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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/
d493011a 74. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
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8
9
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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
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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.
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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
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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.
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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
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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().
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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 and incur unnecessary overhead on Alpha CPUs.
260
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:
264
265 rcu_read_lock();
266 p = rcu_dereference(head.next);
267 rcu_read_unlock();
4357fb57 268 x = p->address; /* BUG!!! */
dd81eca8 269 rcu_read_lock();
4357fb57 270 y = p->data; /* BUG!!! */
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271 rcu_read_unlock();
272
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
278 locking.
279
280 As with rcu_assign_pointer(), an important function of
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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
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286 primitives, such as list_for_each_entry_rcu().
287
288The following diagram shows how each API communicates among the
289reader, updater, and reclaimer.
290
291
292 rcu_assign_pointer()
293 +--------+
294 +---------------------->| reader |---------+
295 | +--------+ |
296 | | |
297 | | | Protect:
298 | | | rcu_read_lock()
299 | | | rcu_read_unlock()
300 | rcu_dereference() | |
301 +---------+ | |
302 | updater |<---------------------+ |
303 +---------+ V
304 | +-----------+
305 +----------------------------------->| reclaimer |
306 +-----------+
307 Defer:
308 synchronize_rcu() & call_rcu()
309
310
311The RCU infrastructure observes the time sequence of rcu_read_lock(),
312rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
313order to determine when (1) synchronize_rcu() invocations may return
314to their callers and (2) call_rcu() callbacks may be invoked. Efficient
315implementations of the RCU infrastructure make heavy use of batching in
316order to amortize their overhead over many uses of the corresponding APIs.
317
318There are no fewer than three RCU mechanisms in the Linux kernel; the
319diagram above shows the first one, which is by far the most commonly used.
320The rcu_dereference() and rcu_assign_pointer() primitives are used for
321all three mechanisms, but different defer and protect primitives are
322used as follows:
323
324 Defer Protect
325
326a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
c598a070 327 call_rcu() rcu_dereference()
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329b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
330 call_rcu_bh() rcu_dereference_bh()
dd81eca8 331
4c54005c 332c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
d07e6d08 333 call_rcu_sched() preempt_disable() / preempt_enable()
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334 local_irq_save() / local_irq_restore()
335 hardirq enter / hardirq exit
336 NMI enter / NMI exit
c598a070 337 rcu_dereference_sched()
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338
339These three mechanisms are used as follows:
340
341a. RCU applied to normal data structures.
342
343b. RCU applied to networking data structures that may be subjected
344 to remote denial-of-service attacks.
345
346c. RCU applied to scheduler and interrupt/NMI-handler tasks.
347
348Again, most uses will be of (a). The (b) and (c) cases are important
349for specialized uses, but are relatively uncommon.
350
351
3523. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
353
354This section shows a simple use of the core RCU API to protect a
d19720a9 355global pointer to a dynamically allocated structure. More-typical
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356uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
357
358 struct foo {
359 int a;
360 char b;
361 long c;
362 };
363 DEFINE_SPINLOCK(foo_mutex);
364
365 struct foo *gbl_foo;
366
367 /*
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.
372 *
373 * Uses rcu_assign_pointer() to ensure that concurrent readers
374 * see the initialized version of the new structure.
375 *
376 * Uses synchronize_rcu() to ensure that any readers that might
377 * have references to the old structure complete before freeing
378 * the old structure.
379 */
380 void foo_update_a(int new_a)
381 {
382 struct foo *new_fp;
383 struct foo *old_fp;
384
de0dfcdf 385 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
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386 spin_lock(&foo_mutex);
387 old_fp = gbl_foo;
388 *new_fp = *old_fp;
389 new_fp->a = new_a;
390 rcu_assign_pointer(gbl_foo, new_fp);
391 spin_unlock(&foo_mutex);
392 synchronize_rcu();
393 kfree(old_fp);
394 }
395
396 /*
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).
403 */
404 int foo_get_a(void)
405 {
406 int retval;
407
408 rcu_read_lock();
409 retval = rcu_dereference(gbl_foo)->a;
410 rcu_read_unlock();
411 return retval;
412 }
413
414So, to sum up:
415
416o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
417 read-side critical sections.
418
419o Within an RCU read-side critical section, use rcu_dereference()
420 to dereference RCU-protected pointers.
421
422o Use some solid scheme (such as locks or semaphores) to
423 keep concurrent updates from interfering with each other.
424
425o 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.
430
431o 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
435 data item.
436
437See checklist.txt for additional rules to follow when using RCU.
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438And again, more-typical uses of RCU may be found in listRCU.txt,
439arrayRCU.txt, and NMI-RCU.txt.
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440
441
4424. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
443
444In the example above, foo_update_a() blocks until a grace period elapses.
445This is quite simple, but in some cases one cannot afford to wait so
446long -- there might be other high-priority work to be done.
447
448In such cases, one uses call_rcu() rather than synchronize_rcu().
449The call_rcu() API is as follows:
450
451 void call_rcu(struct rcu_head * head,
452 void (*func)(struct rcu_head *head));
453
454This function invokes func(head) after a grace period has elapsed.
455This invocation might happen from either softirq or process context,
456so the function is not permitted to block. The foo struct needs to
457have an rcu_head structure added, perhaps as follows:
458
459 struct foo {
460 int a;
461 char b;
462 long c;
463 struct rcu_head rcu;
464 };
465
466The foo_update_a() function might then be written as follows:
467
468 /*
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.
473 *
474 * Uses rcu_assign_pointer() to ensure that concurrent readers
475 * see the initialized version of the new structure.
476 *
477 * Uses call_rcu() to ensure that any readers that might have
478 * references to the old structure complete before freeing the
479 * old structure.
480 */
481 void foo_update_a(int new_a)
482 {
483 struct foo *new_fp;
484 struct foo *old_fp;
485
de0dfcdf 486 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
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487 spin_lock(&foo_mutex);
488 old_fp = gbl_foo;
489 *new_fp = *old_fp;
490 new_fp->a = new_a;
491 rcu_assign_pointer(gbl_foo, new_fp);
492 spin_unlock(&foo_mutex);
493 call_rcu(&old_fp->rcu, foo_reclaim);
494 }
495
496The foo_reclaim() function might appear as follows:
497
498 void foo_reclaim(struct rcu_head *rp)
499 {
500 struct foo *fp = container_of(rp, struct foo, rcu);
501
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502 foo_cleanup(fp->a);
503
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504 kfree(fp);
505 }
506
507The container_of() primitive is a macro that, given a pointer into a
508struct, the type of the struct, and the pointed-to field within the
509struct, returns a pointer to the beginning of the struct.
510
511The use of call_rcu() permits the caller of foo_update_a() to
512immediately regain control, without needing to worry further about the
513old version of the newly updated element. It also clearly shows the
514RCU distinction between updater, namely foo_update_a(), and reclaimer,
515namely foo_reclaim().
516
517The summary of advice is the same as for the previous section, except
518that we are now using call_rcu() rather than synchronize_rcu():
519
520o 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
524 data item.
525
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526If the callback for call_rcu() is not doing anything more than calling
527kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
528to avoid having to write your own callback:
529
530 kfree_rcu(old_fp, rcu);
531
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532Again, see checklist.txt for additional rules governing the use of RCU.
533
534
5355. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
536
537One of the nice things about RCU is that it has extremely simple "toy"
538implementations that are a good first step towards understanding the
539production-quality implementations in the Linux kernel. This section
540presents two such "toy" implementations of RCU, one that is implemented
541in terms of familiar locking primitives, and another that more closely
542resembles "classic" RCU. Both are way too simple for real-world use,
543lacking both functionality and performance. However, they are useful
544in getting a feel for how RCU works. See kernel/rcupdate.c for a
545production-quality implementation, and see:
546
547 http://www.rdrop.com/users/paulmck/RCU
548
549for papers describing the Linux kernel RCU implementation. The OLS'01
550and OLS'02 papers are a good introduction, and the dissertation provides
d19720a9 551more details on the current implementation as of early 2004.
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552
553
5545A. "TOY" IMPLEMENTATION #1: LOCKING
555
556This section presents a "toy" RCU implementation that is based on
557familiar locking primitives. Its overhead makes it a non-starter for
558real-life use, as does its lack of scalability. It is also unsuitable
559for realtime use, since it allows scheduling latency to "bleed" from
560one read-side critical section to another.
561
562However, it is probably the easiest implementation to relate to, so is
563a good starting point.
564
565It is extremely simple:
566
567 static DEFINE_RWLOCK(rcu_gp_mutex);
568
569 void rcu_read_lock(void)
570 {
571 read_lock(&rcu_gp_mutex);
572 }
573
574 void rcu_read_unlock(void)
575 {
576 read_unlock(&rcu_gp_mutex);
577 }
578
579 void synchronize_rcu(void)
580 {
581 write_lock(&rcu_gp_mutex);
582 write_unlock(&rcu_gp_mutex);
583 }
584
585[You can ignore rcu_assign_pointer() and rcu_dereference() without
586missing much. But here they are anyway. And whatever you do, don't
587forget about them when submitting patches making use of RCU!]
588
589 #define rcu_assign_pointer(p, v) ({ \
590 smp_wmb(); \
591 (p) = (v); \
592 })
593
594 #define rcu_dereference(p) ({ \
595 typeof(p) _________p1 = p; \
596 smp_read_barrier_depends(); \
597 (_________p1); \
598 })
599
600
601The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
602and release a global reader-writer lock. The synchronize_rcu()
603primitive write-acquires this same lock, then immediately releases
604it. This means that once synchronize_rcu() exits, all RCU read-side
53cb4726 605critical sections that were in progress before synchronize_rcu() was
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606called are guaranteed to have completed -- there is no way that
607synchronize_rcu() would have been able to write-acquire the lock
608otherwise.
609
610It is possible to nest rcu_read_lock(), since reader-writer locks may
611be recursively acquired. Note also that rcu_read_lock() is immune
612from deadlock (an important property of RCU). The reason for this is
613that the only thing that can block rcu_read_lock() is a synchronize_rcu().
614But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
615so there can be no deadlock cycle.
616
617Quick 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?
620
621
6225B. "TOY" EXAMPLE #2: CLASSIC RCU
623
624This section presents a "toy" RCU implementation that is based on
625"classic RCU". It is also short on performance (but only for updates) and
626on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
627kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
628are the same as those shown in the preceding section, so they are omitted.
629
630 void rcu_read_lock(void) { }
631
632 void rcu_read_unlock(void) { }
633
634 void synchronize_rcu(void)
635 {
636 int cpu;
637
3c30a752 638 for_each_possible_cpu(cpu)
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639 run_on(cpu);
640 }
641
642Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
643This is the great strength of classic RCU in a non-preemptive kernel:
644read-side overhead is precisely zero, at least on non-Alpha CPUs.
645And there is absolutely no way that rcu_read_lock() can possibly
646participate in a deadlock cycle!
647
648The implementation of synchronize_rcu() simply schedules itself on each
649CPU in turn. The run_on() primitive can be implemented straightforwardly
650in terms of the sched_setaffinity() primitive. Of course, a somewhat less
651"toy" implementation would restore the affinity upon completion rather
652than just leaving all tasks running on the last CPU, but when I said
653"toy", I meant -toy-!
654
655So how the heck is this supposed to work???
656
657Remember that it is illegal to block while in an RCU read-side critical
658section. Therefore, if a given CPU executes a context switch, we know
659that it must have completed all preceding RCU read-side critical sections.
660Once -all- CPUs have executed a context switch, then -all- preceding
661RCU read-side critical sections will have completed.
662
663So, suppose that we remove a data item from its structure and then invoke
664synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
665that there are no RCU read-side critical sections holding a reference
666to that data item, so we can safely reclaim it.
667
668Quick Quiz #2: Give an example where Classic RCU's read-side
669 overhead is -negative-.
670
671Quick 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???
674
675
6766. ANALOGY WITH READER-WRITER LOCKING
677
678Although RCU can be used in many different ways, a very common use of
679RCU is analogous to reader-writer locking. The following unified
680diff shows how closely related RCU and reader-writer locking can be.
681
682 @@ -13,15 +14,15 @@
683 struct list_head *lp;
684 struct el *p;
685
686 - read_lock();
687 - list_for_each_entry(p, head, lp) {
688 + rcu_read_lock();
689 + list_for_each_entry_rcu(p, head, lp) {
690 if (p->key == key) {
691 *result = p->data;
692 - read_unlock();
693 + rcu_read_unlock();
694 return 1;
695 }
696 }
697 - read_unlock();
698 + rcu_read_unlock();
699 return 0;
700 }
701
702 @@ -29,15 +30,16 @@
703 {
704 struct el *p;
705
706 - write_lock(&listmutex);
707 + spin_lock(&listmutex);
708 list_for_each_entry(p, head, lp) {
709 if (p->key == key) {
82a854ec 710 - list_del(&p->list);
dd81eca8 711 - write_unlock(&listmutex);
82a854ec 712 + list_del_rcu(&p->list);
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713 + spin_unlock(&listmutex);
714 + synchronize_rcu();
715 kfree(p);
716 return 1;
717 }
718 }
719 - write_unlock(&listmutex);
720 + spin_unlock(&listmutex);
721 return 0;
722 }
723
724Or, for those who prefer a side-by-side listing:
725
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 */
732 7 }; 7 };
733 8 spinlock_t listmutex; 8 spinlock_t listmutex;
734 9 struct el head; 9 struct el head;
735
736 1 int search(long key, int *result) 1 int search(long key, int *result)
737 2 { 2 {
738 3 struct list_head *lp; 3 struct list_head *lp;
739 4 struct el *p; 4 struct el *p;
740 5 5
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;
74510 read_unlock(); 10 rcu_read_unlock();
74611 return 1; 11 return 1;
74712 } 12 }
74813 } 13 }
74914 read_unlock(); 14 rcu_read_unlock();
75015 return 0; 15 return 0;
75116 } 16 }
752
753 1 int delete(long key) 1 int delete(long key)
754 2 { 2 {
755 3 struct el *p; 3 struct el *p;
756 4 4
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) {
82a854ec 760 8 list_del(&p->list); 8 list_del_rcu(&p->list);
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761 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
762 10 synchronize_rcu();
76310 kfree(p); 11 kfree(p);
76411 return 1; 12 return 1;
76512 } 13 }
76613 } 14 }
76714 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
76815 return 0; 16 return 0;
76916 } 17 }
770
771Either way, the differences are quite small. Read-side locking moves
772to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
670e9f34 773a reader-writer lock to a simple spinlock, and a synchronize_rcu()
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774precedes the kfree().
775
776However, there is one potential catch: the read-side and update-side
777critical sections can now run concurrently. In many cases, this will
778not be a problem, but it is necessary to check carefully regardless.
779For example, if multiple independent list updates must be seen as
780a single atomic update, converting to RCU will require special care.
781
782Also, the presence of synchronize_rcu() means that the RCU version of
783delete() can now block. If this is a problem, there is a callback-based
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784mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
785be used in place of synchronize_rcu().
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786
787
7887. FULL LIST OF RCU APIs
789
790The RCU APIs are documented in docbook-format header comments in the
791Linux-kernel source code, but it helps to have a full list of the
792APIs, since there does not appear to be a way to categorize them
793in docbook. Here is the list, by category.
794
c598a070 795RCU list traversal:
dd81eca8 796
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797 list_entry_rcu
798 list_first_entry_rcu
799 list_next_rcu
32300751 800 list_for_each_entry_rcu
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801 list_for_each_entry_continue_rcu
802 hlist_first_rcu
803 hlist_next_rcu
804 hlist_pprev_rcu
32300751 805 hlist_for_each_entry_rcu
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806 hlist_for_each_entry_rcu_bh
807 hlist_for_each_entry_continue_rcu
808 hlist_for_each_entry_continue_rcu_bh
809 hlist_nulls_first_rcu
240ebbf8 810 hlist_nulls_for_each_entry_rcu
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811 hlist_bl_first_rcu
812 hlist_bl_for_each_entry_rcu
dd81eca8 813
32300751 814RCU pointer/list update:
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815
816 rcu_assign_pointer
817 list_add_rcu
818 list_add_tail_rcu
819 list_del_rcu
820 list_replace_rcu
1d023284 821 hlist_add_behind_rcu
32300751 822 hlist_add_before_rcu
dd81eca8 823 hlist_add_head_rcu
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824 hlist_del_rcu
825 hlist_del_init_rcu
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826 hlist_replace_rcu
827 list_splice_init_rcu()
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828 hlist_nulls_del_init_rcu
829 hlist_nulls_del_rcu
830 hlist_nulls_add_head_rcu
831 hlist_bl_add_head_rcu
832 hlist_bl_del_init_rcu
833 hlist_bl_del_rcu
834 hlist_bl_set_first_rcu
dd81eca8 835
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836RCU: Critical sections Grace period Barrier
837
838 rcu_read_lock synchronize_net rcu_barrier
839 rcu_read_unlock synchronize_rcu
c598a070 840 rcu_dereference synchronize_rcu_expedited
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841 rcu_read_lock_held call_rcu
842 rcu_dereference_check kfree_rcu
843 rcu_dereference_protected
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844
845bh: Critical sections Grace period Barrier
846
847 rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
240ebbf8 848 rcu_read_unlock_bh synchronize_rcu_bh
c598a070 849 rcu_dereference_bh synchronize_rcu_bh_expedited
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850 rcu_dereference_bh_check
851 rcu_dereference_bh_protected
852 rcu_read_lock_bh_held
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853
854sched: Critical sections Grace period Barrier
855
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856 rcu_read_lock_sched synchronize_sched rcu_barrier_sched
857 rcu_read_unlock_sched call_rcu_sched
858 [preempt_disable] synchronize_sched_expedited
859 [and friends]
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860 rcu_read_lock_sched_notrace
861 rcu_read_unlock_sched_notrace
c598a070 862 rcu_dereference_sched
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863 rcu_dereference_sched_check
864 rcu_dereference_sched_protected
865 rcu_read_lock_sched_held
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866
867
868SRCU: Critical sections Grace period Barrier
869
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870 srcu_read_lock synchronize_srcu srcu_barrier
871 srcu_read_unlock call_srcu
99f88919 872 srcu_dereference synchronize_srcu_expedited
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873 srcu_dereference_check
874 srcu_read_lock_held
dd81eca8 875
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876SRCU: Initialization/cleanup
877 init_srcu_struct
878 cleanup_srcu_struct
dd81eca8 879
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880All: lockdep-checked RCU-protected pointer access
881
d07e6d08 882 rcu_access_index
50aec002 883 rcu_access_pointer
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884 rcu_dereference_index_check
885 rcu_dereference_raw
886 rcu_lockdep_assert
887 rcu_sleep_check
888 RCU_NONIDLE
50aec002 889
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890See the comment headers in the source code (or the docbook generated
891from them) for more information.
892
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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
99f88919 899b. What about the -rt patchset? If readers would need to block
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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
99f88919 904c. Do you need to treat NMI handlers, hardirq handlers,
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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?
2aef619c 908 If so, RCU-sched is the only choice that will work for you.
fea65126 909
99f88919 910d. Do you need RCU grace periods to complete even in the face
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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
99f88919 915e. Is your workload too update-intensive for normal use of
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916 RCU, but inappropriate for other synchronization mechanisms?
917 If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
918
99f88919 919f. Do you need read-side critical sections that are respected
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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
99f88919 924g. Otherwise, use RCU.
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925
926Of course, this all assumes that you have determined that RCU is in fact
927the right tool for your job.
928
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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
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940 "problematic_lock", disabling irq via
941 spin_lock_irqsave().
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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
d19720a9 1028Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
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1029
1030
1031For more information, see http://www.rdrop.com/users/paulmck/RCU.