ia64/linux-2.6.18-xen.hg

view Documentation/RCU/whatisRCU.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 What is RCU?
3 RCU is a synchronization mechanism that was added to the Linux kernel
4 during the 2.5 development effort that is optimized for read-mostly
5 situations. Although RCU is actually quite simple once you understand it,
6 getting there can sometimes be a challenge. Part of the problem is that
7 most of the past descriptions of RCU have been written with the mistaken
8 assumption that there is "one true way" to describe RCU. Instead,
9 the experience has been that different people must take different paths
10 to arrive at an understanding of RCU. This document provides several
11 different paths, as follows:
13 1. RCU OVERVIEW
14 2. WHAT IS RCU'S CORE API?
15 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
16 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
17 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
18 6. ANALOGY WITH READER-WRITER LOCKING
19 7. FULL LIST OF RCU APIs
20 8. ANSWERS TO QUICK QUIZZES
22 People who prefer starting with a conceptual overview should focus on
23 Section 1, though most readers will profit by reading this section at
24 some point. People who prefer to start with an API that they can then
25 experiment with should focus on Section 2. People who prefer to start
26 with example uses should focus on Sections 3 and 4. People who need to
27 understand the RCU implementation should focus on Section 5, then dive
28 into the kernel source code. People who reason best by analogy should
29 focus on Section 6. Section 7 serves as an index to the docbook API
30 documentation, and Section 8 is the traditional answer key.
32 So, start with the section that makes the most sense to you and your
33 preferred method of learning. If you need to know everything about
34 everything, feel free to read the whole thing -- but if you are really
35 that type of person, you have perused the source code and will therefore
36 never need this document anyway. ;-)
39 1. RCU OVERVIEW
41 The basic idea behind RCU is to split updates into "removal" and
42 "reclamation" phases. The removal phase removes references to data items
43 within a data structure (possibly by replacing them with references to
44 new versions of these data items), and can run concurrently with readers.
45 The reason that it is safe to run the removal phase concurrently with
46 readers is the semantics of modern CPUs guarantee that readers will see
47 either the old or the new version of the data structure rather than a
48 partially updated reference. The reclamation phase does the work of reclaiming
49 (e.g., freeing) the data items removed from the data structure during the
50 removal phase. Because reclaiming data items can disrupt any readers
51 concurrently referencing those data items, the reclamation phase must
52 not start until readers no longer hold references to those data items.
54 Splitting the update into removal and reclamation phases permits the
55 updater to perform the removal phase immediately, and to defer the
56 reclamation phase until all readers active during the removal phase have
57 completed, either by blocking until they finish or by registering a
58 callback that is invoked after they finish. Only readers that are active
59 during the removal phase need be considered, because any reader starting
60 after the removal phase will be unable to gain a reference to the removed
61 data items, and therefore cannot be disrupted by the reclamation phase.
63 So the typical RCU update sequence goes something like the following:
65 a. Remove pointers to a data structure, so that subsequent
66 readers cannot gain a reference to it.
68 b. Wait for all previous readers to complete their RCU read-side
69 critical sections.
71 c. At this point, there cannot be any readers who hold references
72 to the data structure, so it now may safely be reclaimed
73 (e.g., kfree()d).
75 Step (b) above is the key idea underlying RCU's deferred destruction.
76 The ability to wait until all readers are done allows RCU readers to
77 use much lighter-weight synchronization, in some cases, absolutely no
78 synchronization at all. In contrast, in more conventional lock-based
79 schemes, readers must use heavy-weight synchronization in order to
80 prevent an updater from deleting the data structure out from under them.
81 This is because lock-based updaters typically update data items in place,
82 and must therefore exclude readers. In contrast, RCU-based updaters
83 typically take advantage of the fact that writes to single aligned
84 pointers are atomic on modern CPUs, allowing atomic insertion, removal,
85 and replacement of data items in a linked structure without disrupting
86 readers. Concurrent RCU readers can then continue accessing the old
87 versions, and can dispense with the atomic operations, memory barriers,
88 and communications cache misses that are so expensive on present-day
89 SMP computer systems, even in absence of lock contention.
91 In the three-step procedure shown above, the updater is performing both
92 the removal and the reclamation step, but it is often helpful for an
93 entirely different thread to do the reclamation, as is in fact the case
94 in the Linux kernel's directory-entry cache (dcache). Even if the same
95 thread performs both the update step (step (a) above) and the reclamation
96 step (step (c) above), it is often helpful to think of them separately.
97 For example, RCU readers and updaters need not communicate at all,
98 but RCU provides implicit low-overhead communication between readers
99 and reclaimers, namely, in step (b) above.
101 So how the heck can a reclaimer tell when a reader is done, given
102 that readers are not doing any sort of synchronization operations???
103 Read on to learn about how RCU's API makes this easy.
106 2. WHAT IS RCU'S CORE API?
108 The core RCU API is quite small:
110 a. rcu_read_lock()
111 b. rcu_read_unlock()
112 c. synchronize_rcu() / call_rcu()
113 d. rcu_assign_pointer()
114 e. rcu_dereference()
116 There are many other members of the RCU API, but the rest can be
117 expressed in terms of these five, though most implementations instead
118 express synchronize_rcu() in terms of the call_rcu() callback API.
120 The five core RCU APIs are described below, the other 18 will be enumerated
121 later. See the kernel docbook documentation for more info, or look directly
122 at the function header comments.
124 rcu_read_lock()
126 void rcu_read_lock(void);
128 Used by a reader to inform the reclaimer that the reader is
129 entering an RCU read-side critical section. It is illegal
130 to block while in an RCU read-side critical section, though
131 kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side
132 critical sections. Any RCU-protected data structure accessed
133 during an RCU read-side critical section is guaranteed to remain
134 unreclaimed for the full duration of that critical section.
135 Reference counts may be used in conjunction with RCU to maintain
136 longer-term references to data structures.
138 rcu_read_unlock()
140 void rcu_read_unlock(void);
142 Used by a reader to inform the reclaimer that the reader is
143 exiting an RCU read-side critical section. Note that RCU
144 read-side critical sections may be nested and/or overlapping.
146 synchronize_rcu()
148 void synchronize_rcu(void);
150 Marks the end of updater code and the beginning of reclaimer
151 code. It does this by blocking until all pre-existing RCU
152 read-side critical sections on all CPUs have completed.
153 Note that synchronize_rcu() will -not- necessarily wait for
154 any subsequent RCU read-side critical sections to complete.
155 For example, consider the following sequence of events:
157 CPU 0 CPU 1 CPU 2
158 ----------------- ------------------------- ---------------
159 1. rcu_read_lock()
160 2. enters synchronize_rcu()
161 3. rcu_read_lock()
162 4. rcu_read_unlock()
163 5. exits synchronize_rcu()
164 6. rcu_read_unlock()
166 To reiterate, synchronize_rcu() waits only for ongoing RCU
167 read-side critical sections to complete, not necessarily for
168 any that begin after synchronize_rcu() is invoked.
170 Of course, synchronize_rcu() does not necessarily return
171 -immediately- after the last pre-existing RCU read-side critical
172 section completes. For one thing, there might well be scheduling
173 delays. For another thing, many RCU implementations process
174 requests in batches in order to improve efficiencies, which can
175 further delay synchronize_rcu().
177 Since synchronize_rcu() is the API that must figure out when
178 readers are done, its implementation is key to RCU. For RCU
179 to be useful in all but the most read-intensive situations,
180 synchronize_rcu()'s overhead must also be quite small.
182 The call_rcu() API is a callback form of synchronize_rcu(),
183 and is described in more detail in a later section. Instead of
184 blocking, it registers a function and argument which are invoked
185 after all ongoing RCU read-side critical sections have completed.
186 This callback variant is particularly useful in situations where
187 it is illegal to block or where update-side performance is
188 critically important.
190 However, the call_rcu() API should not be used lightly, as use
191 of the synchronize_rcu() API generally results in simpler code.
192 In addition, the synchronize_rcu() API has the nice property
193 of automatically limiting update rate should grace periods
194 be delayed. This property results in system resilience in face
195 of denial-of-service attacks. Code using call_rcu() should limit
196 update rate in order to gain this same sort of resilience. See
197 checklist.txt for some approaches to limiting the update rate.
199 rcu_assign_pointer()
201 typeof(p) rcu_assign_pointer(p, typeof(p) v);
203 Yes, rcu_assign_pointer() -is- implemented as a macro, though it
204 would be cool to be able to declare a function in this manner.
205 (Compiler experts will no doubt disagree.)
207 The updater uses this function to assign a new value to an
208 RCU-protected pointer, in order to safely communicate the change
209 in value from the updater to the reader. This function returns
210 the new value, and also executes any memory-barrier instructions
211 required for a given CPU architecture.
213 Perhaps just as important, it serves to document (1) which
214 pointers are protected by RCU and (2) the point at which a
215 given structure becomes accessible to other CPUs. That said,
216 rcu_assign_pointer() is most frequently used indirectly, via
217 the _rcu list-manipulation primitives such as list_add_rcu().
219 rcu_dereference()
221 typeof(p) rcu_dereference(p);
223 Like rcu_assign_pointer(), rcu_dereference() must be implemented
224 as a macro.
226 The reader uses rcu_dereference() to fetch an RCU-protected
227 pointer, which returns a value that may then be safely
228 dereferenced. Note that rcu_deference() does not actually
229 dereference the pointer, instead, it protects the pointer for
230 later dereferencing. It also executes any needed memory-barrier
231 instructions for a given CPU architecture. Currently, only Alpha
232 needs memory barriers within rcu_dereference() -- on other CPUs,
233 it compiles to nothing, not even a compiler directive.
235 Common coding practice uses rcu_dereference() to copy an
236 RCU-protected pointer to a local variable, then dereferences
237 this local variable, for example as follows:
239 p = rcu_dereference(head.next);
240 return p->data;
242 However, in this case, one could just as easily combine these
243 into one statement:
245 return rcu_dereference(head.next)->data;
247 If you are going to be fetching multiple fields from the
248 RCU-protected structure, using the local variable is of
249 course preferred. Repeated rcu_dereference() calls look
250 ugly and incur unnecessary overhead on Alpha CPUs.
252 Note that the value returned by rcu_dereference() is valid
253 only within the enclosing RCU read-side critical section.
254 For example, the following is -not- legal:
256 rcu_read_lock();
257 p = rcu_dereference(head.next);
258 rcu_read_unlock();
259 x = p->address;
260 rcu_read_lock();
261 y = p->data;
262 rcu_read_unlock();
264 Holding a reference from one RCU read-side critical section
265 to another is just as illegal as holding a reference from
266 one lock-based critical section to another! Similarly,
267 using a reference outside of the critical section in which
268 it was acquired is just as illegal as doing so with normal
269 locking.
271 As with rcu_assign_pointer(), an important function of
272 rcu_dereference() is to document which pointers are protected by
273 RCU, in particular, flagging a pointer that is subject to changing
274 at any time, including immediately after the rcu_dereference().
275 And, again like rcu_assign_pointer(), rcu_dereference() is
276 typically used indirectly, via the _rcu list-manipulation
277 primitives, such as list_for_each_entry_rcu().
279 The following diagram shows how each API communicates among the
280 reader, updater, and reclaimer.
283 rcu_assign_pointer()
284 +--------+
285 +---------------------->| reader |---------+
286 | +--------+ |
287 | | |
288 | | | Protect:
289 | | | rcu_read_lock()
290 | | | rcu_read_unlock()
291 | rcu_dereference() | |
292 +---------+ | |
293 | updater |<---------------------+ |
294 +---------+ V
295 | +-----------+
296 +----------------------------------->| reclaimer |
297 +-----------+
298 Defer:
299 synchronize_rcu() & call_rcu()
302 The RCU infrastructure observes the time sequence of rcu_read_lock(),
303 rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
304 order to determine when (1) synchronize_rcu() invocations may return
305 to their callers and (2) call_rcu() callbacks may be invoked. Efficient
306 implementations of the RCU infrastructure make heavy use of batching in
307 order to amortize their overhead over many uses of the corresponding APIs.
309 There are no fewer than three RCU mechanisms in the Linux kernel; the
310 diagram above shows the first one, which is by far the most commonly used.
311 The rcu_dereference() and rcu_assign_pointer() primitives are used for
312 all three mechanisms, but different defer and protect primitives are
313 used as follows:
315 Defer Protect
317 a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
318 call_rcu()
320 b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
322 c. synchronize_sched() preempt_disable() / preempt_enable()
323 local_irq_save() / local_irq_restore()
324 hardirq enter / hardirq exit
325 NMI enter / NMI exit
327 These three mechanisms are used as follows:
329 a. RCU applied to normal data structures.
331 b. RCU applied to networking data structures that may be subjected
332 to remote denial-of-service attacks.
334 c. RCU applied to scheduler and interrupt/NMI-handler tasks.
336 Again, most uses will be of (a). The (b) and (c) cases are important
337 for specialized uses, but are relatively uncommon.
340 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
342 This section shows a simple use of the core RCU API to protect a
343 global pointer to a dynamically allocated structure. More-typical
344 uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
346 struct foo {
347 int a;
348 char b;
349 long c;
350 };
351 DEFINE_SPINLOCK(foo_mutex);
353 struct foo *gbl_foo;
355 /*
356 * Create a new struct foo that is the same as the one currently
357 * pointed to by gbl_foo, except that field "a" is replaced
358 * with "new_a". Points gbl_foo to the new structure, and
359 * frees up the old structure after a grace period.
360 *
361 * Uses rcu_assign_pointer() to ensure that concurrent readers
362 * see the initialized version of the new structure.
363 *
364 * Uses synchronize_rcu() to ensure that any readers that might
365 * have references to the old structure complete before freeing
366 * the old structure.
367 */
368 void foo_update_a(int new_a)
369 {
370 struct foo *new_fp;
371 struct foo *old_fp;
373 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
374 spin_lock(&foo_mutex);
375 old_fp = gbl_foo;
376 *new_fp = *old_fp;
377 new_fp->a = new_a;
378 rcu_assign_pointer(gbl_foo, new_fp);
379 spin_unlock(&foo_mutex);
380 synchronize_rcu();
381 kfree(old_fp);
382 }
384 /*
385 * Return the value of field "a" of the current gbl_foo
386 * structure. Use rcu_read_lock() and rcu_read_unlock()
387 * to ensure that the structure does not get deleted out
388 * from under us, and use rcu_dereference() to ensure that
389 * we see the initialized version of the structure (important
390 * for DEC Alpha and for people reading the code).
391 */
392 int foo_get_a(void)
393 {
394 int retval;
396 rcu_read_lock();
397 retval = rcu_dereference(gbl_foo)->a;
398 rcu_read_unlock();
399 return retval;
400 }
402 So, to sum up:
404 o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
405 read-side critical sections.
407 o Within an RCU read-side critical section, use rcu_dereference()
408 to dereference RCU-protected pointers.
410 o Use some solid scheme (such as locks or semaphores) to
411 keep concurrent updates from interfering with each other.
413 o Use rcu_assign_pointer() to update an RCU-protected pointer.
414 This primitive protects concurrent readers from the updater,
415 -not- concurrent updates from each other! You therefore still
416 need to use locking (or something similar) to keep concurrent
417 rcu_assign_pointer() primitives from interfering with each other.
419 o Use synchronize_rcu() -after- removing a data element from an
420 RCU-protected data structure, but -before- reclaiming/freeing
421 the data element, in order to wait for the completion of all
422 RCU read-side critical sections that might be referencing that
423 data item.
425 See checklist.txt for additional rules to follow when using RCU.
426 And again, more-typical uses of RCU may be found in listRCU.txt,
427 arrayRCU.txt, and NMI-RCU.txt.
430 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
432 In the example above, foo_update_a() blocks until a grace period elapses.
433 This is quite simple, but in some cases one cannot afford to wait so
434 long -- there might be other high-priority work to be done.
436 In such cases, one uses call_rcu() rather than synchronize_rcu().
437 The call_rcu() API is as follows:
439 void call_rcu(struct rcu_head * head,
440 void (*func)(struct rcu_head *head));
442 This function invokes func(head) after a grace period has elapsed.
443 This invocation might happen from either softirq or process context,
444 so the function is not permitted to block. The foo struct needs to
445 have an rcu_head structure added, perhaps as follows:
447 struct foo {
448 int a;
449 char b;
450 long c;
451 struct rcu_head rcu;
452 };
454 The foo_update_a() function might then be written as follows:
456 /*
457 * Create a new struct foo that is the same as the one currently
458 * pointed to by gbl_foo, except that field "a" is replaced
459 * with "new_a". Points gbl_foo to the new structure, and
460 * frees up the old structure after a grace period.
461 *
462 * Uses rcu_assign_pointer() to ensure that concurrent readers
463 * see the initialized version of the new structure.
464 *
465 * Uses call_rcu() to ensure that any readers that might have
466 * references to the old structure complete before freeing the
467 * old structure.
468 */
469 void foo_update_a(int new_a)
470 {
471 struct foo *new_fp;
472 struct foo *old_fp;
474 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
475 spin_lock(&foo_mutex);
476 old_fp = gbl_foo;
477 *new_fp = *old_fp;
478 new_fp->a = new_a;
479 rcu_assign_pointer(gbl_foo, new_fp);
480 spin_unlock(&foo_mutex);
481 call_rcu(&old_fp->rcu, foo_reclaim);
482 }
484 The foo_reclaim() function might appear as follows:
486 void foo_reclaim(struct rcu_head *rp)
487 {
488 struct foo *fp = container_of(rp, struct foo, rcu);
490 kfree(fp);
491 }
493 The container_of() primitive is a macro that, given a pointer into a
494 struct, the type of the struct, and the pointed-to field within the
495 struct, returns a pointer to the beginning of the struct.
497 The use of call_rcu() permits the caller of foo_update_a() to
498 immediately regain control, without needing to worry further about the
499 old version of the newly updated element. It also clearly shows the
500 RCU distinction between updater, namely foo_update_a(), and reclaimer,
501 namely foo_reclaim().
503 The summary of advice is the same as for the previous section, except
504 that we are now using call_rcu() rather than synchronize_rcu():
506 o Use call_rcu() -after- removing a data element from an
507 RCU-protected data structure in order to register a callback
508 function that will be invoked after the completion of all RCU
509 read-side critical sections that might be referencing that
510 data item.
512 Again, see checklist.txt for additional rules governing the use of RCU.
515 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
517 One of the nice things about RCU is that it has extremely simple "toy"
518 implementations that are a good first step towards understanding the
519 production-quality implementations in the Linux kernel. This section
520 presents two such "toy" implementations of RCU, one that is implemented
521 in terms of familiar locking primitives, and another that more closely
522 resembles "classic" RCU. Both are way too simple for real-world use,
523 lacking both functionality and performance. However, they are useful
524 in getting a feel for how RCU works. See kernel/rcupdate.c for a
525 production-quality implementation, and see:
527 http://www.rdrop.com/users/paulmck/RCU
529 for papers describing the Linux kernel RCU implementation. The OLS'01
530 and OLS'02 papers are a good introduction, and the dissertation provides
531 more details on the current implementation as of early 2004.
534 5A. "TOY" IMPLEMENTATION #1: LOCKING
536 This section presents a "toy" RCU implementation that is based on
537 familiar locking primitives. Its overhead makes it a non-starter for
538 real-life use, as does its lack of scalability. It is also unsuitable
539 for realtime use, since it allows scheduling latency to "bleed" from
540 one read-side critical section to another.
542 However, it is probably the easiest implementation to relate to, so is
543 a good starting point.
545 It is extremely simple:
547 static DEFINE_RWLOCK(rcu_gp_mutex);
549 void rcu_read_lock(void)
550 {
551 read_lock(&rcu_gp_mutex);
552 }
554 void rcu_read_unlock(void)
555 {
556 read_unlock(&rcu_gp_mutex);
557 }
559 void synchronize_rcu(void)
560 {
561 write_lock(&rcu_gp_mutex);
562 write_unlock(&rcu_gp_mutex);
563 }
565 [You can ignore rcu_assign_pointer() and rcu_dereference() without
566 missing much. But here they are anyway. And whatever you do, don't
567 forget about them when submitting patches making use of RCU!]
569 #define rcu_assign_pointer(p, v) ({ \
570 smp_wmb(); \
571 (p) = (v); \
572 })
574 #define rcu_dereference(p) ({ \
575 typeof(p) _________p1 = p; \
576 smp_read_barrier_depends(); \
577 (_________p1); \
578 })
581 The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
582 and release a global reader-writer lock. The synchronize_rcu()
583 primitive write-acquires this same lock, then immediately releases
584 it. This means that once synchronize_rcu() exits, all RCU read-side
585 critical sections that were in progress before synchonize_rcu() was
586 called are guaranteed to have completed -- there is no way that
587 synchronize_rcu() would have been able to write-acquire the lock
588 otherwise.
590 It is possible to nest rcu_read_lock(), since reader-writer locks may
591 be recursively acquired. Note also that rcu_read_lock() is immune
592 from deadlock (an important property of RCU). The reason for this is
593 that the only thing that can block rcu_read_lock() is a synchronize_rcu().
594 But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
595 so there can be no deadlock cycle.
597 Quick Quiz #1: Why is this argument naive? How could a deadlock
598 occur when using this algorithm in a real-world Linux
599 kernel? How could this deadlock be avoided?
602 5B. "TOY" EXAMPLE #2: CLASSIC RCU
604 This section presents a "toy" RCU implementation that is based on
605 "classic RCU". It is also short on performance (but only for updates) and
606 on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
607 kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
608 are the same as those shown in the preceding section, so they are omitted.
610 void rcu_read_lock(void) { }
612 void rcu_read_unlock(void) { }
614 void synchronize_rcu(void)
615 {
616 int cpu;
618 for_each_possible_cpu(cpu)
619 run_on(cpu);
620 }
622 Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
623 This is the great strength of classic RCU in a non-preemptive kernel:
624 read-side overhead is precisely zero, at least on non-Alpha CPUs.
625 And there is absolutely no way that rcu_read_lock() can possibly
626 participate in a deadlock cycle!
628 The implementation of synchronize_rcu() simply schedules itself on each
629 CPU in turn. The run_on() primitive can be implemented straightforwardly
630 in terms of the sched_setaffinity() primitive. Of course, a somewhat less
631 "toy" implementation would restore the affinity upon completion rather
632 than just leaving all tasks running on the last CPU, but when I said
633 "toy", I meant -toy-!
635 So how the heck is this supposed to work???
637 Remember that it is illegal to block while in an RCU read-side critical
638 section. Therefore, if a given CPU executes a context switch, we know
639 that it must have completed all preceding RCU read-side critical sections.
640 Once -all- CPUs have executed a context switch, then -all- preceding
641 RCU read-side critical sections will have completed.
643 So, suppose that we remove a data item from its structure and then invoke
644 synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
645 that there are no RCU read-side critical sections holding a reference
646 to that data item, so we can safely reclaim it.
648 Quick Quiz #2: Give an example where Classic RCU's read-side
649 overhead is -negative-.
651 Quick Quiz #3: If it is illegal to block in an RCU read-side
652 critical section, what the heck do you do in
653 PREEMPT_RT, where normal spinlocks can block???
656 6. ANALOGY WITH READER-WRITER LOCKING
658 Although RCU can be used in many different ways, a very common use of
659 RCU is analogous to reader-writer locking. The following unified
660 diff shows how closely related RCU and reader-writer locking can be.
662 @@ -13,15 +14,15 @@
663 struct list_head *lp;
664 struct el *p;
666 - read_lock();
667 - list_for_each_entry(p, head, lp) {
668 + rcu_read_lock();
669 + list_for_each_entry_rcu(p, head, lp) {
670 if (p->key == key) {
671 *result = p->data;
672 - read_unlock();
673 + rcu_read_unlock();
674 return 1;
675 }
676 }
677 - read_unlock();
678 + rcu_read_unlock();
679 return 0;
680 }
682 @@ -29,15 +30,16 @@
683 {
684 struct el *p;
686 - write_lock(&listmutex);
687 + spin_lock(&listmutex);
688 list_for_each_entry(p, head, lp) {
689 if (p->key == key) {
690 - list_del(&p->list);
691 - write_unlock(&listmutex);
692 + list_del_rcu(&p->list);
693 + spin_unlock(&listmutex);
694 + synchronize_rcu();
695 kfree(p);
696 return 1;
697 }
698 }
699 - write_unlock(&listmutex);
700 + spin_unlock(&listmutex);
701 return 0;
702 }
704 Or, for those who prefer a side-by-side listing:
706 1 struct el { 1 struct el {
707 2 struct list_head list; 2 struct list_head list;
708 3 long key; 3 long key;
709 4 spinlock_t mutex; 4 spinlock_t mutex;
710 5 int data; 5 int data;
711 6 /* Other data fields */ 6 /* Other data fields */
712 7 }; 7 };
713 8 spinlock_t listmutex; 8 spinlock_t listmutex;
714 9 struct el head; 9 struct el head;
716 1 int search(long key, int *result) 1 int search(long key, int *result)
717 2 { 2 {
718 3 struct list_head *lp; 3 struct list_head *lp;
719 4 struct el *p; 4 struct el *p;
720 5 5
721 6 read_lock(); 6 rcu_read_lock();
722 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
723 8 if (p->key == key) { 8 if (p->key == key) {
724 9 *result = p->data; 9 *result = p->data;
725 10 read_unlock(); 10 rcu_read_unlock();
726 11 return 1; 11 return 1;
727 12 } 12 }
728 13 } 13 }
729 14 read_unlock(); 14 rcu_read_unlock();
730 15 return 0; 15 return 0;
731 16 } 16 }
733 1 int delete(long key) 1 int delete(long key)
734 2 { 2 {
735 3 struct el *p; 3 struct el *p;
736 4 4
737 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
738 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
739 7 if (p->key == key) { 7 if (p->key == key) {
740 8 list_del(&p->list); 8 list_del_rcu(&p->list);
741 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
742 10 synchronize_rcu();
743 10 kfree(p); 11 kfree(p);
744 11 return 1; 12 return 1;
745 12 } 13 }
746 13 } 14 }
747 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
748 15 return 0; 16 return 0;
749 16 } 17 }
751 Either way, the differences are quite small. Read-side locking moves
752 to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
753 from a reader-writer lock to a simple spinlock, and a synchronize_rcu()
754 precedes the kfree().
756 However, there is one potential catch: the read-side and update-side
757 critical sections can now run concurrently. In many cases, this will
758 not be a problem, but it is necessary to check carefully regardless.
759 For example, if multiple independent list updates must be seen as
760 a single atomic update, converting to RCU will require special care.
762 Also, the presence of synchronize_rcu() means that the RCU version of
763 delete() can now block. If this is a problem, there is a callback-based
764 mechanism that never blocks, namely call_rcu(), that can be used in
765 place of synchronize_rcu().
768 7. FULL LIST OF RCU APIs
770 The RCU APIs are documented in docbook-format header comments in the
771 Linux-kernel source code, but it helps to have a full list of the
772 APIs, since there does not appear to be a way to categorize them
773 in docbook. Here is the list, by category.
775 Markers for RCU read-side critical sections:
777 rcu_read_lock
778 rcu_read_unlock
779 rcu_read_lock_bh
780 rcu_read_unlock_bh
782 RCU pointer/list traversal:
784 rcu_dereference
785 list_for_each_rcu (to be deprecated in favor of
786 list_for_each_entry_rcu)
787 list_for_each_entry_rcu
788 list_for_each_continue_rcu (to be deprecated in favor of new
789 list_for_each_entry_continue_rcu)
790 hlist_for_each_entry_rcu
792 RCU pointer update:
794 rcu_assign_pointer
795 list_add_rcu
796 list_add_tail_rcu
797 list_del_rcu
798 list_replace_rcu
799 hlist_del_rcu
800 hlist_add_head_rcu
802 RCU grace period:
804 synchronize_net
805 synchronize_sched
806 synchronize_rcu
807 call_rcu
808 call_rcu_bh
810 See the comment headers in the source code (or the docbook generated
811 from them) for more information.
814 8. ANSWERS TO QUICK QUIZZES
816 Quick Quiz #1: Why is this argument naive? How could a deadlock
817 occur when using this algorithm in a real-world Linux
818 kernel? [Referring to the lock-based "toy" RCU
819 algorithm.]
821 Answer: Consider the following sequence of events:
823 1. CPU 0 acquires some unrelated lock, call it
824 "problematic_lock", disabling irq via
825 spin_lock_irqsave().
827 2. CPU 1 enters synchronize_rcu(), write-acquiring
828 rcu_gp_mutex.
830 3. CPU 0 enters rcu_read_lock(), but must wait
831 because CPU 1 holds rcu_gp_mutex.
833 4. CPU 1 is interrupted, and the irq handler
834 attempts to acquire problematic_lock.
836 The system is now deadlocked.
838 One way to avoid this deadlock is to use an approach like
839 that of CONFIG_PREEMPT_RT, where all normal spinlocks
840 become blocking locks, and all irq handlers execute in
841 the context of special tasks. In this case, in step 4
842 above, the irq handler would block, allowing CPU 1 to
843 release rcu_gp_mutex, avoiding the deadlock.
845 Even in the absence of deadlock, this RCU implementation
846 allows latency to "bleed" from readers to other
847 readers through synchronize_rcu(). To see this,
848 consider task A in an RCU read-side critical section
849 (thus read-holding rcu_gp_mutex), task B blocked
850 attempting to write-acquire rcu_gp_mutex, and
851 task C blocked in rcu_read_lock() attempting to
852 read_acquire rcu_gp_mutex. Task A's RCU read-side
853 latency is holding up task C, albeit indirectly via
854 task B.
856 Realtime RCU implementations therefore use a counter-based
857 approach where tasks in RCU read-side critical sections
858 cannot be blocked by tasks executing synchronize_rcu().
860 Quick Quiz #2: Give an example where Classic RCU's read-side
861 overhead is -negative-.
863 Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
864 kernel where a routing table is used by process-context
865 code, but can be updated by irq-context code (for example,
866 by an "ICMP REDIRECT" packet). The usual way of handling
867 this would be to have the process-context code disable
868 interrupts while searching the routing table. Use of
869 RCU allows such interrupt-disabling to be dispensed with.
870 Thus, without RCU, you pay the cost of disabling interrupts,
871 and with RCU you don't.
873 One can argue that the overhead of RCU in this
874 case is negative with respect to the single-CPU
875 interrupt-disabling approach. Others might argue that
876 the overhead of RCU is merely zero, and that replacing
877 the positive overhead of the interrupt-disabling scheme
878 with the zero-overhead RCU scheme does not constitute
879 negative overhead.
881 In real life, of course, things are more complex. But
882 even the theoretical possibility of negative overhead for
883 a synchronization primitive is a bit unexpected. ;-)
885 Quick Quiz #3: If it is illegal to block in an RCU read-side
886 critical section, what the heck do you do in
887 PREEMPT_RT, where normal spinlocks can block???
889 Answer: Just as PREEMPT_RT permits preemption of spinlock
890 critical sections, it permits preemption of RCU
891 read-side critical sections. It also permits
892 spinlocks blocking while in RCU read-side critical
893 sections.
895 Why the apparent inconsistency? Because it is it
896 possible to use priority boosting to keep the RCU
897 grace periods short if need be (for example, if running
898 short of memory). In contrast, if blocking waiting
899 for (say) network reception, there is no way to know
900 what should be boosted. Especially given that the
901 process we need to boost might well be a human being
902 who just went out for a pizza or something. And although
903 a computer-operated cattle prod might arouse serious
904 interest, it might also provoke serious objections.
905 Besides, how does the computer know what pizza parlor
906 the human being went to???
909 ACKNOWLEDGEMENTS
911 My thanks to the people who helped make this human-readable, including
912 Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
915 For more information, see http://www.rdrop.com/users/paulmck/RCU.