading this section at
33 some point. People who prefer to start with an API that they can then
34 experiment with should focus on Section 2. People who prefer to start
35 with example uses should focus on Sections 3 and 4. People who need to
36 understand the RCU implementation should focus on Section 5, then dive
37 into the kernel source code. People who reason best by analogy should
38 focus on Section 6. Section 7 serves as an index to the docbook API
39 documentation, and Section 8 is the traditional answer key.
41 So, start with the section that makes the most sense to you and your
42 preferred method of learning. If you need to know everything about
43 everything, feel free to read the whole thing -- but if you are really
44 that type of person, you have perused the source code and will therefore
45 never need this document anyway. ;-)
50 The basic idea behind RCU is to split updates into "removal" and
51 "reclamation" phases. The removal phase removes references to data items
52 within a data structure (possibly by replacing them with references to
53 new versions of these data items), and can run concurrently with readers.
54 The reason that it is safe to run the removal phase concurrently with
55 readers is the semantics of modern CPUs guarantee that readers will see
56 either the old or the new version of the data structure rather than a
57 partially updated reference. The reclamation phase does the work of reclaiming
58 (e.g., freeing) the data items removed from the data structure during the
59 removal phase. Because reclaiming data items can disrupt any readers
60 concurrently referencing those data items, the reclamation phase must
61 not start until readers no longer hold references to those data items.
63 Splitting the update into removal and reclamation phases permits the
64 updater to perform the removal phase immediately, and to defer the
65 reclamation phase until all readers active during the removal phase have
66 completed, either by blocking until they finish or by registering a
67 callback that is invoked after they finish. Only readers that are active
68 during the removal phase need be considered, because any reader starting
69 after the removal phase will be unable to gain a reference to the removed
70 data items, and therefore cannot be disrupted by the reclamation phase.
72 So the typical RCU update sequence goes something like the following:
74 a. Remove pointers to a data structure, so that subsequent
75 readers cannot gain a reference to it.
77 b. Wait for all previous readers to complete their RCU read-side
80 c. At this point, there cannot be any readers who hold references
81 to the data structure, so it now may safely be reclaimed
84 Step (b) above is the key idea underlying RCU's deferred destruction.
85 The ability to wait until all readers are done allows RCU readers to
86 use much lighter-weight synchronization, in some cases, absolutely no
87 synchronization at all. In contrast, in more conventional lock-based
88 schemes, readers must use heavy-weight synchronization in order to
89 prevent an updater from deleting the data structure out from under them.
90 This is because lock-based updaters typically update data items in place,
91 and must therefore exclude readers. In contrast, RCU-based updaters
92 typically take advantage of the fact that writes to single aligned
93 pointers are atomic on modern CPUs, allowing atomic insertion, removal,
94 and replacement of data items in a linked structure without disrupting
95 readers. Concurrent RCU readers can then continue accessing the old
96 versions, and can dispense with the atomic operations, memory barriers,
97 and communications cache misses that are so expensive on present-day
98 SMP computer systems, even in absence of lock contention.
100 In the three-step procedure shown above, the updater is performing both
101 the removal and the reclamation step, but it is often helpful for an
102 entirely different thread to do the reclamation, as is in fact the case
103 in the Linux kernel's directory-entry cache (dcache). Even if the same
104 thread performs both the update step (step (a) above) and the reclamation
105 step (step (c) above), it is often helpful to think of them separately.
106 For example, RCU readers and updaters need not communicate at all,
107 but RCU provides implicit low-overhead communication between readers
108 and reclaimers, namely, in step (b) above.
110 So how the heck can a reclaimer tell when a reader is done, given
111 that readers are not doing any sort of synchronization operations???
112 Read on to learn about how RCU's API makes this easy.
115 2. WHAT IS RCU'S CORE API?
117 The core RCU API is quite small:
121 c. synchronize_rcu() / call_rcu()
122 d. rcu_assign_pointer()
125 There are many other members of the RCU API, but the rest can be
126 expressed in terms of these five, though most implementations instead
127 express synchronize_rcu() in terms of the call_rcu() callback API.
129 The five core RCU APIs are described below, the other 18 will be enumerated
130 later. See the kernel docbook documentation for more info, or look directly
131 at the function header comments.
135 void rcu_read_lock(void);
137 Used by a reader to inform the reclaimer that the reader is
138 entering an RCU read-side critical section. It is illegal
139 to block while in an RCU read-side critical section, though
140 kernels built with CONFIG_TREE_PREEMPT_RCU can preempt RCU
141 read-side critical sections. Any RCU-protected data structure
142 accessed during an RCU read-side critical section is guaranteed to
143 remain unreclaimed for the full duration of that critical section.
144 Reference counts may be used in conjunction with RCU to maintain
145 longer-term references to data structures.
149 void rcu_read_unlock(void);
151 Used by a reader to inform the reclaimer that the reader is
152 exiting an RCU read-side critical section. Note that RCU
153 read-side critical sections may be nested and/or overlapping.
157 void synchronize_rcu(void);
159 Marks the end of updater code and the beginning of reclaimer
160 code. It does this by blocking until all pre-existing RCU
161 read-side critical sections on all CPUs have completed.
162 Note that synchronize_rcu() will -not- necessarily wait for
163 any subsequent RCU read-side critical sections to complete.
164 For example, consider the following sequence of events:
167 ----------------- ------------------------- ---------------
169 2. enters synchronize_rcu()
172 5. exits synchronize_rcu()
175 To reiterate, synchronize_rcu() waits only for ongoing RCU
176 read-side critical sections to complete, not necessarily for
177 any that begin after synchronize_rcu() is invoked.
179 Of course, synchronize_rcu() does not necessarily return
180 -immediately- after the last pre-existing RCU read-side critical
181 section completes. For one thing, there might well be scheduling
182 delays. For another thing, many RCU implementations process
183 requests in batches in order to improve efficiencies, which can
184 further delay synchronize_rcu().
186 Since synchronize_rcu() is the API that must figure out when
187 readers are done, its implementation is key to RCU. For RCU
188 to be useful in all but the most read-intensive situations,
189 synchronize_rcu()'s overhead must also be quite small.
191 The call_rcu() API is a callback form of synchronize_rcu(),
192 and is described in more detail in a later section. Instead of
193 blocking, it registers a function and argument which are invoked
194 after all ongoing RCU read-side critical sections have completed.
195 This callback variant is particularly useful in situations where
196 it is illegal to block or where update-side performance is
197 critically important.
199 However, the call_rcu() API should not be used lightly, as use
200 of the synchronize_rcu() API generally results in simpler code.
201 In addition, the synchronize_rcu() API has the nice property
202 of automatically limiting update rate should grace periods
203 be delayed. This property results in system resilience in face
204 of denial-of-service attacks. Code using call_rcu() should limit
205 update rate in order to gain this same sort of resilience. See
206 checklist.txt for some approaches to limiting the update rate.
210 typeof(p) rcu_assign_pointer(p, typeof(p) v);
212 Yes, rcu_assign_pointer() -is- implemented as a macro, though it
213 would be cool to be able to declare a function in this manner.
214 (Compiler experts will no doubt disagree.)
216 The updater uses this function to assign a new value to an
217 RCU-protected pointer, in order to safely communicate the change
218 in value from the updater to the reader. This function returns
219 the new value, and also executes any memory-barrier instructions
220 required for a given CPU architecture.
222 Perhaps just as important, it serves to document (1) which
223 pointers are protected by RCU and (2) the point at which a
224 given structure becomes accessible to other CPUs. That said,
225 rcu_assign_pointer() is most frequently used indirectly, via
226 the _rcu list-manipulation primitives such as list_add_rcu().
230 typeof(p) rcu_dereference(p);
232 Like rcu_assign_pointer(), rcu_dereference() must be implemented
235 The reader uses rcu_dereference() to fetch an RCU-protected
236 pointer, which returns a value that may then be safely
237 dereferenced. Note that rcu_deference() does not actually
238 dereference the pointer, instead, it protects the pointer for
239 later dereferencing. It also executes any needed memory-barrier
240 instructions for a given CPU architecture. Currently, only Alpha
241 needs memory barriers within rcu_dereference() -- on other CPUs,
242 it compiles to nothing, not even a compiler directive.
244 Common coding practice uses rcu_dereference() to copy an
245 RCU-protected pointer to a local variable, then dereferences
246 this local variable, for example as follows:
248 p = rcu_dereference(head.next);
251 However, in this case, one could just as easily combine these
254 return rcu_dereference(head.next)->data;
256 If you are going to be fetching multiple fields from the
257 RCU-protected structure, using the local variable is of
258 course preferred. Repeated rcu_dereference() calls look
259 ugly and incur unnecessary overhead on Alpha CPUs.
261 Note that the value returned by rcu_dereference() is valid
262 only within the enclosing RCU read-side critical section.
263 For example, the following is -not- legal:
266 p = rcu_dereference(head.next);
273 Holding a reference from one RCU read-side critical section
274 to another is just as illegal as holding a reference from
275 one lock-based critical section to another! Similarly,
276 using a reference outside of the critical section in which
277 it was acquired is just as illegal as doing so with normal
280 As with rcu_assign_pointer(), an important function of
281 rcu_dereference() is to document which pointers are protected by
282 RCU, in particular, flagging a pointer that is subject to changing
283 at any time, including immediately after the rcu_dereference().
284 And, again like rcu_assign_pointer(), rcu_dereference() is
285 typically used indirectly, via the _rcu list-manipulation
286 primitives, such as list_for_each_entry_rcu().
288 The following diagram shows how each API communicates among the
289 reader, updater, and reclaimer.
294 +---------------------->| reader |---------+
298 | | | rcu_read_lock()
299 | | | rcu_read_unlock()
300 | rcu_dereference() | |
302 | updater |<---------------------+ |
305 +----------------------------------->| reclaimer |
308 synchronize_rcu() & call_rcu()
311 The RCU infrastructure observes the time sequence of rcu_read_lock(),
312 rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
313 order to determine when (1) synchronize_rcu() invocations may return
314 to their callers and (2) call_rcu() callbacks may be invoked. Efficient
315 implementations of the RCU infrastructure make heavy use of batching in
316 order to amortize their overhead over many uses of the corresponding APIs.
318 There are no fewer than three RCU mechanisms in the Linux kernel; the
319 diagram above shows the first one, which is by far the most commonly used.
320 The rcu_dereference() and rcu_assign_pointer() primitives are used for
321 all three mechanisms, but different defer and protect primitives are
326 a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
327 call_rcu() rcu_dereference()
329 b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
332 c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
333 preempt_disable() / preempt_enable()
334 local_irq_save() / local_irq_restore()
335 hardirq enter / hardirq exit
337 rcu_dereference_sched()
339 These three mechanisms are used as follows:
341 a. RCU applied to normal data structures.
343 b. RCU applied to networking data structures that may be subjected
344 to remote denial-of-service attacks.
346 c. RCU applied to scheduler and interrupt/NMI-handler tasks.
348 Again, most uses will be of (a). The (b) and (c) cases are important
349 for specialized uses, but are relatively uncommon.
352 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
354 This section shows a simple use of the core RCU API to protect a
355 global pointer to a dynamically allocated structure. More-typical
356 uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
363 DEFINE_SPINLOCK(foo_mutex);
368 * Create a new struct foo that is the same as the one currently
369 * pointed to by gbl_foo, except that field "a" is replaced
370 * with "new_a". Points gbl_foo to the new structure, and
371 * frees up the old structure after a grace period.
373 * Uses rcu_assign_pointer() to ensure that concurrent readers
374 * see the initialized version of the new structure.
376 * Uses synchronize_rcu() to ensure that any readers that might
377 * have references to the old structure complete before freeing
380 void foo_update_a(int new_a)
385 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
386 spin_lock(&foo_mutex);
390 rcu_assign_pointer(gbl_foo, new_fp);
391 spin_unlock(&foo_mutex);
397 * Return the value of field "a" of the current gbl_foo
398 * structure. Use rcu_read_lock() and rcu_read_unlock()
399 * to ensure that the structure does not get deleted out
400 * from under us, and use rcu_dereference() to ensure that
401 * we see the initialized version of the structure (important
402 * for DEC Alpha and for people reading the code).
409 retval = rcu_dereference(gbl_foo)->a;
416 o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
417 read-side critical sections.
419 o Within an RCU read-side critical section, use rcu_dereference()
420 to dereference RCU-protected pointers.
422 o Use some solid scheme (such as locks or semaphores) to
423 keep concurrent updates from interfering with each other.
425 o Use rcu_assign_pointer() to update an RCU-protected pointer.
426 This primitive protects concurrent readers from the updater,
427 -not- concurrent updates from each other! You therefore still
428 need to use locking (or something similar) to keep concurrent
429 rcu_assign_pointer() primitives from interfering with each other.
431 o Use synchronize_rcu() -after- removing a data element from an
432 RCU-protected data structure, but -before- reclaiming/freeing
433 the data element, in order to wait for the completion of all
434 RCU read-side critical sections that might be referencing that
437 See checklist.txt for additional rules to follow when using RCU.
438 And again, more-typical uses of RCU may be found in listRCU.txt,
439 arrayRCU.txt, and NMI-RCU.txt.
442 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
444 In the example above, foo_update_a() blocks until a grace period elapses.
445 This is quite simple, but in some cases one cannot afford to wait so
446 long -- there might be other high-priority work to be done.
448 In such cases, one uses call_rcu() rather than synchronize_rcu().
449 The call_rcu() API is as follows:
451 void call_rcu(struct rcu_head * head,
452 void (*func)(struct rcu_head *head));
454 This function invokes func(head) after a grace period has elapsed.
455 This invocation might happen from either softirq or process context,
456 so the function is not permitted to block. The foo struct needs to
457 have an rcu_head structure added, perhaps as follows:
466 The foo_update_a() function might then be written as follows:
469 * Create a new struct foo that is the same as the one currently
470 * pointed to by gbl_foo, except that field "a" is replaced
471 * with "new_a". Points gbl_foo to the new structure, and
472 * frees up the old structure after a grace period.
474 * Uses rcu_assign_pointer() to ensure that concurrent readers
475 * see the initialized version of the new structure.
477 * Uses call_rcu() to ensure that any readers that might have
478 * references to the old structure complete before freeing the
481 void foo_update_a(int new_a)
486 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
487 spin_lock(&foo_mutex);
491 rcu_assign_pointer(gbl_foo, new_fp);
492 spin_unlock(&foo_mutex);
493 call_rcu(&old_fp->rcu, foo_reclaim);
496 The foo_reclaim() function might appear as follows:
498 void foo_reclaim(struct rcu_head *rp)
500 struct foo *fp = container_of(rp, struct foo, rcu);
507 The container_of() primitive is a macro that, given a pointer into a
508 struct, the type of the struct, and the pointed-to field within the
509 struct, returns a pointer to the beginning of the struct.
511 The use of call_rcu() permits the caller of foo_update_a() to
512 immediately regain control, without needing to worry further about the
513 old version of the newly updated element. It also clearly shows the
514 RCU distinction between updater, namely foo_update_a(), and reclaimer,
515 namely foo_reclaim().
517 The summary of advice is the same as for the previous section, except
518 that we are now using call_rcu() rather than synchronize_rcu():
520 o Use call_rcu() -after- removing a data element from an
521 RCU-protected data structure in order to register a callback
522 function that will be invoked after the completion of all RCU
523 read-side critical sections that might be referencing that
526 If the callback for call_rcu() is not doing anything more than calling
527 kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
528 to avoid having to write your own callback:
530 kfree_rcu(old_fp, rcu);
532 Again, see checklist.txt for additional rules governing the use of RCU.
535 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
537 One of the nice things about RCU is that it has extremely simple "toy"
538 implementations that are a good first step towards understanding the
539 production-quality implementations in the Linux kernel. This section
540 presents two such "toy" implementations of RCU, one that is implemented
541 in terms of familiar locking primitives, and another that more closely
542 resembles "classic" RCU. Both are way too simple for real-world use,
543 lacking both functionality and performance. However, they are useful
544 in getting a feel for how RCU works. See kernel/rcupdate.c for a
545 production-quality implementation, and see:
547 http://www.rdrop.com/users/paulmck/RCU
549 for papers describing the Linux kernel RCU implementation. The OLS'01
550 and OLS'02 papers are a good introduction, and the dissertation provides
551 more details on the current implementation as of early 2004.
554 5A. "TOY" IMPLEMENTATION #1: LOCKING
556 This section presents a "toy" RCU implementation that is based on
557 familiar locking primitives. Its overhead makes it a non-starter for
558 real-life use, as does its lack of scalability. It is also unsuitable
559 for realtime use, since it allows scheduling latency to "bleed" from
560 one read-side critical section to another.
562 However, it is probably the easiest implementation to relate to, so is
563 a good starting point.
565 It is extremely simple:
567 static DEFINE_RWLOCK(rcu_gp_mutex);
569 void rcu_read_lock(void)
571 read_lock(&rcu_gp_mutex);
574 void rcu_read_unlock(void)
576 read_unlock(&rcu_gp_mutex);
579 void synchronize_rcu(void)
581 write_lock(&rcu_gp_mutex);
582 write_unlock(&rcu_gp_mutex);
585 [You can ignore rcu_assign_pointer() and rcu_dereference() without
586 missing much. But here they are anyway. And whatever you do, don't
587 forget about them when submitting patches making use of RCU!]
589 #define rcu_assign_pointer(p, v) ({ \
594 #define rcu_dereference(p) ({ \
595 typeof(p) _________p1 = p; \
596 smp_read_barrier_depends(); \
601 The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
602 and release a global reader-writer lock. The synchronize_rcu()
603 primitive write-acquires this same lock, then immediately releases
604 it. This means that once synchronize_rcu() exits, all RCU read-side
605 critical sections that were in progress before synchronize_rcu() was
606 called are guaranteed to have completed -- there is no way that
607 synchronize_rcu() would have been able to write-acquire the lock
610 It is possible to nest rcu_read_lock(), since reader-writer locks may
611 be recursively acquired. Note also that rcu_read_lock() is immune
612 from deadlock (an important property of RCU). The reason for this is
613 that the only thing that can block rcu_read_lock() is a synchronize_rcu().
614 But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
615 so there can be no deadlock cycle.
617 Quick Quiz #1: Why is this argument naive? How could a deadlock
618 occur when using this algorithm in a real-world Linux
619 kernel? How could this deadlock be avoided?
622 5B. "TOY" EXAMPLE #2: CLASSIC RCU
624 This section presents a "toy" RCU implementation that is based on
625 "classic RCU". It is also short on performance (but only for updates) and
626 on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
627 kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
628 are the same as those shown in the preceding section, so they are omitted.
630 void rcu_read_lock(void) { }
632 void rcu_read_unlock(void) { }
634 void synchronize_rcu(void)
638 for_each_possible_cpu(cpu)
642 Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
643 This is the great strength of classic RCU in a non-preemptive kernel:
644 read-side overhead is precisely zero, at least on non-Alpha CPUs.
645 And there is absolutely no way that rcu_read_lock() can possibly
646 participate in a deadlock cycle!
648 The implementation of synchronize_rcu() simply schedules itself on each
649 CPU in turn. The run_on() primitive can be implemented straightforwardly
650 in terms of the sched_setaffinity() primitive. Of course, a somewhat less
651 "toy" implementation would restore the affinity upon completion rather
652 than just leaving all tasks running on the last CPU, but when I said
653 "toy", I meant -toy-!
655 So how the heck is this supposed to work???
657 Remember that it is illegal to block while in an RCU read-side critical
658 section. Therefore, if a given CPU executes a context switch, we know
659 that it must have completed all preceding RCU read-side critical sections.
660 Once -all- CPUs have executed a context switch, then -all- preceding
661 RCU read-side critical sections will have completed.
663 So, suppose that we remove a data item from its structure and then invoke
664 synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
665 that there are no RCU read-side critical sections holding a reference
666 to that data item, so we can safely reclaim it.
668 Quick Quiz #2: Give an example where Classic RCU's read-side
669 overhead is -negative-.
671 Quick Quiz #3: If it is illegal to block in an RCU read-side
672 critical section, what the heck do you do in
673 PREEMPT_RT, where normal spinlocks can block???
676 6. ANALOGY WITH READER-WRITER LOCKING
678 Although RCU can be used in many different ways, a very common use of
679 RCU is analogous to reader-writer locking. The following unified
680 diff shows how closely related RCU and reader-writer locking can be.
683 struct list_head *lp;
687 - list_for_each_entry(p, head, lp) {
689 + list_for_each_entry_rcu(p, head, lp) {
706 - write_lock(&listmutex);
707 + spin_lock(&listmutex);
708 list_for_each_entry(p, head, lp) {
710 - list_del(&p->list);
711 - write_unlock(&listmutex);
712 + list_del_rcu(&p->list);
713 + spin_unlock(&listmutex);
719 - write_unlock(&listmutex);
720 + spin_unlock(&listmutex);
724 Or, for those who prefer a side-by-side listing:
726 1 struct el { 1 struct el {
727 2 struct list_head list; 2 struct list_head list;
728 3 long key; 3 long key;
729 4 spinlock_t mutex; 4 spinlock_t mutex;
730 5 int data; 5 int data;
731 6 /* Other data fields */ 6 /* Other data fields */
733 8 spinlock_t listmutex; 8 spinlock_t listmutex;
734 9 struct el head; 9 struct el head;
736 1 int search(long key, int *result) 1 int search(long key, int *result)
738 3 struct list_head *lp; 3 struct list_head *lp;
739 4 struct el *p; 4 struct el *p;
741 6 read_lock(); 6 rcu_read_lock();
742 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
743 8 if (p->key == key) { 8 if (p->key == key) {
744 9 *result = p->data; 9 *result = p->data;
745 10 read_unlock(); 10 rcu_read_unlock();
746 11 return 1; 11 return 1;
749 14 read_unlock(); 14 rcu_read_unlock();
750 15 return 0; 15 return 0;
753 1 int delete(long key) 1 int delete(long key)
755 3 struct el *p; 3 struct el *p;
757 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
758 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
759 7 if (p->key == key) { 7 if (p->key == key) {
760 8 list_del(&p->list); 8 list_del_rcu(&p->list);
761 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
762 10 synchronize_rcu();
763 10 kfree(p); 11 kfree(p);
764 11 return 1; 12 return 1;
767 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
768 15 return 0; 16 return 0;
771 Either way, the differences are quite small. Read-side locking moves
772 to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
773 a reader-writer lock to a simple spinlock, and a synchronize_rcu()
774 precedes the kfree().
776 However, there is one potential catch: the read-side and update-side
777 critical sections can now run concurrently. In many cases, this will
778 not be a problem, but it is necessary to check carefully regardless.
779 For example, if multiple independent list updates must be seen as
780 a single atomic update, converting to RCU will require special care.
782 Also, the presence of synchronize_rcu() means that the RCU version of
783 delete() can now block. If this is a problem, there is a callback-based
784 mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
785 be used in place of synchronize_rcu().
788 7. FULL LIST OF RCU APIs
790 The RCU APIs are documented in docbook-format header comments in the
791 Linux-kernel source code, but it helps to have a full list of the
792 APIs, since there does not appear to be a way to categorize them
793 in docbook. Here is the list, by category.
797 list_for_each_entry_rcu
798 hlist_for_each_entry_rcu
799 hlist_nulls_for_each_entry_rcu
800 list_for_each_entry_continue_rcu
802 RCU pointer/list update:
814 list_splice_init_rcu()
816 RCU: Critical sections Grace period Barrier
818 rcu_read_lock synchronize_net rcu_barrier
819 rcu_read_unlock synchronize_rcu
820 rcu_dereference synchronize_rcu_expedited
825 bh: Critical sections Grace period Barrier
827 rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
828 rcu_read_unlock_bh synchronize_rcu_bh
829 rcu_dereference_bh synchronize_rcu_bh_expedited
832 sched: Critical sections Grace period Barrier
834 rcu_read_lock_sched synchronize_sched rcu_barrier_sched
835 rcu_read_unlock_sched call_rcu_sched
836 [preempt_disable] synchronize_sched_expedited
838 rcu_dereference_sched
841 SRCU: Critical sections Grace period Barrier
843 srcu_read_lock synchronize_srcu srcu_barrier
844 srcu_read_unlock call_srcu
845 srcu_read_lock_raw synchronize_srcu_expedited
849 SRCU: Initialization/cleanup
853 All: lockdep-checked RCU-protected pointer access
855 rcu_dereference_check
856 rcu_dereference_protected
859 See the comment headers in the source code (or the docbook generated
860 from them) for more information.
862 However, given that there are no fewer than four families of RCU APIs
863 in the Linux kernel, how do you choose which one to use? The following
866 a. Will readers need to block? If so, you need SRCU.
868 b. Is it necessary to start a read-side critical section in a
869 hardirq handler or exception handler, and then to complete
870 this read-side critical section in the task that was
871 interrupted? If so, you need SRCU's srcu_read_lock_raw() and
872 srcu_read_unlock_raw() primitives.
874 c. What about the -rt patchset? If readers would need to block
875 in an non-rt kernel, you need SRCU. If readers would block
876 in a -rt kernel, but not in a non-rt kernel, SRCU is not
879 d. Do you need to treat NMI handlers, hardirq handlers,
880 and code segments with preemption disabled (whether
881 via preempt_disable(), local_irq_save(), local_bh_disable(),
882 or some other mechanism) as if they were explicit RCU readers?
883 If so, RCU-sched is the only choice that will work for you.
885 e. Do you need RCU grace periods to complete even in the face
886 of softirq monopolization of one or more of the CPUs? For
887 example, is your code subject to network-based denial-of-service
888 attacks? If so, you need RCU-bh.
890 f. Is your workload too update-intensive for normal use of
891 RCU, but inappropriate for other synchronization mechanisms?
892 If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
894 g. Do you need read-side critical sections that are respected
895 even though they are in the middle of the idle loop, during
896 user-mode execution, or on an offlined CPU? If so, SRCU is the
897 only choice that will work for you.
899 h. Otherwise, use RCU.
901 Of course, this all assumes that you have determined that RCU is in fact
902 the right tool for your job.
905 8. ANSWERS TO QUICK QUIZZES
907 Quick Quiz #1: Why is this argument naive? How could a deadlock
908 occur when using this algorithm in a real-world Linux
909 kernel? [Referring to the lock-based "toy" RCU
912 Answer: Consider the following sequence of events:
914 1. CPU 0 acquires some unrelated lock, call it
915 "problematic_lock", disabling irq via
918 2. CPU 1 enters synchronize_rcu(), write-acquiring
921 3. CPU 0 enters rcu_read_lock(), but must wait
922 because CPU 1 holds rcu_gp_mutex.
924 4. CPU 1 is interrupted, and the irq handler
925 attempts to acquire problematic_lock.
927 The system is now deadlocked.
929 One way to avoid this deadlock is to use an approach like
930 that of CONFIG_PREEMPT_RT, where all normal spinlocks
931 become blocking locks, and all irq handlers execute in
932 the context of special tasks. In this case, in step 4
933 above, the irq handler would block, allowing CPU 1 to
934 release rcu_gp_mutex, avoiding the deadlock.
936 Even in the absence of deadlock, this RCU implementation
937 allows latency to "bleed" from readers to other
938 readers through synchronize_rcu(). To see this,
939 consider task A in an RCU read-side critical section
940 (thus read-holding rcu_gp_mutex), task B blocked
941 attempting to write-acquire rcu_gp_mutex, and
942 task C blocked in rcu_read_lock() attempting to
943 read_acquire rcu_gp_mutex. Task A's RCU read-side
944 latency is holding up task C, albeit indirectly via
947 Realtime RCU implementations therefore use a counter-based
948 approach where tasks in RCU read-side critical sections
949 cannot be blocked by tasks executing synchronize_rcu().
951 Quick Quiz #2: Give an example where Classic RCU's read-side
952 overhead is -negative-.
954 Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
955 kernel where a routing table is used by process-context
956 code, but can be updated by irq-context code (for example,
957 by an "ICMP REDIRECT" packet). The usual way of handling
958 this would be to have the process-context code disable
959 interrupts while searching the routing table. Use of
960 RCU allows such interrupt-disabling to be dispensed with.
961 Thus, without RCU, you pay the cost of disabling interrupts,
962 and with RCU you don't.
964 One can argue that the overhead of RCU in this
965 case is negative with respect to the single-CPU
966 interrupt-disabling approach. Others might argue that
967 the overhead of RCU is merely zero, and that replacing
968 the positive overhead of the interrupt-disabling scheme
969 with the zero-overhead RCU scheme does not constitute
972 In real life, of course, things are more complex. But
973 even the theoretical possibility of negative overhead for
974 a synchronization primitive is a bit unexpected. ;-)
976 Quick Quiz #3: If it is illegal to block in an RCU read-side
977 critical section, what the heck do you do in
978 PREEMPT_RT, where normal spinlocks can block???
980 Answer: Just as PREEMPT_RT permits preemption of spinlock
981 critical sections, it permits preemption of RCU
982 read-side critical sections. It also permits
983 spinlocks blocking while in RCU read-side critical
986 Why the apparent inconsistency? Because it is it
987 possible to use priority boosting to keep the RCU
988 grace periods short if need be (for example, if running
989 short of memory). In contrast, if blocking waiting
990 for (say) network reception, there is no way to know
991 what should be boosted. Especially given that the
992 process we need to boost might well be a human being
993 who just went out for a pizza or something. And although
994 a computer-operated cattle prod might arouse serious
995 interest, it might also provoke serious objections.
996 Besides, how does the computer know what pizza parlor
997 the human being went to???
1002 My thanks to the people who helped make this human-readable, including
1003 Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1006 For more information, see http://www.rdrop.com/users/paulmck/RCU.