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1Please note that the "What is RCU?" LWN series is an excellent place
2to start learning about RCU:
3
41. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
52. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
63. RCU part 3: the RCU API http://lwn.net/Articles/264090/
74. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
8
9
10What is RCU?
11
12RCU is a synchronization mechanism that was added to the Linux kernel
13during the 2.5 development effort that is optimized for read-mostly
14situations. Although RCU is actually quite simple once you understand it,
15getting there can sometimes be a challenge. Part of the problem is that
16most of the past descriptions of RCU have been written with the mistaken
17assumption that there is "one true way" to describe RCU. Instead,
18the experience has been that different people must take different paths
19to arrive at an understanding of RCU. This document provides several
20different paths, as follows:
21
221. RCU OVERVIEW
232. WHAT IS RCU'S CORE API?
243. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
254. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
265. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
276. ANALOGY WITH READER-WRITER LOCKING
287. FULL LIST OF RCU APIs
298. ANSWERS TO QUICK QUIZZES
30
31People who prefer starting with a conceptual overview should focus on
32Section 1, though most readers will profit by reading this section at
33some point. People who prefer to start with an API that they can then
34experiment with should focus on Section 2. People who prefer to start
35with example uses should focus on Sections 3 and 4. People who need to
36understand the RCU implementation should focus on Section 5, then dive
37into the kernel source code. People who reason best by analogy should
38focus on Section 6. Section 7 serves as an index to the docbook API
39documentation, and Section 8 is the traditional answer key.
40
41So, start with the section that makes the most sense to you and your
42preferred method of learning. If you need to know everything about
43everything, feel free to read the whole thing -- but if you are really
44that type of person, you have perused the source code and will therefore
45never need this document anyway. ;-)
46
47
481. RCU OVERVIEW
49
50The basic idea behind RCU is to split updates into "removal" and
51"reclamation" phases. The removal phase removes references to data items
52within a data structure (possibly by replacing them with references to
53new versions of these data items), and can run concurrently with readers.
54The reason that it is safe to run the removal phase concurrently with
55readers is the semantics of modern CPUs guarantee that readers will see
56either the old or the new version of the data structure rather than a
57partially updated reference. The reclamation phase does the work of reclaiming
58(e.g., freeing) the data items removed from the data structure during the
59removal phase. Because reclaiming data items can disrupt any readers
60concurrently referencing those data items, the reclamation phase must
61not start until readers no longer hold references to those data items.
62
63Splitting the update into removal and reclamation phases permits the
64updater to perform the removal phase immediately, and to defer the
65reclamation phase until all readers active during the removal phase have
66completed, either by blocking until they finish or by registering a
67callback that is invoked after they finish. Only readers that are active
68during the removal phase need be considered, because any reader starting
69after the removal phase will be unable to gain a reference to the removed
70data items, and therefore cannot be disrupted by the reclamation phase.
71
72So the typical RCU update sequence goes something like the following:
73
74a. Remove pointers to a data structure, so that subsequent
75 readers cannot gain a reference to it.
76
77b. Wait for all previous readers to complete their RCU read-side
78 critical sections.
79
80c. At this point, there cannot be any readers who hold references
81 to the data structure, so it now may safely be reclaimed
82 (e.g., kfree()d).
83
84Step (b) above is the key idea underlying RCU's deferred destruction.
85The ability to wait until all readers are done allows RCU readers to
86use much lighter-weight synchronization, in some cases, absolutely no
87synchronization at all. In contrast, in more conventional lock-based
88schemes, readers must use heavy-weight synchronization in order to
89prevent an updater from deleting the data structure out from under them.
90This is because lock-based updaters typically update data items in place,
91and must therefore exclude readers. In contrast, RCU-based updaters
92typically take advantage of the fact that writes to single aligned
93pointers are atomic on modern CPUs, allowing atomic insertion, removal,
94and replacement of data items in a linked structure without disrupting
95readers. Concurrent RCU readers can then continue accessing the old
96versions, and can dispense with the atomic operations, memory barriers,
97and communications cache misses that are so expensive on present-day
98SMP computer systems, even in absence of lock contention.
99
100In the three-step procedure shown above, the updater is performing both
101the removal and the reclamation step, but it is often helpful for an
102entirely different thread to do the reclamation, as is in fact the case
103in the Linux kernel's directory-entry cache (dcache). Even if the same
104thread performs both the update step (step (a) above) and the reclamation
105step (step (c) above), it is often helpful to think of them separately.
106For example, RCU readers and updaters need not communicate at all,
107but RCU provides implicit low-overhead communication between readers
108and reclaimers, namely, in step (b) above.
109
110So how the heck can a reclaimer tell when a reader is done, given
111that readers are not doing any sort of synchronization operations???
112Read on to learn about how RCU's API makes this easy.
113
114
1152. WHAT IS RCU'S CORE API?
116
117The core RCU API is quite small:
118
119a. rcu_read_lock()
120b. rcu_read_unlock()
121c. synchronize_rcu() / call_rcu()
122d. rcu_assign_pointer()
123e. rcu_dereference()
124
125There are many other members of the RCU API, but the rest can be
126expressed in terms of these five, though most implementations instead
127express synchronize_rcu() in terms of the call_rcu() callback API.
128
129The five core RCU APIs are described below, the other 18 will be enumerated
130later. See the kernel docbook documentation for more info, or look directly
131at the function header comments.
132
133rcu_read_lock()
134
135 void rcu_read_lock(void);
136
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_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.
146
147rcu_read_unlock()
148
149 void rcu_read_unlock(void);
150
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.
154
155synchronize_rcu()
156
157 void synchronize_rcu(void);
158
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:
165
166 CPU 0 CPU 1 CPU 2
167 ----------------- ------------------------- ---------------
168 1. rcu_read_lock()
169 2. enters synchronize_rcu()
170 3. rcu_read_lock()
171 4. rcu_read_unlock()
172 5. exits synchronize_rcu()
173 6. rcu_read_unlock()
174
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.
178
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().
185
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.
190
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.
198
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.
207
208rcu_assign_pointer()
209
210 typeof(p) rcu_assign_pointer(p, typeof(p) v);
211
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.)
215
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.
221
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().
227
228rcu_dereference()
229
230 typeof(p) rcu_dereference(p);
231
232 Like rcu_assign_pointer(), rcu_dereference() must be implemented
233 as a macro.
234
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.
243
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:
247
248 p = rcu_dereference(head.next);
249 return p->data;
250
251 However, in this case, one could just as easily combine these
252 into one statement:
253
254 return rcu_dereference(head.next)->data;
255
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, do not guarantee that the same pointer will be returned
260 if an update happened while in the critical section, and incur
261 unnecessary overhead on Alpha CPUs.
262
263 Note that the value returned by rcu_dereference() is valid
264 only within the enclosing RCU read-side critical section.
265 For example, the following is -not- legal:
266
267 rcu_read_lock();
268 p = rcu_dereference(head.next);
269 rcu_read_unlock();
270 x = p->address; /* BUG!!! */
271 rcu_read_lock();
272 y = p->data; /* BUG!!! */
273 rcu_read_unlock();
274
275 Holding a reference from one RCU read-side critical section
276 to another is just as illegal as holding a reference from
277 one lock-based critical section to another! Similarly,
278 using a reference outside of the critical section in which
279 it was acquired is just as illegal as doing so with normal
280 locking.
281
282 As with rcu_assign_pointer(), an important function of
283 rcu_dereference() is to document which pointers are protected by
284 RCU, in particular, flagging a pointer that is subject to changing
285 at any time, including immediately after the rcu_dereference().
286 And, again like rcu_assign_pointer(), rcu_dereference() is
287 typically used indirectly, via the _rcu list-manipulation
288 primitives, such as list_for_each_entry_rcu().
289
290The following diagram shows how each API communicates among the
291reader, updater, and reclaimer.
292
293
294 rcu_assign_pointer()
295 +--------+
296 +---------------------->| reader |---------+
297 | +--------+ |
298 | | |
299 | | | Protect:
300 | | | rcu_read_lock()
301 | | | rcu_read_unlock()
302 | rcu_dereference() | |
303 +---------+ | |
304 | updater |<---------------------+ |
305 +---------+ V
306 | +-----------+
307 +----------------------------------->| reclaimer |
308 +-----------+
309 Defer:
310 synchronize_rcu() & call_rcu()
311
312
313The RCU infrastructure observes the time sequence of rcu_read_lock(),
314rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
315order to determine when (1) synchronize_rcu() invocations may return
316to their callers and (2) call_rcu() callbacks may be invoked. Efficient
317implementations of the RCU infrastructure make heavy use of batching in
318order to amortize their overhead over many uses of the corresponding APIs.
319
320There are no fewer than three RCU mechanisms in the Linux kernel; the
321diagram above shows the first one, which is by far the most commonly used.
322The rcu_dereference() and rcu_assign_pointer() primitives are used for
323all three mechanisms, but different defer and protect primitives are
324used as follows:
325
326 Defer Protect
327
328a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
329 call_rcu() rcu_dereference()
330
331b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
332 call_rcu_bh() rcu_dereference_bh()
333
334c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
335 call_rcu_sched() preempt_disable() / preempt_enable()
336 local_irq_save() / local_irq_restore()
337 hardirq enter / hardirq exit
338 NMI enter / NMI exit
339 rcu_dereference_sched()
340
341These three mechanisms are used as follows:
342
343a. RCU applied to normal data structures.
344
345b. RCU applied to networking data structures that may be subjected
346 to remote denial-of-service attacks.
347
348c. RCU applied to scheduler and interrupt/NMI-handler tasks.
349
350Again, most uses will be of (a). The (b) and (c) cases are important
351for specialized uses, but are relatively uncommon.
352
353
3543. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
355
356This section shows a simple use of the core RCU API to protect a
357global pointer to a dynamically allocated structure. More-typical
358uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
359
360 struct foo {
361 int a;
362 char b;
363 long c;
364 };
365 DEFINE_SPINLOCK(foo_mutex);
366
367 struct foo __rcu *gbl_foo;
368
369 /*
370 * Create a new struct foo that is the same as the one currently
371 * pointed to by gbl_foo, except that field "a" is replaced
372 * with "new_a". Points gbl_foo to the new structure, and
373 * frees up the old structure after a grace period.
374 *
375 * Uses rcu_assign_pointer() to ensure that concurrent readers
376 * see the initialized version of the new structure.
377 *
378 * Uses synchronize_rcu() to ensure that any readers that might
379 * have references to the old structure complete before freeing
380 * the old structure.
381 */
382 void foo_update_a(int new_a)
383 {
384 struct foo *new_fp;
385 struct foo *old_fp;
386
387 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
388 spin_lock(&foo_mutex);
389 old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
390 *new_fp = *old_fp;
391 new_fp->a = new_a;
392 rcu_assign_pointer(gbl_foo, new_fp);
393 spin_unlock(&foo_mutex);
394 synchronize_rcu();
395 kfree(old_fp);
396 }
397
398 /*
399 * Return the value of field "a" of the current gbl_foo
400 * structure. Use rcu_read_lock() and rcu_read_unlock()
401 * to ensure that the structure does not get deleted out
402 * from under us, and use rcu_dereference() to ensure that
403 * we see the initialized version of the structure (important
404 * for DEC Alpha and for people reading the code).
405 */
406 int foo_get_a(void)
407 {
408 int retval;
409
410 rcu_read_lock();
411 retval = rcu_dereference(gbl_foo)->a;
412 rcu_read_unlock();
413 return retval;
414 }
415
416So, to sum up:
417
418o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
419 read-side critical sections.
420
421o Within an RCU read-side critical section, use rcu_dereference()
422 to dereference RCU-protected pointers.
423
424o Use some solid scheme (such as locks or semaphores) to
425 keep concurrent updates from interfering with each other.
426
427o Use rcu_assign_pointer() to update an RCU-protected pointer.
428 This primitive protects concurrent readers from the updater,
429 -not- concurrent updates from each other! You therefore still
430 need to use locking (or something similar) to keep concurrent
431 rcu_assign_pointer() primitives from interfering with each other.
432
433o Use synchronize_rcu() -after- removing a data element from an
434 RCU-protected data structure, but -before- reclaiming/freeing
435 the data element, in order to wait for the completion of all
436 RCU read-side critical sections that might be referencing that
437 data item.
438
439See checklist.txt for additional rules to follow when using RCU.
440And again, more-typical uses of RCU may be found in listRCU.txt,
441arrayRCU.txt, and NMI-RCU.txt.
442
443
4444. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
445
446In the example above, foo_update_a() blocks until a grace period elapses.
447This is quite simple, but in some cases one cannot afford to wait so
448long -- there might be other high-priority work to be done.
449
450In such cases, one uses call_rcu() rather than synchronize_rcu().
451The call_rcu() API is as follows:
452
453 void call_rcu(struct rcu_head * head,
454 void (*func)(struct rcu_head *head));
455
456This function invokes func(head) after a grace period has elapsed.
457This invocation might happen from either softirq or process context,
458so the function is not permitted to block. The foo struct needs to
459have an rcu_head structure added, perhaps as follows:
460
461 struct foo {
462 int a;
463 char b;
464 long c;
465 struct rcu_head rcu;
466 };
467
468The foo_update_a() function might then be written as follows:
469
470 /*
471 * Create a new struct foo that is the same as the one currently
472 * pointed to by gbl_foo, except that field "a" is replaced
473 * with "new_a". Points gbl_foo to the new structure, and
474 * frees up the old structure after a grace period.
475 *
476 * Uses rcu_assign_pointer() to ensure that concurrent readers
477 * see the initialized version of the new structure.
478 *
479 * Uses call_rcu() to ensure that any readers that might have
480 * references to the old structure complete before freeing the
481 * old structure.
482 */
483 void foo_update_a(int new_a)
484 {
485 struct foo *new_fp;
486 struct foo *old_fp;
487
488 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
489 spin_lock(&foo_mutex);
490 old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
491 *new_fp = *old_fp;
492 new_fp->a = new_a;
493 rcu_assign_pointer(gbl_foo, new_fp);
494 spin_unlock(&foo_mutex);
495 call_rcu(&old_fp->rcu, foo_reclaim);
496 }
497
498The foo_reclaim() function might appear as follows:
499
500 void foo_reclaim(struct rcu_head *rp)
501 {
502 struct foo *fp = container_of(rp, struct foo, rcu);
503
504 foo_cleanup(fp->a);
505
506 kfree(fp);
507 }
508
509The container_of() primitive is a macro that, given a pointer into a
510struct, the type of the struct, and the pointed-to field within the
511struct, returns a pointer to the beginning of the struct.
512
513The use of call_rcu() permits the caller of foo_update_a() to
514immediately regain control, without needing to worry further about the
515old version of the newly updated element. It also clearly shows the
516RCU distinction between updater, namely foo_update_a(), and reclaimer,
517namely foo_reclaim().
518
519The summary of advice is the same as for the previous section, except
520that we are now using call_rcu() rather than synchronize_rcu():
521
522o Use call_rcu() -after- removing a data element from an
523 RCU-protected data structure in order to register a callback
524 function that will be invoked after the completion of all RCU
525 read-side critical sections that might be referencing that
526 data item.
527
528If the callback for call_rcu() is not doing anything more than calling
529kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
530to avoid having to write your own callback:
531
532 kfree_rcu(old_fp, rcu);
533
534Again, see checklist.txt for additional rules governing the use of RCU.
535
536
5375. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
538
539One of the nice things about RCU is that it has extremely simple "toy"
540implementations that are a good first step towards understanding the
541production-quality implementations in the Linux kernel. This section
542presents two such "toy" implementations of RCU, one that is implemented
543in terms of familiar locking primitives, and another that more closely
544resembles "classic" RCU. Both are way too simple for real-world use,
545lacking both functionality and performance. However, they are useful
546in getting a feel for how RCU works. See kernel/rcupdate.c for a
547production-quality implementation, and see:
548
549 http://www.rdrop.com/users/paulmck/RCU
550
551for papers describing the Linux kernel RCU implementation. The OLS'01
552and OLS'02 papers are a good introduction, and the dissertation provides
553more details on the current implementation as of early 2004.
554
555
5565A. "TOY" IMPLEMENTATION #1: LOCKING
557
558This section presents a "toy" RCU implementation that is based on
559familiar locking primitives. Its overhead makes it a non-starter for
560real-life use, as does its lack of scalability. It is also unsuitable
561for realtime use, since it allows scheduling latency to "bleed" from
562one read-side critical section to another.
563
564However, it is probably the easiest implementation to relate to, so is
565a good starting point.
566
567It is extremely simple:
568
569 static DEFINE_RWLOCK(rcu_gp_mutex);
570
571 void rcu_read_lock(void)
572 {
573 read_lock(&rcu_gp_mutex);
574 }
575
576 void rcu_read_unlock(void)
577 {
578 read_unlock(&rcu_gp_mutex);
579 }
580
581 void synchronize_rcu(void)
582 {
583 write_lock(&rcu_gp_mutex);
584 write_unlock(&rcu_gp_mutex);
585 }
586
587[You can ignore rcu_assign_pointer() and rcu_dereference() without
588missing much. But here they are anyway. And whatever you do, don't
589forget about them when submitting patches making use of RCU!]
590
591 #define rcu_assign_pointer(p, v) ({ \
592 smp_wmb(); \
593 (p) = (v); \
594 })
595
596 #define rcu_dereference(p) ({ \
597 typeof(p) _________p1 = p; \
598 smp_read_barrier_depends(); \
599 (_________p1); \
600 })
601
602
603The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
604and release a global reader-writer lock. The synchronize_rcu()
605primitive write-acquires this same lock, then immediately releases
606it. This means that once synchronize_rcu() exits, all RCU read-side
607critical sections that were in progress before synchronize_rcu() was
608called are guaranteed to have completed -- there is no way that
609synchronize_rcu() would have been able to write-acquire the lock
610otherwise.
611
612It is possible to nest rcu_read_lock(), since reader-writer locks may
613be recursively acquired. Note also that rcu_read_lock() is immune
614from deadlock (an important property of RCU). The reason for this is
615that the only thing that can block rcu_read_lock() is a synchronize_rcu().
616But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
617so there can be no deadlock cycle.
618
619Quick Quiz #1: Why is this argument naive? How could a deadlock
620 occur when using this algorithm in a real-world Linux
621 kernel? How could this deadlock be avoided?
622
623
6245B. "TOY" EXAMPLE #2: CLASSIC RCU
625
626This section presents a "toy" RCU implementation that is based on
627"classic RCU". It is also short on performance (but only for updates) and
628on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
629kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
630are the same as those shown in the preceding section, so they are omitted.
631
632 void rcu_read_lock(void) { }
633
634 void rcu_read_unlock(void) { }
635
636 void synchronize_rcu(void)
637 {
638 int cpu;
639
640 for_each_possible_cpu(cpu)
641 run_on(cpu);
642 }
643
644Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
645This is the great strength of classic RCU in a non-preemptive kernel:
646read-side overhead is precisely zero, at least on non-Alpha CPUs.
647And there is absolutely no way that rcu_read_lock() can possibly
648participate in a deadlock cycle!
649
650The implementation of synchronize_rcu() simply schedules itself on each
651CPU in turn. The run_on() primitive can be implemented straightforwardly
652in terms of the sched_setaffinity() primitive. Of course, a somewhat less
653"toy" implementation would restore the affinity upon completion rather
654than just leaving all tasks running on the last CPU, but when I said
655"toy", I meant -toy-!
656
657So how the heck is this supposed to work???
658
659Remember that it is illegal to block while in an RCU read-side critical
660section. Therefore, if a given CPU executes a context switch, we know
661that it must have completed all preceding RCU read-side critical sections.
662Once -all- CPUs have executed a context switch, then -all- preceding
663RCU read-side critical sections will have completed.
664
665So, suppose that we remove a data item from its structure and then invoke
666synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
667that there are no RCU read-side critical sections holding a reference
668to that data item, so we can safely reclaim it.
669
670Quick Quiz #2: Give an example where Classic RCU's read-side
671 overhead is -negative-.
672
673Quick Quiz #3: If it is illegal to block in an RCU read-side
674 critical section, what the heck do you do in
675 PREEMPT_RT, where normal spinlocks can block???
676
677
6786. ANALOGY WITH READER-WRITER LOCKING
679
680Although RCU can be used in many different ways, a very common use of
681RCU is analogous to reader-writer locking. The following unified
682diff shows how closely related RCU and reader-writer locking can be.
683
684 @@ -13,15 +14,15 @@
685 struct list_head *lp;
686 struct el *p;
687
688 - read_lock();
689 - list_for_each_entry(p, head, lp) {
690 + rcu_read_lock();
691 + list_for_each_entry_rcu(p, head, lp) {
692 if (p->key == key) {
693 *result = p->data;
694 - read_unlock();
695 + rcu_read_unlock();
696 return 1;
697 }
698 }
699 - read_unlock();
700 + rcu_read_unlock();
701 return 0;
702 }
703
704 @@ -29,15 +30,16 @@
705 {
706 struct el *p;
707
708 - write_lock(&listmutex);
709 + spin_lock(&listmutex);
710 list_for_each_entry(p, head, lp) {
711 if (p->key == key) {
712 - list_del(&p->list);
713 - write_unlock(&listmutex);
714 + list_del_rcu(&p->list);
715 + spin_unlock(&listmutex);
716 + synchronize_rcu();
717 kfree(p);
718 return 1;
719 }
720 }
721 - write_unlock(&listmutex);
722 + spin_unlock(&listmutex);
723 return 0;
724 }
725
726Or, for those who prefer a side-by-side listing:
727
728 1 struct el { 1 struct el {
729 2 struct list_head list; 2 struct list_head list;
730 3 long key; 3 long key;
731 4 spinlock_t mutex; 4 spinlock_t mutex;
732 5 int data; 5 int data;
733 6 /* Other data fields */ 6 /* Other data fields */
734 7 }; 7 };
735 8 spinlock_t listmutex; 8 spinlock_t listmutex;
736 9 struct el head; 9 struct el head;
737
738 1 int search(long key, int *result) 1 int search(long key, int *result)
739 2 { 2 {
740 3 struct list_head *lp; 3 struct list_head *lp;
741 4 struct el *p; 4 struct el *p;
742 5 5
743 6 read_lock(); 6 rcu_read_lock();
744 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
745 8 if (p->key == key) { 8 if (p->key == key) {
746 9 *result = p->data; 9 *result = p->data;
74710 read_unlock(); 10 rcu_read_unlock();
74811 return 1; 11 return 1;
74912 } 12 }
75013 } 13 }
75114 read_unlock(); 14 rcu_read_unlock();
75215 return 0; 15 return 0;
75316 } 16 }
754
755 1 int delete(long key) 1 int delete(long key)
756 2 { 2 {
757 3 struct el *p; 3 struct el *p;
758 4 4
759 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
760 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
761 7 if (p->key == key) { 7 if (p->key == key) {
762 8 list_del(&p->list); 8 list_del_rcu(&p->list);
763 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
764 10 synchronize_rcu();
76510 kfree(p); 11 kfree(p);
76611 return 1; 12 return 1;
76712 } 13 }
76813 } 14 }
76914 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
77015 return 0; 16 return 0;
77116 } 17 }
772
773Either way, the differences are quite small. Read-side locking moves
774to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
775a reader-writer lock to a simple spinlock, and a synchronize_rcu()
776precedes the kfree().
777
778However, there is one potential catch: the read-side and update-side
779critical sections can now run concurrently. In many cases, this will
780not be a problem, but it is necessary to check carefully regardless.
781For example, if multiple independent list updates must be seen as
782a single atomic update, converting to RCU will require special care.
783
784Also, the presence of synchronize_rcu() means that the RCU version of
785delete() can now block. If this is a problem, there is a callback-based
786mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
787be used in place of synchronize_rcu().
788
789
7907. FULL LIST OF RCU APIs
791
792The RCU APIs are documented in docbook-format header comments in the
793Linux-kernel source code, but it helps to have a full list of the
794APIs, since there does not appear to be a way to categorize them
795in docbook. Here is the list, by category.
796
797RCU list traversal:
798
799 list_entry_rcu
800 list_first_entry_rcu
801 list_next_rcu
802 list_for_each_entry_rcu
803 list_for_each_entry_continue_rcu
804 hlist_first_rcu
805 hlist_next_rcu
806 hlist_pprev_rcu
807 hlist_for_each_entry_rcu
808 hlist_for_each_entry_rcu_bh
809 hlist_for_each_entry_continue_rcu
810 hlist_for_each_entry_continue_rcu_bh
811 hlist_nulls_first_rcu
812 hlist_nulls_for_each_entry_rcu
813 hlist_bl_first_rcu
814 hlist_bl_for_each_entry_rcu
815
816RCU pointer/list update:
817
818 rcu_assign_pointer
819 list_add_rcu
820 list_add_tail_rcu
821 list_del_rcu
822 list_replace_rcu
823 hlist_add_behind_rcu
824 hlist_add_before_rcu
825 hlist_add_head_rcu
826 hlist_del_rcu
827 hlist_del_init_rcu
828 hlist_replace_rcu
829 list_splice_init_rcu()
830 hlist_nulls_del_init_rcu
831 hlist_nulls_del_rcu
832 hlist_nulls_add_head_rcu
833 hlist_bl_add_head_rcu
834 hlist_bl_del_init_rcu
835 hlist_bl_del_rcu
836 hlist_bl_set_first_rcu
837
838RCU: Critical sections Grace period Barrier
839
840 rcu_read_lock synchronize_net rcu_barrier
841 rcu_read_unlock synchronize_rcu
842 rcu_dereference synchronize_rcu_expedited
843 rcu_read_lock_held call_rcu
844 rcu_dereference_check kfree_rcu
845 rcu_dereference_protected
846
847bh: Critical sections Grace period Barrier
848
849 rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
850 rcu_read_unlock_bh synchronize_rcu_bh
851 rcu_dereference_bh synchronize_rcu_bh_expedited
852 rcu_dereference_bh_check
853 rcu_dereference_bh_protected
854 rcu_read_lock_bh_held
855
856sched: Critical sections Grace period Barrier
857
858 rcu_read_lock_sched synchronize_sched rcu_barrier_sched
859 rcu_read_unlock_sched call_rcu_sched
860 [preempt_disable] synchronize_sched_expedited
861 [and friends]
862 rcu_read_lock_sched_notrace
863 rcu_read_unlock_sched_notrace
864 rcu_dereference_sched
865 rcu_dereference_sched_check
866 rcu_dereference_sched_protected
867 rcu_read_lock_sched_held
868
869
870SRCU: Critical sections Grace period Barrier
871
872 srcu_read_lock synchronize_srcu srcu_barrier
873 srcu_read_unlock call_srcu
874 srcu_dereference synchronize_srcu_expedited
875 srcu_dereference_check
876 srcu_read_lock_held
877
878SRCU: Initialization/cleanup
879 init_srcu_struct
880 cleanup_srcu_struct
881
882All: lockdep-checked RCU-protected pointer access
883
884 rcu_access_pointer
885 rcu_dereference_raw
886 RCU_LOCKDEP_WARN
887 rcu_sleep_check
888 RCU_NONIDLE
889
890See the comment headers in the source code (or the docbook generated
891from them) for more information.
892
893However, given that there are no fewer than four families of RCU APIs
894in the Linux kernel, how do you choose which one to use? The following
895list can be helpful:
896
897a. Will readers need to block? If so, you need SRCU.
898
899b. What about the -rt patchset? If readers would need to block
900 in an non-rt kernel, you need SRCU. If readers would block
901 in a -rt kernel, but not in a non-rt kernel, SRCU is not
902 necessary.
903
904c. Do you need to treat NMI handlers, hardirq handlers,
905 and code segments with preemption disabled (whether
906 via preempt_disable(), local_irq_save(), local_bh_disable(),
907 or some other mechanism) as if they were explicit RCU readers?
908 If so, RCU-sched is the only choice that will work for you.
909
910d. Do you need RCU grace periods to complete even in the face
911 of softirq monopolization of one or more of the CPUs? For
912 example, is your code subject to network-based denial-of-service
913 attacks? If so, you need RCU-bh.
914
915e. Is your workload too update-intensive for normal use of
916 RCU, but inappropriate for other synchronization mechanisms?
917 If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
918
919f. Do you need read-side critical sections that are respected
920 even though they are in the middle of the idle loop, during
921 user-mode execution, or on an offlined CPU? If so, SRCU is the
922 only choice that will work for you.
923
924g. Otherwise, use RCU.
925
926Of course, this all assumes that you have determined that RCU is in fact
927the right tool for your job.
928
929
9308. ANSWERS TO QUICK QUIZZES
931
932Quick Quiz #1: Why is this argument naive? How could a deadlock
933 occur when using this algorithm in a real-world Linux
934 kernel? [Referring to the lock-based "toy" RCU
935 algorithm.]
936
937Answer: Consider the following sequence of events:
938
939 1. CPU 0 acquires some unrelated lock, call it
940 "problematic_lock", disabling irq via
941 spin_lock_irqsave().
942
943 2. CPU 1 enters synchronize_rcu(), write-acquiring
944 rcu_gp_mutex.
945
946 3. CPU 0 enters rcu_read_lock(), but must wait
947 because CPU 1 holds rcu_gp_mutex.
948
949 4. CPU 1 is interrupted, and the irq handler
950 attempts to acquire problematic_lock.
951
952 The system is now deadlocked.
953
954 One way to avoid this deadlock is to use an approach like
955 that of CONFIG_PREEMPT_RT, where all normal spinlocks
956 become blocking locks, and all irq handlers execute in
957 the context of special tasks. In this case, in step 4
958 above, the irq handler would block, allowing CPU 1 to
959 release rcu_gp_mutex, avoiding the deadlock.
960
961 Even in the absence of deadlock, this RCU implementation
962 allows latency to "bleed" from readers to other
963 readers through synchronize_rcu(). To see this,
964 consider task A in an RCU read-side critical section
965 (thus read-holding rcu_gp_mutex), task B blocked
966 attempting to write-acquire rcu_gp_mutex, and
967 task C blocked in rcu_read_lock() attempting to
968 read_acquire rcu_gp_mutex. Task A's RCU read-side
969 latency is holding up task C, albeit indirectly via
970 task B.
971
972 Realtime RCU implementations therefore use a counter-based
973 approach where tasks in RCU read-side critical sections
974 cannot be blocked by tasks executing synchronize_rcu().
975
976Quick Quiz #2: Give an example where Classic RCU's read-side
977 overhead is -negative-.
978
979Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
980 kernel where a routing table is used by process-context
981 code, but can be updated by irq-context code (for example,
982 by an "ICMP REDIRECT" packet). The usual way of handling
983 this would be to have the process-context code disable
984 interrupts while searching the routing table. Use of
985 RCU allows such interrupt-disabling to be dispensed with.
986 Thus, without RCU, you pay the cost of disabling interrupts,
987 and with RCU you don't.
988
989 One can argue that the overhead of RCU in this
990 case is negative with respect to the single-CPU
991 interrupt-disabling approach. Others might argue that
992 the overhead of RCU is merely zero, and that replacing
993 the positive overhead of the interrupt-disabling scheme
994 with the zero-overhead RCU scheme does not constitute
995 negative overhead.
996
997 In real life, of course, things are more complex. But
998 even the theoretical possibility of negative overhead for
999 a synchronization primitive is a bit unexpected. ;-)
1000
1001Quick Quiz #3: If it is illegal to block in an RCU read-side
1002 critical section, what the heck do you do in
1003 PREEMPT_RT, where normal spinlocks can block???
1004
1005Answer: Just as PREEMPT_RT permits preemption of spinlock
1006 critical sections, it permits preemption of RCU
1007 read-side critical sections. It also permits
1008 spinlocks blocking while in RCU read-side critical
1009 sections.
1010
1011 Why the apparent inconsistency? Because it is it
1012 possible to use priority boosting to keep the RCU
1013 grace periods short if need be (for example, if running
1014 short of memory). In contrast, if blocking waiting
1015 for (say) network reception, there is no way to know
1016 what should be boosted. Especially given that the
1017 process we need to boost might well be a human being
1018 who just went out for a pizza or something. And although
1019 a computer-operated cattle prod might arouse serious
1020 interest, it might also provoke serious objections.
1021 Besides, how does the computer know what pizza parlor
1022 the human being went to???
1023
1024
1025ACKNOWLEDGEMENTS
1026
1027My thanks to the people who helped make this human-readable, including
1028Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1029
1030
1031For more information, see http://www.rdrop.com/users/paulmck/RCU.