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7 <h1>A Tour Through RCU's Requirements</h1>
9 <p>Copyright IBM Corporation, 2015</p>
10 <p>Author: Paul E. McKenney</p>
11 <p><i>The initial version of this document appeared in the
12 <a href="https://lwn.net/">LWN</a> articles
13 <a href="https://lwn.net/Articles/652156/">here</a>,
14 <a href="https://lwn.net/Articles/652677/">here</a>, and
15 <a href="https://lwn.net/Articles/653326/">here</a>.</i></p>
20 Read-copy update (RCU) is a synchronization mechanism that is often
21 used as a replacement for reader-writer locking.
22 RCU is unusual in that updaters do not block readers,
23 which means that RCU's read-side primitives can be exceedingly fast
25 In addition, updaters can make useful forward progress concurrently
27 However, all this concurrency between RCU readers and updaters does raise
28 the question of exactly what RCU readers are doing, which in turn
29 raises the question of exactly what RCU's requirements are.
32 This document therefore summarizes RCU's requirements, and can be thought
33 of as an informal, high-level specification for RCU.
34 It is important to understand that RCU's specification is primarily
36 in fact, I learned about many of these requirements the hard way.
37 This situation might cause some consternation, however, not only
38 has this learning process been a lot of fun, but it has also been
39 a great privilege to work with so many people willing to apply
40 technologies in interesting new ways.
43 All that aside, here are the categories of currently known RCU requirements:
47 <li> <a href="#Fundamental Requirements">
48 Fundamental Requirements</a>
49 <li> <a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a>
50 <li> <a href="#Parallelism Facts of Life">
51 Parallelism Facts of Life</a>
52 <li> <a href="#Quality-of-Implementation Requirements">
53 Quality-of-Implementation Requirements</a>
54 <li> <a href="#Linux Kernel Complications">
55 Linux Kernel Complications</a>
56 <li> <a href="#Software-Engineering Requirements">
57 Software-Engineering Requirements</a>
58 <li> <a href="#Other RCU Flavors">
60 <li> <a href="#Possible Future Changes">
61 Possible Future Changes</a>
65 This is followed by a <a href="#Summary">summary</a>,
66 which is in turn followed by the inevitable
67 <a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
69 <h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
72 RCU's fundamental requirements are the closest thing RCU has to hard
73 mathematical requirements.
77 <li> <a href="#Grace-Period Guarantee">
78 Grace-Period Guarantee</a>
79 <li> <a href="#Publish-Subscribe Guarantee">
80 Publish-Subscribe Guarantee</a>
81 <li> <a href="#Memory-Barrier Guarantees">
82 Memory-Barrier Guarantees</a>
83 <li> <a href="#RCU Primitives Guaranteed to Execute Unconditionally">
84 RCU Primitives Guaranteed to Execute Unconditionally</a>
85 <li> <a href="#Guaranteed Read-to-Write Upgrade">
86 Guaranteed Read-to-Write Upgrade</a>
89 <h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3>
92 RCU's grace-period guarantee is unusual in being premeditated:
93 Jack Slingwine and I had this guarantee firmly in mind when we started
94 work on RCU (then called “rclock”) in the early 1990s.
95 That said, the past two decades of experience with RCU have produced
96 a much more detailed understanding of this guarantee.
99 RCU's grace-period guarantee allows updaters to wait for the completion
100 of all pre-existing RCU read-side critical sections.
101 An RCU read-side critical section
102 begins with the marker <tt>rcu_read_lock()</tt> and ends with
103 the marker <tt>rcu_read_unlock()</tt>.
104 These markers may be nested, and RCU treats a nested set as one
105 big RCU read-side critical section.
106 Production-quality implementations of <tt>rcu_read_lock()</tt> and
107 <tt>rcu_read_unlock()</tt> are extremely lightweight, and in
108 fact have exactly zero overhead in Linux kernels built for production
109 use with <tt>CONFIG_PREEMPT=n</tt>.
112 This guarantee allows ordering to be enforced with extremely low
113 overhead to readers, for example:
127 11 void thread1(void)
130 14 synchronize_rcu();
137 Because the <tt>synchronize_rcu()</tt> on line 14 waits for
138 all pre-existing readers, any instance of <tt>thread0()</tt> that
139 loads a value of zero from <tt>x</tt> must complete before
140 <tt>thread1()</tt> stores to <tt>y</tt>, so that instance must
141 also load a value of zero from <tt>y</tt>.
142 Similarly, any instance of <tt>thread0()</tt> that loads a value of
143 one from <tt>y</tt> must have started after the
144 <tt>synchronize_rcu()</tt> started, and must therefore also load
145 a value of one from <tt>x</tt>.
146 Therefore, the outcome:
149 (r1 == 0 && r2 == 1)
156 You said that updaters can make useful forward progress concurrently
157 with readers, but pre-existing readers will block
158 <tt>synchronize_rcu()</tt>!!!
159 Just who are you trying to fool???
161 First, if updaters do not wish to be blocked by readers, they can use
162 <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
164 Second, even when using <tt>synchronize_rcu()</tt>, the other
165 update-side code does run concurrently with readers, whether pre-existing
170 This scenario resembles one of the first uses of RCU in
171 <a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>,
172 which managed a distributed lock manager's transition into
173 a state suitable for handling recovery from node failure,
174 more or less as follows:
178 1 #define STATE_NORMAL 0
179 2 #define STATE_WANT_RECOVERY 1
180 3 #define STATE_RECOVERING 2
181 4 #define STATE_WANT_NORMAL 3
183 6 int state = STATE_NORMAL;
185 8 void do_something_dlm(void)
190 13 state_snap = READ_ONCE(state);
191 14 if (state_snap == STATE_NORMAL)
194 17 do_something_carefully();
195 18 rcu_read_unlock();
198 21 void start_recovery(void)
200 23 WRITE_ONCE(state, STATE_WANT_RECOVERY);
201 24 synchronize_rcu();
202 25 WRITE_ONCE(state, STATE_RECOVERING);
204 27 WRITE_ONCE(state, STATE_WANT_NORMAL);
205 28 synchronize_rcu();
206 29 WRITE_ONCE(state, STATE_NORMAL);
212 The RCU read-side critical section in <tt>do_something_dlm()</tt>
213 works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt>
214 to guarantee that <tt>do_something()</tt> never runs concurrently
215 with <tt>recovery()</tt>, but with little or no synchronization
216 overhead in <tt>do_something_dlm()</tt>.
219 Why is the <tt>synchronize_rcu()</tt> on line 28 needed?
221 Without that extra grace period, memory reordering could result in
222 <tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
223 concurrently with the last bits of <tt>recovery()</tt>.
227 In order to avoid fatal problems such as deadlocks,
228 an RCU read-side critical section must not contain calls to
229 <tt>synchronize_rcu()</tt>.
230 Similarly, an RCU read-side critical section must not
231 contain anything that waits, directly or indirectly, on completion of
232 an invocation of <tt>synchronize_rcu()</tt>.
235 Although RCU's grace-period guarantee is useful in and of itself, with
236 <a href="https://lwn.net/Articles/573497/">quite a few use cases</a>,
237 it would be good to be able to use RCU to coordinate read-side
238 access to linked data structures.
239 For this, the grace-period guarantee is not sufficient, as can
240 be seen in function <tt>add_gp_buggy()</tt> below.
241 We will look at the reader's code later, but in the meantime, just think of
242 the reader as locklessly picking up the <tt>gp</tt> pointer,
243 and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the
244 <tt>->a</tt> and <tt>->b</tt> fields.
248 1 bool add_gp_buggy(int a, int b)
250 3 p = kmalloc(sizeof(*p), GFP_KERNEL);
253 6 spin_lock(&gp_lock);
254 7 if (rcu_access_pointer(gp)) {
255 8 spin_unlock(&gp_lock);
260 13 gp = p; /* ORDERING BUG */
261 14 spin_unlock(&gp_lock);
268 The problem is that both the compiler and weakly ordered CPUs are within
269 their rights to reorder this code as follows:
273 1 bool add_gp_buggy_optimized(int a, int b)
275 3 p = kmalloc(sizeof(*p), GFP_KERNEL);
278 6 spin_lock(&gp_lock);
279 7 if (rcu_access_pointer(gp)) {
280 8 spin_unlock(&gp_lock);
283 <b>11 gp = p; /* ORDERING BUG */
286 14 spin_unlock(&gp_lock);
293 If an RCU reader fetches <tt>gp</tt> just after
294 <tt>add_gp_buggy_optimized</tt> executes line 11,
295 it will see garbage in the <tt>->a</tt> and <tt>->b</tt>
297 And this is but one of many ways in which compiler and hardware optimizations
299 Therefore, we clearly need some way to prevent the compiler and the CPU from
300 reordering in this manner, which brings us to the publish-subscribe
301 guarantee discussed in the next section.
303 <h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3>
306 RCU's publish-subscribe guarantee allows data to be inserted
307 into a linked data structure without disrupting RCU readers.
308 The updater uses <tt>rcu_assign_pointer()</tt> to insert the
309 new data, and readers use <tt>rcu_dereference()</tt> to
310 access data, whether new or old.
311 The following shows an example of insertion:
315 1 bool add_gp(int a, int b)
317 3 p = kmalloc(sizeof(*p), GFP_KERNEL);
320 6 spin_lock(&gp_lock);
321 7 if (rcu_access_pointer(gp)) {
322 8 spin_unlock(&gp_lock);
327 13 rcu_assign_pointer(gp, p);
328 14 spin_unlock(&gp_lock);
335 The <tt>rcu_assign_pointer()</tt> on line 13 is conceptually
336 equivalent to a simple assignment statement, but also guarantees
337 that its assignment will
338 happen after the two assignments in lines 11 and 12,
339 similar to the C11 <tt>memory_order_release</tt> store operation.
340 It also prevents any number of “interesting” compiler
341 optimizations, for example, the use of <tt>gp</tt> as a scratch
342 location immediately preceding the assignment.
345 But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
346 two assignments to <tt>p->a</tt> and <tt>p->b</tt>
347 from being reordered.
348 Can't that also cause problems?
351 The readers cannot see either of these two fields until
352 the assignment to <tt>gp</tt>, by which time both fields are
354 So reordering the assignments
355 to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly
360 It is tempting to assume that the reader need not do anything special
361 to control its accesses to the RCU-protected data,
362 as shown in <tt>do_something_gp_buggy()</tt> below:
366 1 bool do_something_gp_buggy(void)
369 4 p = gp; /* OPTIMIZATIONS GALORE!!! */
371 6 do_something(p->a, p->b);
375 10 rcu_read_unlock();
382 However, this temptation must be resisted because there are a
383 surprisingly large number of ways that the compiler
385 <a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>)
386 can trip this code up.
387 For but one example, if the compiler were short of registers, it
388 might choose to refetch from <tt>gp</tt> rather than keeping
389 a separate copy in <tt>p</tt> as follows:
393 1 bool do_something_gp_buggy_optimized(void)
396 4 if (gp) { /* OPTIMIZATIONS GALORE!!! */
397 <b> 5 do_something(gp->a, gp->b);</b>
408 If this function ran concurrently with a series of updates that
409 replaced the current structure with a new one,
410 the fetches of <tt>gp->a</tt>
411 and <tt>gp->b</tt> might well come from two different structures,
412 which could cause serious confusion.
413 To prevent this (and much else besides), <tt>do_something_gp()</tt> uses
414 <tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>:
418 1 bool do_something_gp(void)
421 4 p = rcu_dereference(gp);
423 6 do_something(p->a, p->b);
427 10 rcu_read_unlock();
434 The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha)
435 memory barriers in the Linux kernel.
437 <a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a>
438 ever appear, then <tt>rcu_dereference()</tt> could be implemented
439 as a <tt>memory_order_consume</tt> load.
440 Regardless of the exact implementation, a pointer fetched by
441 <tt>rcu_dereference()</tt> may not be used outside of the
442 outermost RCU read-side critical section containing that
443 <tt>rcu_dereference()</tt>, unless protection of
444 the corresponding data element has been passed from RCU to some
445 other synchronization mechanism, most commonly locking or
446 <a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>.
449 In short, updaters use <tt>rcu_assign_pointer()</tt> and readers
450 use <tt>rcu_dereference()</tt>, and these two RCU API elements
451 work together to ensure that readers have a consistent view of
452 newly added data elements.
455 Of course, it is also necessary to remove elements from RCU-protected
456 data structures, for example, using the following process:
459 <li> Remove the data element from the enclosing structure.
460 <li> Wait for all pre-existing RCU read-side critical sections
461 to complete (because only pre-existing readers can possibly have
462 a reference to the newly removed data element).
463 <li> At this point, only the updater has a reference to the
464 newly removed data element, so it can safely reclaim
465 the data element, for example, by passing it to <tt>kfree()</tt>.
468 This process is implemented by <tt>remove_gp_synchronous()</tt>:
472 1 bool remove_gp_synchronous(void)
476 5 spin_lock(&gp_lock);
477 6 p = rcu_access_pointer(gp);
479 8 spin_unlock(&gp_lock);
482 11 rcu_assign_pointer(gp, NULL);
483 12 spin_unlock(&gp_lock);
484 13 synchronize_rcu();
492 This function is straightforward, with line 13 waiting for a grace
493 period before line 14 frees the old data element.
494 This waiting ensures that readers will reach line 7 of
495 <tt>do_something_gp()</tt> before the data element referenced by
497 The <tt>rcu_access_pointer()</tt> on line 6 is similar to
498 <tt>rcu_dereference()</tt>, except that:
501 <li> The value returned by <tt>rcu_access_pointer()</tt>
502 cannot be dereferenced.
503 If you want to access the value pointed to as well as
504 the pointer itself, use <tt>rcu_dereference()</tt>
505 instead of <tt>rcu_access_pointer()</tt>.
506 <li> The call to <tt>rcu_access_pointer()</tt> need not be
508 In contrast, <tt>rcu_dereference()</tt> must either be
509 within an RCU read-side critical section or in a code
510 segment where the pointer cannot change, for example, in
511 code protected by the corresponding update-side lock.
515 Without the <tt>rcu_dereference()</tt> or the
516 <tt>rcu_access_pointer()</tt>, what destructive optimizations
517 might the compiler make use of?
519 Let's start with what happens to <tt>do_something_gp()</tt>
520 if it fails to use <tt>rcu_dereference()</tt>.
521 It could reuse a value formerly fetched from this same pointer.
522 It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
523 manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
524 mash-up of two distince pointer values.
525 It might even use value-speculation optimizations, where it makes a wrong
526 guess, but by the time it gets around to checking the value, an update
527 has changed the pointer to match the wrong guess.
528 Too bad about any dereferences that returned pre-initialization garbage
532 For <tt>remove_gp_synchronous()</tt>, as long as all modifications
533 to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
534 the above optimizations are harmless.
536 with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
537 <tt>sparse</tt> will complain if you
538 define <tt>gp</tt> with <tt>__rcu</tt> and then
539 access it without using
540 either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
544 In short, RCU's publish-subscribe guarantee is provided by the combination
545 of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>.
546 This guarantee allows data elements to be safely added to RCU-protected
547 linked data structures without disrupting RCU readers.
548 This guarantee can be used in combination with the grace-period
549 guarantee to also allow data elements to be removed from RCU-protected
550 linked data structures, again without disrupting RCU readers.
553 This guarantee was only partially premeditated.
554 DYNIX/ptx used an explicit memory barrier for publication, but had nothing
555 resembling <tt>rcu_dereference()</tt> for subscription, nor did it
556 have anything resembling the <tt>smp_read_barrier_depends()</tt>
557 that was later subsumed into <tt>rcu_dereference()</tt>.
558 The need for these operations made itself known quite suddenly at a
559 late-1990s meeting with the DEC Alpha architects, back in the days when
560 DEC was still a free-standing company.
561 It took the Alpha architects a good hour to convince me that any sort
562 of barrier would ever be needed, and it then took me a good <i>two</i> hours
563 to convince them that their documentation did not make this point clear.
564 More recent work with the C and C++ standards committees have provided
565 much education on tricks and traps from the compiler.
566 In short, compilers were much less tricky in the early 1990s, but in
567 2015, don't even think about omitting <tt>rcu_dereference()</tt>!
569 <h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3>
572 The previous section's simple linked-data-structure scenario clearly
573 demonstrates the need for RCU's stringent memory-ordering guarantees on
574 systems with more than one CPU:
577 <li> Each CPU that has an RCU read-side critical section that
578 begins before <tt>synchronize_rcu()</tt> starts is
579 guaranteed to execute a full memory barrier between the time
580 that the RCU read-side critical section ends and the time that
581 <tt>synchronize_rcu()</tt> returns.
582 Without this guarantee, a pre-existing RCU read-side critical section
583 might hold a reference to the newly removed <tt>struct foo</tt>
584 after the <tt>kfree()</tt> on line 14 of
585 <tt>remove_gp_synchronous()</tt>.
586 <li> Each CPU that has an RCU read-side critical section that ends
587 after <tt>synchronize_rcu()</tt> returns is guaranteed
588 to execute a full memory barrier between the time that
589 <tt>synchronize_rcu()</tt> begins and the time that the RCU
590 read-side critical section begins.
591 Without this guarantee, a later RCU read-side critical section
592 running after the <tt>kfree()</tt> on line 14 of
593 <tt>remove_gp_synchronous()</tt> might
594 later run <tt>do_something_gp()</tt> and find the
595 newly deleted <tt>struct foo</tt>.
596 <li> If the task invoking <tt>synchronize_rcu()</tt> remains
597 on a given CPU, then that CPU is guaranteed to execute a full
598 memory barrier sometime during the execution of
599 <tt>synchronize_rcu()</tt>.
600 This guarantee ensures that the <tt>kfree()</tt> on
601 line 14 of <tt>remove_gp_synchronous()</tt> really does
602 execute after the removal on line 11.
603 <li> If the task invoking <tt>synchronize_rcu()</tt> migrates
604 among a group of CPUs during that invocation, then each of the
605 CPUs in that group is guaranteed to execute a full memory barrier
606 sometime during the execution of <tt>synchronize_rcu()</tt>.
607 This guarantee also ensures that the <tt>kfree()</tt> on
608 line 14 of <tt>remove_gp_synchronous()</tt> really does
609 execute after the removal on
610 line 11, but also in the case where the thread executing the
611 <tt>synchronize_rcu()</tt> migrates in the meantime.
615 Given that multiple CPUs can start RCU read-side critical sections
616 at any time without any ordering whatsoever, how can RCU possibly tell whether
617 or not a given RCU read-side critical section starts before a
618 given instance of <tt>synchronize_rcu()</tt>?
620 If RCU cannot tell whether or not a given
621 RCU read-side critical section starts before a
622 given instance of <tt>synchronize_rcu()</tt>,
623 then it must assume that the RCU read-side critical section
625 In other words, a given instance of <tt>synchronize_rcu()</tt>
626 can avoid waiting on a given RCU read-side critical section only
627 if it can prove that <tt>synchronize_rcu()</tt> started first.
631 The first and second guarantees require unbelievably strict ordering!
632 Are all these memory barriers <i> really</i> required?
634 Yes, they really are required.
635 To see why the first guarantee is required, consider the following
639 <li> CPU 1: <tt>rcu_read_lock()</tt>
640 <li> CPU 1: <tt>q = rcu_dereference(gp);
641 /* Very likely to return p. */</tt>
642 <li> CPU 0: <tt>list_del_rcu(p);</tt>
643 <li> CPU 0: <tt>synchronize_rcu()</tt> starts.
644 <li> CPU 1: <tt>do_something_with(q->a);
645 /* No smp_mb(), so might happen after kfree(). */</tt>
646 <li> CPU 1: <tt>rcu_read_unlock()</tt>
647 <li> CPU 0: <tt>synchronize_rcu()</tt> returns.
648 <li> CPU 0: <tt>kfree(p);</tt>
652 Therefore, there absolutely must be a full memory barrier between the
653 end of the RCU read-side critical section and the end of the
657 The sequence of events demonstrating the necessity of the second rule
661 <li> CPU 0: <tt>list_del_rcu(p);</tt>
662 <li> CPU 0: <tt>synchronize_rcu()</tt> starts.
663 <li> CPU 1: <tt>rcu_read_lock()</tt>
664 <li> CPU 1: <tt>q = rcu_dereference(gp);
665 /* Might return p if no memory barrier. */</tt>
666 <li> CPU 0: <tt>synchronize_rcu()</tt> returns.
667 <li> CPU 0: <tt>kfree(p);</tt>
668 <li> CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt>
669 <li> CPU 1: <tt>rcu_read_unlock()</tt>
673 And similarly, without a memory barrier between the beginning of the
674 grace period and the beginning of the RCU read-side critical section,
675 CPU 1 might end up accessing the freelist.
678 The “as if” rule of course applies, so that any implementation
679 that acts as if the appropriate memory barriers were in place is a
680 correct implementation.
681 That said, it is much easier to fool yourself into believing that you have
682 adhered to the as-if rule than it is to actually adhere to it!
686 Note that these memory-barrier requirements do not replace the fundamental
687 RCU requirement that a grace period wait for all pre-existing readers.
688 On the contrary, the memory barriers called out in this section must operate in
689 such a way as to <i>enforce</i> this fundamental requirement.
690 Of course, different implementations enforce this requirement in different
691 ways, but enforce it they must.
693 <h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3>
696 The common-case RCU primitives are unconditional.
697 They are invoked, they do their job, and they return, with no possibility
698 of error, and no need to retry.
699 This is a key RCU design philosophy.
702 However, this philosophy is pragmatic rather than pigheaded.
703 If someone comes up with a good justification for a particular conditional
704 RCU primitive, it might well be implemented and added.
705 After all, this guarantee was reverse-engineered, not premeditated.
706 The unconditional nature of the RCU primitives was initially an
707 accident of implementation, and later experience with synchronization
708 primitives with conditional primitives caused me to elevate this
709 accident to a guarantee.
710 Therefore, the justification for adding a conditional primitive to
711 RCU would need to be based on detailed and compelling use cases.
713 <h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3>
716 As far as RCU is concerned, it is always possible to carry out an
717 update within an RCU read-side critical section.
718 For example, that RCU read-side critical section might search for
719 a given data element, and then might acquire the update-side
720 spinlock in order to update that element, all while remaining
721 in that RCU read-side critical section.
722 Of course, it is necessary to exit the RCU read-side critical section
723 before invoking <tt>synchronize_rcu()</tt>, however, this
724 inconvenience can be avoided through use of the
725 <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
726 described later in this document.
729 But how does the upgrade-to-write operation exclude other readers?
731 It doesn't, just like normal RCU updates, which also do not exclude
736 This guarantee allows lookup code to be shared between read-side
737 and update-side code, and was premeditated, appearing in the earliest
738 DYNIX/ptx RCU documentation.
740 <h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2>
743 RCU provides extremely lightweight readers, and its read-side guarantees,
744 though quite useful, are correspondingly lightweight.
745 It is therefore all too easy to assume that RCU is guaranteeing more
747 Of course, the list of things that RCU does not guarantee is infinitely
748 long, however, the following sections list a few non-guarantees that
749 have caused confusion.
750 Except where otherwise noted, these non-guarantees were premeditated.
753 <li> <a href="#Readers Impose Minimal Ordering">
754 Readers Impose Minimal Ordering</a>
755 <li> <a href="#Readers Do Not Exclude Updaters">
756 Readers Do Not Exclude Updaters</a>
757 <li> <a href="#Updaters Only Wait For Old Readers">
758 Updaters Only Wait For Old Readers</a>
759 <li> <a href="#Grace Periods Don't Partition Read-Side Critical Sections">
760 Grace Periods Don't Partition Read-Side Critical Sections</a>
761 <li> <a href="#Read-Side Critical Sections Don't Partition Grace Periods">
762 Read-Side Critical Sections Don't Partition Grace Periods</a>
763 <li> <a href="#Disabling Preemption Does Not Block Grace Periods">
764 Disabling Preemption Does Not Block Grace Periods</a>
767 <h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
770 Reader-side markers such as <tt>rcu_read_lock()</tt> and
771 <tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees
772 except through their interaction with the grace-period APIs such as
773 <tt>synchronize_rcu()</tt>.
774 To see this, consider the following pair of threads:
788 11 void thread1(void)
791 14 r1 = READ_ONCE(y);
792 15 rcu_read_unlock();
794 17 r2 = READ_ONCE(x);
795 18 rcu_read_unlock();
801 After <tt>thread0()</tt> and <tt>thread1()</tt> execute
802 concurrently, it is quite possible to have
806 (r1 == 1 && r2 == 0)
810 (that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>),
811 which would not be possible if <tt>rcu_read_lock()</tt> and
812 <tt>rcu_read_unlock()</tt> had much in the way of ordering
814 But they do not, so the CPU is within its rights
815 to do significant reordering.
816 This is by design: Any significant ordering constraints would slow down
817 these fast-path APIs.
820 Can't the compiler also reorder this code?
822 No, the volatile casts in <tt>READ_ONCE()</tt> and
823 <tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
824 this particular case.
827 <h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
830 Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt>
832 All they do is to prevent grace periods from ending.
833 The following example illustrates this:
842 6 do_something_with_nonzero_x();
844 8 WARN_ON(!r2); /* BUG!!! */
846 10 rcu_read_unlock();
849 13 void thread1(void)
851 15 spin_lock(&my_lock);
854 18 spin_unlock(&my_lock);
860 If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt>
861 excluded the <tt>thread1()</tt> function's update,
862 the <tt>WARN_ON()</tt> could never fire.
863 But the fact is that <tt>rcu_read_lock()</tt> does not exclude
864 much of anything aside from subsequent grace periods, of which
865 <tt>thread1()</tt> has none, so the
866 <tt>WARN_ON()</tt> can and does fire.
868 <h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3>
871 It might be tempting to assume that after <tt>synchronize_rcu()</tt>
872 completes, there are no readers executing.
873 This temptation must be avoided because
874 new readers can start immediately after <tt>synchronize_rcu()</tt>
875 starts, and <tt>synchronize_rcu()</tt> is under no
876 obligation to wait for these new readers.
879 Suppose that synchronize_rcu() did wait until all readers had completed.
880 Would the updater be able to rely on this?
883 Even if <tt>synchronize_rcu()</tt> were to wait until
884 all readers had completed, a new reader might start immediately after
885 <tt>synchronize_rcu()</tt> completed.
886 Therefore, the code following
887 <tt>synchronize_rcu()</tt> cannot rely on there being no readers
891 <h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
892 Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
895 It is tempting to assume that if any part of one RCU read-side critical
896 section precedes a given grace period, and if any part of another RCU
897 read-side critical section follows that same grace period, then all of
898 the first RCU read-side critical section must precede all of the second.
899 However, this just isn't the case: A single grace period does not
900 partition the set of RCU read-side critical sections.
901 An example of this situation can be illustrated as follows, where
902 <tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero:
916 11 r1 = READ_ONCE(a);
917 12 synchronize_rcu();
921 16 void thread2(void)
924 19 r2 = READ_ONCE(b);
925 20 r3 = READ_ONCE(c);
926 21 rcu_read_unlock();
932 It turns out that the outcome:
936 (r1 == 1 && r2 == 0 && r3 == 1)
940 is entirely possible.
941 The following figure show how this can happen, with each circled
942 <tt>QS</tt> indicating the point at which RCU recorded a
943 <i>quiescent state</i> for each thread, that is, a state in which
944 RCU knows that the thread cannot be in the midst of an RCU read-side
945 critical section that started before the current grace period:
947 <p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p>
950 If it is necessary to partition RCU read-side critical sections in this
951 manner, it is necessary to use two grace periods, where the first
952 grace period is known to end before the second grace period starts:
966 11 r1 = READ_ONCE(a);
967 12 synchronize_rcu();
971 16 void thread2(void)
973 18 r2 = READ_ONCE(c);
974 19 synchronize_rcu();
978 23 void thread3(void)
981 26 r3 = READ_ONCE(b);
982 27 r4 = READ_ONCE(d);
983 28 rcu_read_unlock();
989 Here, if <tt>(r1 == 1)</tt>, then
990 <tt>thread0()</tt>'s write to <tt>b</tt> must happen
991 before the end of <tt>thread1()</tt>'s grace period.
992 If in addition <tt>(r4 == 1)</tt>, then
993 <tt>thread3()</tt>'s read from <tt>b</tt> must happen
994 after the beginning of <tt>thread2()</tt>'s grace period.
995 If it is also the case that <tt>(r2 == 1)</tt>, then the
996 end of <tt>thread1()</tt>'s grace period must precede the
997 beginning of <tt>thread2()</tt>'s grace period.
998 This mean that the two RCU read-side critical sections cannot overlap,
999 guaranteeing that <tt>(r3 == 1)</tt>.
1000 As a result, the outcome:
1004 (r1 == 1 && r2 == 1 && r3 == 0 && r4 == 1)
1011 This non-requirement was also non-premeditated, but became apparent
1012 when studying RCU's interaction with memory ordering.
1014 <h3><a name="Read-Side Critical Sections Don't Partition Grace Periods">
1015 Read-Side Critical Sections Don't Partition Grace Periods</a></h3>
1018 It is also tempting to assume that if an RCU read-side critical section
1019 happens between a pair of grace periods, then those grace periods cannot
1021 However, this temptation leads nowhere good, as can be illustrated by
1022 the following, with all variables initially zero:
1026 1 void thread0(void)
1031 6 rcu_read_unlock();
1034 9 void thread1(void)
1036 11 r1 = READ_ONCE(a);
1037 12 synchronize_rcu();
1038 13 WRITE_ONCE(c, 1);
1041 16 void thread2(void)
1044 19 WRITE_ONCE(d, 1);
1045 20 r2 = READ_ONCE(c);
1046 21 rcu_read_unlock();
1049 24 void thread3(void)
1051 26 r3 = READ_ONCE(d);
1052 27 synchronize_rcu();
1053 28 WRITE_ONCE(e, 1);
1056 31 void thread4(void)
1059 34 r4 = READ_ONCE(b);
1060 35 r5 = READ_ONCE(e);
1061 36 rcu_read_unlock();
1067 In this case, the outcome:
1071 (r1 == 1 && r2 == 1 && r3 == 1 && r4 == 0 && r5 == 1)
1075 is entirely possible, as illustrated below:
1077 <p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p>
1080 Again, an RCU read-side critical section can overlap almost all of a
1081 given grace period, just so long as it does not overlap the entire
1083 As a result, an RCU read-side critical section cannot partition a pair
1084 of RCU grace periods.
1087 How long a sequence of grace periods, each separated by an RCU read-side
1088 critical section, would be required to partition the RCU read-side
1089 critical sections at the beginning and end of the chain?
1091 In theory, an infinite number.
1092 In practice, an unknown number that is sensitive to both implementation
1093 details and timing considerations.
1094 Therefore, even in practice, RCU users must abide by the theoretical rather
1095 than the practical answer.
1098 <h3><a name="Disabling Preemption Does Not Block Grace Periods">
1099 Disabling Preemption Does Not Block Grace Periods</a></h3>
1102 There was a time when disabling preemption on any given CPU would block
1103 subsequent grace periods.
1104 However, this was an accident of implementation and is not a requirement.
1105 And in the current Linux-kernel implementation, disabling preemption
1106 on a given CPU in fact does not block grace periods, as Oleg Nesterov
1107 <a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
1110 If you need a preempt-disable region to block grace periods, you need to add
1111 <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
1116 1 preempt_disable();
1119 4 rcu_read_unlock();
1122 7 /* Spinlocks implicitly disable preemption. */
1123 8 spin_lock(&mylock);
1126 11 rcu_read_unlock();
1127 12 spin_unlock(&mylock);
1132 In theory, you could enter the RCU read-side critical section first,
1133 but it is more efficient to keep the entire RCU read-side critical
1134 section contained in the preempt-disable region as shown above.
1135 Of course, RCU read-side critical sections that extend outside of
1136 preempt-disable regions will work correctly, but such critical sections
1137 can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
1139 And no, this is <i>not</i> an invitation to enclose all of your RCU
1140 read-side critical sections within preempt-disable regions, because
1141 doing so would degrade real-time response.
1144 This non-requirement appeared with preemptible RCU.
1145 If you need a grace period that waits on non-preemptible code regions, use
1146 <a href="#Sched Flavor">RCU-sched</a>.
1148 <h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
1151 These parallelism facts of life are by no means specific to RCU, but
1152 the RCU implementation must abide by them.
1153 They therefore bear repeating:
1156 <li> Any CPU or task may be delayed at any time,
1157 and any attempts to avoid these delays by disabling
1158 preemption, interrupts, or whatever are completely futile.
1159 This is most obvious in preemptible user-level
1160 environments and in virtualized environments (where
1161 a given guest OS's VCPUs can be preempted at any time by
1162 the underlying hypervisor), but can also happen in bare-metal
1163 environments due to ECC errors, NMIs, and other hardware
1165 Although a delay of more than about 20 seconds can result
1166 in splats, the RCU implementation is obligated to use
1167 algorithms that can tolerate extremely long delays, but where
1168 “extremely long” is not long enough to allow
1169 wrap-around when incrementing a 64-bit counter.
1170 <li> Both the compiler and the CPU can reorder memory accesses.
1171 Where it matters, RCU must use compiler directives and
1172 memory-barrier instructions to preserve ordering.
1173 <li> Conflicting writes to memory locations in any given cache line
1174 will result in expensive cache misses.
1175 Greater numbers of concurrent writes and more-frequent
1176 concurrent writes will result in more dramatic slowdowns.
1177 RCU is therefore obligated to use algorithms that have
1178 sufficient locality to avoid significant performance and
1179 scalability problems.
1180 <li> As a rough rule of thumb, only one CPU's worth of processing
1181 may be carried out under the protection of any given exclusive
1183 RCU must therefore use scalable locking designs.
1184 <li> Counters are finite, especially on 32-bit systems.
1185 RCU's use of counters must therefore tolerate counter wrap,
1186 or be designed such that counter wrap would take way more
1187 time than a single system is likely to run.
1188 An uptime of ten years is quite possible, a runtime
1189 of a century much less so.
1190 As an example of the latter, RCU's dyntick-idle nesting counter
1191 allows 54 bits for interrupt nesting level (this counter
1192 is 64 bits even on a 32-bit system).
1193 Overflowing this counter requires 2<sup>54</sup>
1194 half-interrupts on a given CPU without that CPU ever going idle.
1195 If a half-interrupt happened every microsecond, it would take
1196 570 years of runtime to overflow this counter, which is currently
1197 believed to be an acceptably long time.
1198 <li> Linux systems can have thousands of CPUs running a single
1199 Linux kernel in a single shared-memory environment.
1200 RCU must therefore pay close attention to high-end scalability.
1204 This last parallelism fact of life means that RCU must pay special
1205 attention to the preceding facts of life.
1206 The idea that Linux might scale to systems with thousands of CPUs would
1207 have been met with some skepticism in the 1990s, but these requirements
1208 would have otherwise have been unsurprising, even in the early 1990s.
1210 <h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2>
1213 These sections list quality-of-implementation requirements.
1214 Although an RCU implementation that ignores these requirements could
1215 still be used, it would likely be subject to limitations that would
1216 make it inappropriate for industrial-strength production use.
1217 Classes of quality-of-implementation requirements are as follows:
1220 <li> <a href="#Specialization">Specialization</a>
1221 <li> <a href="#Performance and Scalability">Performance and Scalability</a>
1222 <li> <a href="#Composability">Composability</a>
1223 <li> <a href="#Corner Cases">Corner Cases</a>
1227 These classes is covered in the following sections.
1229 <h3><a name="Specialization">Specialization</a></h3>
1232 RCU is and always has been intended primarily for read-mostly situations, as
1233 illustrated by the following figure.
1234 This means that RCU's read-side primitives are optimized, often at the
1235 expense of its update-side primitives.
1237 <p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
1240 This focus on read-mostly situations means that RCU must interoperate
1241 with other synchronization primitives.
1242 For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt>
1243 examples discussed earlier use RCU to protect readers and locking to
1244 coordinate updaters.
1245 However, the need extends much farther, requiring that a variety of
1246 synchronization primitives be legal within RCU read-side critical sections,
1247 including spinlocks, sequence locks, atomic operations, reference
1248 counters, and memory barriers.
1251 What about sleeping locks?
1253 These are forbidden within Linux-kernel RCU read-side critical sections
1254 because it is not legal to place a quiescent state (in this case,
1255 voluntary context switch) within an RCU read-side critical section.
1256 However, sleeping locks may be used within userspace RCU read-side critical
1257 sections, and also within Linux-kernel sleepable RCU
1258 <a href="#Sleepable RCU">(SRCU)</a>
1259 read-side critical sections.
1260 In addition, the -rt patchset turns spinlocks into a sleeping locks so
1261 that the corresponding critical sections can be preempted, which
1262 also means that these sleeplockified spinlocks (but not other sleeping locks!)
1263 may be acquire within -rt-Linux-kernel RCU read-side critical sections.
1266 Note that it <i>is</i> legal for a normal RCU read-side critical section
1267 to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
1268 but only as long as it does not loop indefinitely attempting to
1269 conditionally acquire that sleeping locks.
1270 The key point is that things like <tt>mutex_trylock()</tt>
1271 either return with the mutex held, or return an error indication if
1272 the mutex was not immediately available.
1273 Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
1277 It often comes as a surprise that many algorithms do not require a
1278 consistent view of data, but many can function in that mode,
1279 with network routing being the poster child.
1280 Internet routing algorithms take significant time to propagate
1281 updates, so that by the time an update arrives at a given system,
1282 that system has been sending network traffic the wrong way for
1283 a considerable length of time.
1284 Having a few threads continue to send traffic the wrong way for a
1285 few more milliseconds is clearly not a problem: In the worst case,
1286 TCP retransmissions will eventually get the data where it needs to go.
1287 In general, when tracking the state of the universe outside of the
1288 computer, some level of inconsistency must be tolerated due to
1289 speed-of-light delays if nothing else.
1292 Furthermore, uncertainty about external state is inherent in many cases.
1293 For example, a pair of veternarians might use heartbeat to determine
1294 whether or not a given cat was alive.
1295 But how long should they wait after the last heartbeat to decide that
1296 the cat is in fact dead?
1297 Waiting less than 400 milliseconds makes no sense because this would
1298 mean that a relaxed cat would be considered to cycle between death
1299 and life more than 100 times per minute.
1300 Moreover, just as with human beings, a cat's heart might stop for
1301 some period of time, so the exact wait period is a judgment call.
1302 One of our pair of veternarians might wait 30 seconds before pronouncing
1303 the cat dead, while the other might insist on waiting a full minute.
1304 The two veternarians would then disagree on the state of the cat during
1305 the final 30 seconds of the minute following the last heartbeat, as
1306 fancifully illustrated below:
1308 <p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
1311 Interestingly enough, this same situation applies to hardware.
1312 When push comes to shove, how do we tell whether or not some
1313 external server has failed?
1314 We send messages to it periodically, and declare it failed if we
1315 don't receive a response within a given period of time.
1316 Policy decisions can usually tolerate short
1317 periods of inconsistency.
1318 The policy was decided some time ago, and is only now being put into
1319 effect, so a few milliseconds of delay is normally inconsequential.
1322 However, there are algorithms that absolutely must see consistent data.
1323 For example, the translation between a user-level SystemV semaphore
1324 ID to the corresponding in-kernel data structure is protected by RCU,
1325 but it is absolutely forbidden to update a semaphore that has just been
1327 In the Linux kernel, this need for consistency is accommodated by acquiring
1328 spinlocks located in the in-kernel data structure from within
1329 the RCU read-side critical section, and this is indicated by the
1330 green box in the figure above.
1331 Many other techniques may be used, and are in fact used within the
1335 In short, RCU is not required to maintain consistency, and other
1336 mechanisms may be used in concert with RCU when consistency is required.
1337 RCU's specialization allows it to do its job extremely well, and its
1338 ability to interoperate with other synchronization mechanisms allows
1339 the right mix of synchronization tools to be used for a given job.
1341 <h3><a name="Performance and Scalability">Performance and Scalability</a></h3>
1344 Energy efficiency is a critical component of performance today,
1345 and Linux-kernel RCU implementations must therefore avoid unnecessarily
1346 awakening idle CPUs.
1347 I cannot claim that this requirement was premeditated.
1348 In fact, I learned of it during a telephone conversation in which I
1349 was given “frank and open” feedback on the importance
1350 of energy efficiency in battery-powered systems and on specific
1351 energy-efficiency shortcomings of the Linux-kernel RCU implementation.
1352 In my experience, the battery-powered embedded community will consider
1353 any unnecessary wakeups to be extremely unfriendly acts.
1354 So much so that mere Linux-kernel-mailing-list posts are
1355 insufficient to vent their ire.
1358 Memory consumption is not particularly important for in most
1359 situations, and has become decreasingly
1360 so as memory sizes have expanded and memory
1361 costs have plummeted.
1362 However, as I learned from Matt Mackall's
1363 <a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a>
1364 efforts, memory footprint is critically important on single-CPU systems with
1365 non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus
1366 <a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a>
1368 Josh Triplett has since taken over the small-memory banner with his
1369 <a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a>
1370 project, which resulted in
1371 <a href="#Sleepable RCU">SRCU</a>
1372 becoming optional for those kernels not needing it.
1375 The remaining performance requirements are, for the most part,
1377 For example, in keeping with RCU's read-side specialization,
1378 <tt>rcu_dereference()</tt> should have negligible overhead (for
1379 example, suppression of a few minor compiler optimizations).
1380 Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and
1381 <tt>rcu_read_unlock()</tt> should have exactly zero overhead.
1384 In preemptible environments, in the case where the RCU read-side
1385 critical section was not preempted (as will be the case for the
1386 highest-priority real-time process), <tt>rcu_read_lock()</tt> and
1387 <tt>rcu_read_unlock()</tt> should have minimal overhead.
1388 In particular, they should not contain atomic read-modify-write
1389 operations, memory-barrier instructions, preemption disabling,
1390 interrupt disabling, or backwards branches.
1391 However, in the case where the RCU read-side critical section was preempted,
1392 <tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts.
1393 This is why it is better to nest an RCU read-side critical section
1394 within a preempt-disable region than vice versa, at least in cases
1395 where that critical section is short enough to avoid unduly degrading
1396 real-time latencies.
1399 The <tt>synchronize_rcu()</tt> grace-period-wait primitive is
1400 optimized for throughput.
1401 It may therefore incur several milliseconds of latency in addition to
1402 the duration of the longest RCU read-side critical section.
1403 On the other hand, multiple concurrent invocations of
1404 <tt>synchronize_rcu()</tt> are required to use batching optimizations
1405 so that they can be satisfied by a single underlying grace-period-wait
1407 For example, in the Linux kernel, it is not unusual for a single
1408 grace-period-wait operation to serve more than
1409 <a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a>
1410 of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation
1411 overhead down to nearly zero.
1412 However, the grace-period optimization is also required to avoid
1413 measurable degradation of real-time scheduling and interrupt latencies.
1416 In some cases, the multi-millisecond <tt>synchronize_rcu()</tt>
1417 latencies are unacceptable.
1418 In these cases, <tt>synchronize_rcu_expedited()</tt> may be used
1419 instead, reducing the grace-period latency down to a few tens of
1420 microseconds on small systems, at least in cases where the RCU read-side
1421 critical sections are short.
1422 There are currently no special latency requirements for
1423 <tt>synchronize_rcu_expedited()</tt> on large systems, but,
1424 consistent with the empirical nature of the RCU specification,
1425 that is subject to change.
1426 However, there most definitely are scalability requirements:
1427 A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096
1428 CPUs should at least make reasonable forward progress.
1429 In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt>
1430 is permitted to impose modest degradation of real-time latency
1431 on non-idle online CPUs.
1432 That said, it will likely be necessary to take further steps to reduce this
1433 degradation, hopefully to roughly that of a scheduling-clock interrupt.
1436 There are a number of situations where even
1437 <tt>synchronize_rcu_expedited()</tt>'s reduced grace-period
1438 latency is unacceptable.
1439 In these situations, the asynchronous <tt>call_rcu()</tt> can be
1440 used in place of <tt>synchronize_rcu()</tt> as follows:
1447 4 struct rcu_head rh;
1450 7 static void remove_gp_cb(struct rcu_head *rhp)
1452 9 struct foo *p = container_of(rhp, struct foo, rh);
1457 14 bool remove_gp_asynchronous(void)
1461 18 spin_lock(&gp_lock);
1462 19 p = rcu_dereference(gp);
1464 21 spin_unlock(&gp_lock);
1467 24 rcu_assign_pointer(gp, NULL);
1468 25 call_rcu(&p->rh, remove_gp_cb);
1469 26 spin_unlock(&gp_lock);
1476 A definition of <tt>struct foo</tt> is finally needed, and appears
1478 The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt>
1479 on line 25, and will be invoked after the end of a subsequent
1481 This gets the same effect as <tt>remove_gp_synchronous()</tt>,
1482 but without forcing the updater to wait for a grace period to elapse.
1483 The <tt>call_rcu()</tt> function may be used in a number of
1484 situations where neither <tt>synchronize_rcu()</tt> nor
1485 <tt>synchronize_rcu_expedited()</tt> would be legal,
1486 including within preempt-disable code, <tt>local_bh_disable()</tt> code,
1487 interrupt-disable code, and interrupt handlers.
1488 However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
1489 The callback function (<tt>remove_gp_cb()</tt> in this case) will be
1490 executed within softirq (software interrupt) environment within the
1492 either within a real softirq handler or under the protection
1493 of <tt>local_bh_disable()</tt>.
1494 In both the Linux kernel and in userspace, it is bad practice to
1495 write an RCU callback function that takes too long.
1496 Long-running operations should be relegated to separate threads or
1497 (in the Linux kernel) workqueues.
1500 Why does line 19 use <tt>rcu_access_pointer()</tt>?
1501 After all, <tt>call_rcu()</tt> on line 25 stores into the
1502 structure, which would interact badly with concurrent insertions.
1503 Doesn't this mean that <tt>rcu_dereference()</tt> is required?
1505 Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes
1506 any changes, including any insertions that <tt>rcu_dereference()</tt>
1507 would protect against.
1508 Therefore, any insertions will be delayed until after <tt>->gp_lock</tt>
1509 is released on line 25, which in turn means that
1510 <tt>rcu_access_pointer()</tt> suffices.
1514 However, all that <tt>remove_gp_cb()</tt> is doing is
1515 invoking <tt>kfree()</tt> on the data element.
1516 This is a common idiom, and is supported by <tt>kfree_rcu()</tt>,
1517 which allows “fire and forget” operation as shown below:
1524 4 struct rcu_head rh;
1527 7 bool remove_gp_faf(void)
1531 11 spin_lock(&gp_lock);
1532 12 p = rcu_dereference(gp);
1534 14 spin_unlock(&gp_lock);
1537 17 rcu_assign_pointer(gp, NULL);
1538 18 kfree_rcu(p, rh);
1539 19 spin_unlock(&gp_lock);
1546 Note that <tt>remove_gp_faf()</tt> simply invokes
1547 <tt>kfree_rcu()</tt> and proceeds, without any need to pay any
1548 further attention to the subsequent grace period and <tt>kfree()</tt>.
1549 It is permissible to invoke <tt>kfree_rcu()</tt> from the same
1550 environments as for <tt>call_rcu()</tt>.
1551 Interestingly enough, DYNIX/ptx had the equivalents of
1552 <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not
1553 <tt>synchronize_rcu()</tt>.
1554 This was due to the fact that RCU was not heavily used within DYNIX/ptx,
1555 so the very few places that needed something like
1556 <tt>synchronize_rcu()</tt> simply open-coded it.
1559 Earlier it was claimed that <tt>call_rcu()</tt> and
1560 <tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
1562 But how can that be correct, given that the invocation of the callback
1563 and the freeing of the memory (respectively) must still wait for
1564 a grace period to elapse?
1566 We could define things this way, but keep in mind that this sort of
1567 definition would say that updates in garbage-collected languages
1568 cannot complete until the next time the garbage collector runs,
1569 which does not seem at all reasonable.
1570 The key point is that in most cases, an updater using either
1571 <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
1572 next update as soon as it has invoked <tt>call_rcu()</tt> or
1573 <tt>kfree_rcu()</tt>, without having to wait for a subsequent
1578 But what if the updater must wait for the completion of code to be
1579 executed after the end of the grace period, but has other tasks
1580 that can be carried out in the meantime?
1581 The polling-style <tt>get_state_synchronize_rcu()</tt> and
1582 <tt>cond_synchronize_rcu()</tt> functions may be used for this
1583 purpose, as shown below:
1587 1 bool remove_gp_poll(void)
1592 6 spin_lock(&gp_lock);
1593 7 p = rcu_access_pointer(gp);
1595 9 spin_unlock(&gp_lock);
1598 12 rcu_assign_pointer(gp, NULL);
1599 13 spin_unlock(&gp_lock);
1600 14 s = get_state_synchronize_rcu();
1601 15 do_something_while_waiting();
1602 16 cond_synchronize_rcu(s);
1610 On line 14, <tt>get_state_synchronize_rcu()</tt> obtains a
1611 “cookie” from RCU,
1612 then line 15 carries out other tasks,
1613 and finally, line 16 returns immediately if a grace period has
1614 elapsed in the meantime, but otherwise waits as required.
1615 The need for <tt>get_state_synchronize_rcu</tt> and
1616 <tt>cond_synchronize_rcu()</tt> has appeared quite recently,
1617 so it is too early to tell whether they will stand the test of time.
1620 RCU thus provides a range of tools to allow updaters to strike the
1621 required tradeoff between latency, flexibility and CPU overhead.
1623 <h3><a name="Composability">Composability</a></h3>
1626 Composability has received much attention in recent years, perhaps in part
1627 due to the collision of multicore hardware with object-oriented techniques
1628 designed in single-threaded environments for single-threaded use.
1629 And in theory, RCU read-side critical sections may be composed, and in
1630 fact may be nested arbitrarily deeply.
1631 In practice, as with all real-world implementations of composable
1632 constructs, there are limitations.
1635 Implementations of RCU for which <tt>rcu_read_lock()</tt>
1636 and <tt>rcu_read_unlock()</tt> generate no code, such as
1637 Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be
1638 nested arbitrarily deeply.
1639 After all, there is no overhead.
1640 Except that if all these instances of <tt>rcu_read_lock()</tt>
1641 and <tt>rcu_read_unlock()</tt> are visible to the compiler,
1642 compilation will eventually fail due to exhausting memory,
1643 mass storage, or user patience, whichever comes first.
1644 If the nesting is not visible to the compiler, as is the case with
1645 mutually recursive functions each in its own translation unit,
1646 stack overflow will result.
1647 If the nesting takes the form of loops, either the control variable
1648 will overflow or (in the Linux kernel) you will get an RCU CPU stall warning.
1649 Nevertheless, this class of RCU implementations is one
1650 of the most composable constructs in existence.
1653 RCU implementations that explicitly track nesting depth
1654 are limited by the nesting-depth counter.
1655 For example, the Linux kernel's preemptible RCU limits nesting to
1657 This should suffice for almost all practical purposes.
1658 That said, a consecutive pair of RCU read-side critical sections
1659 between which there is an operation that waits for a grace period
1660 cannot be enclosed in another RCU read-side critical section.
1661 This is because it is not legal to wait for a grace period within
1662 an RCU read-side critical section: To do so would result either
1664 in RCU implicitly splitting the enclosing RCU read-side critical
1665 section, neither of which is conducive to a long-lived and prosperous
1669 It is worth noting that RCU is not alone in limiting composability.
1670 For example, many transactional-memory implementations prohibit
1671 composing a pair of transactions separated by an irrevocable
1672 operation (for example, a network receive operation).
1673 For another example, lock-based critical sections can be composed
1674 surprisingly freely, but only if deadlock is avoided.
1677 In short, although RCU read-side critical sections are highly composable,
1678 care is required in some situations, just as is the case for any other
1679 composable synchronization mechanism.
1681 <h3><a name="Corner Cases">Corner Cases</a></h3>
1684 A given RCU workload might have an endless and intense stream of
1685 RCU read-side critical sections, perhaps even so intense that there
1686 was never a point in time during which there was not at least one
1687 RCU read-side critical section in flight.
1688 RCU cannot allow this situation to block grace periods: As long as
1689 all the RCU read-side critical sections are finite, grace periods
1690 must also be finite.
1693 That said, preemptible RCU implementations could potentially result
1694 in RCU read-side critical sections being preempted for long durations,
1695 which has the effect of creating a long-duration RCU read-side
1697 This situation can arise only in heavily loaded systems, but systems using
1698 real-time priorities are of course more vulnerable.
1699 Therefore, RCU priority boosting is provided to help deal with this
1701 That said, the exact requirements on RCU priority boosting will likely
1702 evolve as more experience accumulates.
1705 Other workloads might have very high update rates.
1706 Although one can argue that such workloads should instead use
1707 something other than RCU, the fact remains that RCU must
1708 handle such workloads gracefully.
1709 This requirement is another factor driving batching of grace periods,
1710 but it is also the driving force behind the checks for large numbers
1711 of queued RCU callbacks in the <tt>call_rcu()</tt> code path.
1712 Finally, high update rates should not delay RCU read-side critical
1713 sections, although some read-side delays can occur when using
1714 <tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use
1715 of <tt>try_stop_cpus()</tt>.
1716 (In the future, <tt>synchronize_rcu_expedited()</tt> will be
1717 converted to use lighter-weight inter-processor interrupts (IPIs),
1718 but this will still disturb readers, though to a much smaller degree.)
1721 Although all three of these corner cases were understood in the early
1722 1990s, a simple user-level test consisting of <tt>close(open(path))</tt>
1724 in the early 2000s suddenly provided a much deeper appreciation of the
1725 high-update-rate corner case.
1726 This test also motivated addition of some RCU code to react to high update
1727 rates, for example, if a given CPU finds itself with more than 10,000
1728 RCU callbacks queued, it will cause RCU to take evasive action by
1729 more aggressively starting grace periods and more aggressively forcing
1730 completion of grace-period processing.
1731 This evasive action causes the grace period to complete more quickly,
1732 but at the cost of restricting RCU's batching optimizations, thus
1733 increasing the CPU overhead incurred by that grace period.
1735 <h2><a name="Software-Engineering Requirements">
1736 Software-Engineering Requirements</a></h2>
1739 Between Murphy's Law and “To err is human”, it is necessary to
1740 guard against mishaps and misuse:
1743 <li> It is all too easy to forget to use <tt>rcu_read_lock()</tt>
1744 everywhere that it is needed, so kernels built with
1745 <tt>CONFIG_PROVE_RCU=y</tt> will spat if
1746 <tt>rcu_dereference()</tt> is used outside of an
1747 RCU read-side critical section.
1748 Update-side code can use <tt>rcu_dereference_protected()</tt>,
1750 <a href="https://lwn.net/Articles/371986/">lockdep expression</a>
1751 to indicate what is providing the protection.
1752 If the indicated protection is not provided, a lockdep splat
1756 Code shared between readers and updaters can use
1757 <tt>rcu_dereference_check()</tt>, which also takes a
1758 lockdep expression, and emits a lockdep splat if neither
1759 <tt>rcu_read_lock()</tt> nor the indicated protection
1761 In addition, <tt>rcu_dereference_raw()</tt> is used in those
1762 (hopefully rare) cases where the required protection cannot
1763 be easily described.
1764 Finally, <tt>rcu_read_lock_held()</tt> is provided to
1765 allow a function to verify that it has been invoked within
1766 an RCU read-side critical section.
1767 I was made aware of this set of requirements shortly after Thomas
1768 Gleixner audited a number of RCU uses.
1769 <li> A given function might wish to check for RCU-related preconditions
1770 upon entry, before using any other RCU API.
1771 The <tt>rcu_lockdep_assert()</tt> does this job,
1772 asserting the expression in kernels having lockdep enabled
1773 and doing nothing otherwise.
1774 <li> It is also easy to forget to use <tt>rcu_assign_pointer()</tt>
1775 and <tt>rcu_dereference()</tt>, perhaps (incorrectly)
1776 substituting a simple assignment.
1777 To catch this sort of error, a given RCU-protected pointer may be
1778 tagged with <tt>__rcu</tt>, after which running sparse
1779 with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain
1780 about simple-assignment accesses to that pointer.
1781 Arnd Bergmann made me aware of this requirement, and also
1783 <a href="https://lwn.net/Articles/376011/">patch series</a>.
1784 <li> Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt>
1785 will splat if a data element is passed to <tt>call_rcu()</tt>
1786 twice in a row, without a grace period in between.
1787 (This error is similar to a double free.)
1788 The corresponding <tt>rcu_head</tt> structures that are
1789 dynamically allocated are automatically tracked, but
1790 <tt>rcu_head</tt> structures allocated on the stack
1791 must be initialized with <tt>init_rcu_head_on_stack()</tt>
1792 and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>.
1793 Similarly, statically allocated non-stack <tt>rcu_head</tt>
1794 structures must be initialized with <tt>init_rcu_head()</tt>
1795 and cleaned up with <tt>destroy_rcu_head()</tt>.
1796 Mathieu Desnoyers made me aware of this requirement, and also
1798 <a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>.
1799 <li> An infinite loop in an RCU read-side critical section will
1800 eventually trigger an RCU CPU stall warning splat, with
1801 the duration of “eventually” being controlled by the
1802 <tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or,
1803 alternatively, by the
1804 <tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs
1806 However, RCU is not obligated to produce this splat
1807 unless there is a grace period waiting on that particular
1808 RCU read-side critical section.
1810 Some extreme workloads might intentionally delay
1811 RCU grace periods, and systems running those workloads can
1812 be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt>
1813 to suppress the splats.
1814 This kernel parameter may also be set via <tt>sysfs</tt>.
1815 Furthermore, RCU CPU stall warnings are counter-productive
1816 during sysrq dumps and during panics.
1817 RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and
1818 <tt>rcu_sysrq_end()</tt> API members to be called before
1819 and after long sysrq dumps.
1820 RCU also supplies the <tt>rcu_panic()</tt> notifier that is
1821 automatically invoked at the beginning of a panic to suppress
1822 further RCU CPU stall warnings.
1825 This requirement made itself known in the early 1990s, pretty
1826 much the first time that it was necessary to debug a CPU stall.
1827 That said, the initial implementation in DYNIX/ptx was quite
1828 generic in comparison with that of Linux.
1829 <li> Although it would be very good to detect pointers leaking out
1830 of RCU read-side critical sections, there is currently no
1831 good way of doing this.
1832 One complication is the need to distinguish between pointers
1833 leaking and pointers that have been handed off from RCU to
1834 some other synchronization mechanism, for example, reference
1836 <li> In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related
1837 information is provided via both debugfs and event tracing.
1838 <li> Open-coded use of <tt>rcu_assign_pointer()</tt> and
1839 <tt>rcu_dereference()</tt> to create typical linked
1840 data structures can be surprisingly error-prone.
1841 Therefore, RCU-protected
1842 <a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a>
1843 and, more recently, RCU-protected
1844 <a href="https://lwn.net/Articles/612100/">hash tables</a>
1846 Many other special-purpose RCU-protected data structures are
1847 available in the Linux kernel and the userspace RCU library.
1848 <li> Some linked structures are created at compile time, but still
1849 require <tt>__rcu</tt> checking.
1850 The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this
1852 <li> It is not necessary to use <tt>rcu_assign_pointer()</tt>
1853 when creating linked structures that are to be published via
1854 a single external pointer.
1855 The <tt>RCU_INIT_POINTER()</tt> macro is provided for
1856 this task and also for assigning <tt>NULL</tt> pointers
1861 This not a hard-and-fast list: RCU's diagnostic capabilities will
1862 continue to be guided by the number and type of usage bugs found
1863 in real-world RCU usage.
1865 <h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2>
1868 The Linux kernel provides an interesting environment for all kinds of
1869 software, including RCU.
1870 Some of the relevant points of interest are as follows:
1873 <li> <a href="#Configuration">Configuration</a>.
1874 <li> <a href="#Firmware Interface">Firmware Interface</a>.
1875 <li> <a href="#Early Boot">Early Boot</a>.
1876 <li> <a href="#Interrupts and NMIs">
1877 Interrupts and non-maskable interrupts (NMIs)</a>.
1878 <li> <a href="#Loadable Modules">Loadable Modules</a>.
1879 <li> <a href="#Hotplug CPU">Hotplug CPU</a>.
1880 <li> <a href="#Scheduler and RCU">Scheduler and RCU</a>.
1881 <li> <a href="#Tracing and RCU">Tracing and RCU</a>.
1882 <li> <a href="#Energy Efficiency">Energy Efficiency</a>.
1883 <li> <a href="#Memory Efficiency">Memory Efficiency</a>.
1884 <li> <a href="#Performance, Scalability, Response Time, and Reliability">
1885 Performance, Scalability, Response Time, and Reliability</a>.
1889 This list is probably incomplete, but it does give a feel for the
1890 most notable Linux-kernel complications.
1891 Each of the following sections covers one of the above topics.
1893 <h3><a name="Configuration">Configuration</a></h3>
1896 RCU's goal is automatic configuration, so that almost nobody
1897 needs to worry about RCU's <tt>Kconfig</tt> options.
1898 And for almost all users, RCU does in fact work well
1899 “out of the box.”
1902 However, there are specialized use cases that are handled by
1903 kernel boot parameters and <tt>Kconfig</tt> options.
1904 Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users
1905 about new <tt>Kconfig</tt> options, which requires almost all of them
1906 be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option.
1909 This all should be quite obvious, but the fact remains that
1910 Linus Torvalds recently had to
1911 <a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a>
1912 me of this requirement.
1914 <h3><a name="Firmware Interface">Firmware Interface</a></h3>
1917 In many cases, kernel obtains information about the system from the
1918 firmware, and sometimes things are lost in translation.
1919 Or the translation is accurate, but the original message is bogus.
1922 For example, some systems' firmware overreports the number of CPUs,
1923 sometimes by a large factor.
1924 If RCU naively believed the firmware, as it used to do,
1925 it would create too many per-CPU kthreads.
1926 Although the resulting system will still run correctly, the extra
1927 kthreads needlessly consume memory and can cause confusion
1928 when they show up in <tt>ps</tt> listings.
1931 RCU must therefore wait for a given CPU to actually come online before
1932 it can allow itself to believe that the CPU actually exists.
1933 The resulting “ghost CPUs” (which are never going to
1934 come online) cause a number of
1935 <a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>.
1937 <h3><a name="Early Boot">Early Boot</a></h3>
1940 The Linux kernel's boot sequence is an interesting process,
1941 and RCU is used early, even before <tt>rcu_init()</tt>
1943 In fact, a number of RCU's primitives can be used as soon as the
1944 initial task's <tt>task_struct</tt> is available and the
1945 boot CPU's per-CPU variables are set up.
1946 The read-side primitives (<tt>rcu_read_lock()</tt>,
1947 <tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>,
1948 and <tt>rcu_access_pointer()</tt>) will operate normally very early on,
1949 as will <tt>rcu_assign_pointer()</tt>.
1952 Although <tt>call_rcu()</tt> may be invoked at any
1953 time during boot, callbacks are not guaranteed to be invoked until after
1954 the scheduler is fully up and running.
1955 This delay in callback invocation is due to the fact that RCU does not
1956 invoke callbacks until it is fully initialized, and this full initialization
1957 cannot occur until after the scheduler has initialized itself to the
1958 point where RCU can spawn and run its kthreads.
1959 In theory, it would be possible to invoke callbacks earlier,
1960 however, this is not a panacea because there would be severe restrictions
1961 on what operations those callbacks could invoke.
1964 Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
1965 <a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
1966 (<a href="#Bottom-Half Flavor">discussed below</a>),
1968 <a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>
1969 will all operate normally
1970 during very early boot, the reason being that there is only one CPU
1971 and preemption is disabled.
1972 This means that the call <tt>synchronize_rcu()</tt> (or friends)
1973 itself is a quiescent
1974 state and thus a grace period, so the early-boot implementation can
1978 Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt>
1979 continue to operate normally through the remainder of boot, courtesy
1980 of the fact that preemption is disabled across their RCU read-side
1981 critical sections and also courtesy of the fact that there is still
1983 However, once the scheduler starts initializing, preemption is enabled.
1984 There is still only a single CPU, but the fact that preemption is enabled
1985 means that the no-op implementation of <tt>synchronize_rcu()</tt> no
1986 longer works in <tt>CONFIG_PREEMPT=y</tt> kernels.
1987 Therefore, as soon as the scheduler starts initializing, the early-boot
1988 fastpath is disabled.
1989 This means that <tt>synchronize_rcu()</tt> switches to its runtime
1990 mode of operation where it posts callbacks, which in turn means that
1991 any call to <tt>synchronize_rcu()</tt> will block until the corresponding
1992 callback is invoked.
1993 Unfortunately, the callback cannot be invoked until RCU's runtime
1994 grace-period machinery is up and running, which cannot happen until
1995 the scheduler has initialized itself sufficiently to allow RCU's
1996 kthreads to be spawned.
1997 Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
1998 initialization can result in deadlock.
2001 So what happens with <tt>synchronize_rcu()</tt> during
2002 scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
2005 In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
2006 maps directly to <tt>synchronize_sched()</tt>.
2007 Therefore, <tt>synchronize_rcu()</tt> works normally throughout
2008 boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
2009 However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
2010 so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
2011 during scheduler initialization.
2015 I learned of these boot-time requirements as a result of a series of
2018 <h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3>
2021 The Linux kernel has interrupts, and RCU read-side critical sections are
2022 legal within interrupt handlers and within interrupt-disabled regions
2023 of code, as are invocations of <tt>call_rcu()</tt>.
2026 Some Linux-kernel architectures can enter an interrupt handler from
2027 non-idle process context, and then just never leave it, instead stealthily
2028 transitioning back to process context.
2029 This trick is sometimes used to invoke system calls from inside the kernel.
2030 These “half-interrupts” mean that RCU has to be very careful
2031 about how it counts interrupt nesting levels.
2032 I learned of this requirement the hard way during a rewrite
2033 of RCU's dyntick-idle code.
2036 The Linux kernel has non-maskable interrupts (NMIs), and
2037 RCU read-side critical sections are legal within NMI handlers.
2038 Thankfully, RCU update-side primitives, including
2039 <tt>call_rcu()</tt>, are prohibited within NMI handlers.
2042 The name notwithstanding, some Linux-kernel architectures
2043 can have nested NMIs, which RCU must handle correctly.
2045 <a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
2046 with this requirement;
2047 he also kindly surprised me with
2048 <a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
2049 that meets this requirement.
2051 <h3><a name="Loadable Modules">Loadable Modules</a></h3>
2054 The Linux kernel has loadable modules, and these modules can
2056 After a given module has been unloaded, any attempt to call
2057 one of its functions results in a segmentation fault.
2058 The module-unload functions must therefore cancel any
2059 delayed calls to loadable-module functions, for example,
2060 any outstanding <tt>mod_timer()</tt> must be dealt with
2061 via <tt>del_timer_sync()</tt> or similar.
2064 Unfortunately, there is no way to cancel an RCU callback;
2065 once you invoke <tt>call_rcu()</tt>, the callback function is
2066 going to eventually be invoked, unless the system goes down first.
2067 Because it is normally considered socially irresponsible to crash the system
2068 in response to a module unload request, we need some other way
2069 to deal with in-flight RCU callbacks.
2072 RCU therefore provides
2073 <tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>,
2074 which waits until all in-flight RCU callbacks have been invoked.
2075 If a module uses <tt>call_rcu()</tt>, its exit function should therefore
2076 prevent any future invocation of <tt>call_rcu()</tt>, then invoke
2077 <tt>rcu_barrier()</tt>.
2078 In theory, the underlying module-unload code could invoke
2079 <tt>rcu_barrier()</tt> unconditionally, but in practice this would
2080 incur unacceptable latencies.
2083 Nikita Danilov noted this requirement for an analogous filesystem-unmount
2084 situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU.
2085 The need for <tt>rcu_barrier()</tt> for module unloading became
2088 <h3><a name="Hotplug CPU">Hotplug CPU</a></h3>
2091 The Linux kernel supports CPU hotplug, which means that CPUs
2093 It is of course illegal to use any RCU API member from an offline CPU.
2094 This requirement was present from day one in DYNIX/ptx, but
2095 on the other hand, the Linux kernel's CPU-hotplug implementation
2096 is “interesting.”
2099 The Linux-kernel CPU-hotplug implementation has notifiers that
2100 are used to allow the various kernel subsystems (including RCU)
2101 to respond appropriately to a given CPU-hotplug operation.
2102 Most RCU operations may be invoked from CPU-hotplug notifiers,
2103 including even normal synchronous grace-period operations
2104 such as <tt>synchronize_rcu()</tt>.
2105 However, expedited grace-period operations such as
2106 <tt>synchronize_rcu_expedited()</tt> are not supported,
2107 due to the fact that current implementations block CPU-hotplug
2108 operations, which could result in deadlock.
2111 In addition, all-callback-wait operations such as
2112 <tt>rcu_barrier()</tt> are also not supported, due to the
2113 fact that there are phases of CPU-hotplug operations where
2114 the outgoing CPU's callbacks will not be invoked until after
2115 the CPU-hotplug operation ends, which could also result in deadlock.
2117 <h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3>
2120 RCU depends on the scheduler, and the scheduler uses RCU to
2121 protect some of its data structures.
2122 This means the scheduler is forbidden from acquiring
2123 the runqueue locks and the priority-inheritance locks
2124 in the middle of an outermost RCU read-side critical section unless either
2125 (1) it releases them before exiting that same
2126 RCU read-side critical section, or
2127 (2) interrupts are disabled across
2128 that entire RCU read-side critical section.
2129 This same prohibition also applies (recursively!) to any lock that is acquired
2130 while holding any lock to which this prohibition applies.
2131 Adhering to this rule prevents preemptible RCU from invoking
2132 <tt>rcu_read_unlock_special()</tt> while either runqueue or
2133 priority-inheritance locks are held, thus avoiding deadlock.
2136 Prior to v4.4, it was only necessary to disable preemption across
2137 RCU read-side critical sections that acquired scheduler locks.
2138 In v4.4, expedited grace periods started using IPIs, and these
2139 IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
2140 Therefore, this expedited-grace-period change required disabling of
2141 interrupts, not just preemption.
2144 For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
2145 implementation must be written carefully to avoid similar deadlocks.
2146 In particular, <tt>rcu_read_unlock()</tt> must tolerate an
2147 interrupt where the interrupt handler invokes both
2148 <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
2149 This possibility requires <tt>rcu_read_unlock()</tt> to use
2150 negative nesting levels to avoid destructive recursion via
2151 interrupt handler's use of RCU.
2154 This pair of mutual scheduler-RCU requirements came as a
2155 <a href="https://lwn.net/Articles/453002/">complete surprise</a>.
2158 As noted above, RCU makes use of kthreads, and it is necessary to
2159 avoid excessive CPU-time accumulation by these kthreads.
2160 This requirement was no surprise, but RCU's violation of it
2161 when running context-switch-heavy workloads when built with
2162 <tt>CONFIG_NO_HZ_FULL=y</tt>
2163 <a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>.
2164 RCU has made good progress towards meeting this requirement, even
2165 for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads,
2166 but there is room for further improvement.
2168 <h3><a name="Tracing and RCU">Tracing and RCU</a></h3>
2171 It is possible to use tracing on RCU code, but tracing itself
2173 For this reason, <tt>rcu_dereference_raw_notrace()</tt>
2174 is provided for use by tracing, which avoids the destructive
2175 recursion that could otherwise ensue.
2176 This API is also used by virtualization in some architectures,
2177 where RCU readers execute in environments in which tracing
2179 The tracing folks both located the requirement and provided the
2180 needed fix, so this surprise requirement was relatively painless.
2182 <h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
2185 Interrupting idle CPUs is considered socially unacceptable,
2186 especially by people with battery-powered embedded systems.
2187 RCU therefore conserves energy by detecting which CPUs are
2188 idle, including tracking CPUs that have been interrupted from idle.
2189 This is a large part of the energy-efficiency requirement,
2190 so I learned of this via an irate phone call.
2193 Because RCU avoids interrupting idle CPUs, it is illegal to
2194 execute an RCU read-side critical section on an idle CPU.
2195 (Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat
2197 The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt>
2198 event tracing is provided to work around this restriction.
2199 In addition, <tt>rcu_is_watching()</tt> may be used to
2200 test whether or not it is currently legal to run RCU read-side
2201 critical sections on this CPU.
2202 I learned of the need for diagnostics on the one hand
2203 and <tt>RCU_NONIDLE()</tt> on the other while inspecting
2205 Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
2206 which is used quite heavily in the idle loop.
2209 It is similarly socially unacceptable to interrupt an
2210 <tt>nohz_full</tt> CPU running in userspace.
2211 RCU must therefore track <tt>nohz_full</tt> userspace
2214 <a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a>
2215 kernels, RCU must separately track idle CPUs on the one hand and
2216 CPUs that are either idle or executing in userspace on the other.
2217 In both cases, RCU must be able to sample state at two points in
2218 time, and be able to determine whether or not some other CPU spent
2219 any time idle and/or executing in userspace.
2222 These energy-efficiency requirements have proven quite difficult to
2223 understand and to meet, for example, there have been more than five
2224 clean-sheet rewrites of RCU's energy-efficiency code, the last of
2225 which was finally able to demonstrate
2226 <a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>.
2228 I learned of many of these requirements via angry phone calls:
2229 Flaming me on the Linux-kernel mailing list was apparently not
2230 sufficient to fully vent their ire at RCU's energy-efficiency bugs!
2232 <h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
2235 Although small-memory non-realtime systems can simply use Tiny RCU,
2236 code size is only one aspect of memory efficiency.
2237 Another aspect is the size of the <tt>rcu_head</tt> structure
2238 used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>.
2239 Although this structure contains nothing more than a pair of pointers,
2240 it does appear in many RCU-protected data structures, including
2241 some that are size critical.
2242 The <tt>page</tt> structure is a case in point, as evidenced by
2243 the many occurrences of the <tt>union</tt> keyword within that structure.
2246 This need for memory efficiency is one reason that RCU uses hand-crafted
2247 singly linked lists to track the <tt>rcu_head</tt> structures that
2248 are waiting for a grace period to elapse.
2249 It is also the reason why <tt>rcu_head</tt> structures do not contain
2250 debug information, such as fields tracking the file and line of the
2251 <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them.
2252 Although this information might appear in debug-only kernel builds at some
2253 point, in the meantime, the <tt>->func</tt> field will often provide
2254 the needed debug information.
2257 However, in some cases, the need for memory efficiency leads to even
2258 more extreme measures.
2259 Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field
2260 shares storage with a great many other structures that are used at
2261 various points in the corresponding page's lifetime.
2262 In order to correctly resolve certain
2263 <a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>,
2264 the Linux kernel's memory-management subsystem needs a particular bit
2265 to remain zero during all phases of grace-period processing,
2266 and that bit happens to map to the bottom bit of the
2267 <tt>rcu_head</tt> structure's <tt>->next</tt> field.
2268 RCU makes this guarantee as long as <tt>call_rcu()</tt>
2269 is used to post the callback, as opposed to <tt>kfree_rcu()</tt>
2270 or some future “lazy”
2271 variant of <tt>call_rcu()</tt> that might one day be created for
2272 energy-efficiency purposes.
2274 <h3><a name="Performance, Scalability, Response Time, and Reliability">
2275 Performance, Scalability, Response Time, and Reliability</a></h3>
2279 <a href="#Performance and Scalability">earlier discussion</a>,
2280 RCU is used heavily by hot code paths in performance-critical
2281 portions of the Linux kernel's networking, security, virtualization,
2282 and scheduling code paths.
2283 RCU must therefore use efficient implementations, especially in its
2284 read-side primitives.
2285 To that end, it would be good if preemptible RCU's implementation
2286 of <tt>rcu_read_lock()</tt> could be inlined, however, doing
2287 this requires resolving <tt>#include</tt> issues with the
2288 <tt>task_struct</tt> structure.
2291 The Linux kernel supports hardware configurations with up to
2292 4096 CPUs, which means that RCU must be extremely scalable.
2293 Algorithms that involve frequent acquisitions of global locks or
2294 frequent atomic operations on global variables simply cannot be
2295 tolerated within the RCU implementation.
2296 RCU therefore makes heavy use of a combining tree based on the
2297 <tt>rcu_node</tt> structure.
2298 RCU is required to tolerate all CPUs continuously invoking any
2299 combination of RCU's runtime primitives with minimal per-operation
2301 In fact, in many cases, increasing load must <i>decrease</i> the
2302 per-operation overhead, witness the batching optimizations for
2303 <tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>,
2304 <tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>.
2305 As a general rule, RCU must cheerfully accept whatever the
2306 rest of the Linux kernel decides to throw at it.
2309 The Linux kernel is used for real-time workloads, especially
2310 in conjunction with the
2311 <a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>.
2312 The real-time-latency response requirements are such that the
2313 traditional approach of disabling preemption across RCU
2314 read-side critical sections is inappropriate.
2315 Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore
2316 use an RCU implementation that allows RCU read-side critical
2317 sections to be preempted.
2318 This requirement made its presence known after users made it
2319 clear that an earlier
2320 <a href="https://lwn.net/Articles/107930/">real-time patch</a>
2321 did not meet their needs, in conjunction with some
2322 <a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a>
2323 encountered by a very early version of the -rt patchset.
2326 In addition, RCU must make do with a sub-100-microsecond real-time latency
2328 In fact, on smaller systems with the -rt patchset, the Linux kernel
2329 provides sub-20-microsecond real-time latencies for the whole kernel,
2331 RCU's scalability and latency must therefore be sufficient for
2332 these sorts of configurations.
2333 To my surprise, the sub-100-microsecond real-time latency budget
2334 <a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf">
2335 applies to even the largest systems [PDF]</a>,
2336 up to and including systems with 4096 CPUs.
2337 This real-time requirement motivated the grace-period kthread, which
2338 also simplified handling of a number of race conditions.
2341 Finally, RCU's status as a synchronization primitive means that
2342 any RCU failure can result in arbitrary memory corruption that can be
2343 extremely difficult to debug.
2344 This means that RCU must be extremely reliable, which in
2345 practice also means that RCU must have an aggressive stress-test
2347 This stress-test suite is called <tt>rcutorture</tt>.
2350 Although the need for <tt>rcutorture</tt> was no surprise,
2351 the current immense popularity of the Linux kernel is posing
2352 interesting—and perhaps unprecedented—validation
2354 To see this, keep in mind that there are well over one billion
2355 instances of the Linux kernel running today, given Android
2356 smartphones, Linux-powered televisions, and servers.
2357 This number can be expected to increase sharply with the advent of
2358 the celebrated Internet of Things.
2361 Suppose that RCU contains a race condition that manifests on average
2362 once per million years of runtime.
2363 This bug will be occurring about three times per <i>day</i> across
2365 RCU could simply hide behind hardware error rates, given that no one
2366 should really expect their smartphone to last for a million years.
2367 However, anyone taking too much comfort from this thought should
2368 consider the fact that in most jurisdictions, a successful multi-year
2369 test of a given mechanism, which might include a Linux kernel,
2370 suffices for a number of types of safety-critical certifications.
2371 In fact, rumor has it that the Linux kernel is already being used
2372 in production for safety-critical applications.
2373 I don't know about you, but I would feel quite bad if a bug in RCU
2375 Which might explain my recent focus on validation and verification.
2377 <h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2>
2380 One of the more surprising things about RCU is that there are now
2381 no fewer than five <i>flavors</i>, or API families.
2382 In addition, the primary flavor that has been the sole focus up to
2383 this point has two different implementations, non-preemptible and
2385 The other four flavors are listed below, with requirements for each
2386 described in a separate section.
2389 <li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
2390 <li> <a href="#Sched Flavor">Sched Flavor</a>
2391 <li> <a href="#Sleepable RCU">Sleepable RCU</a>
2392 <li> <a href="#Tasks RCU">Tasks RCU</a>
2395 <h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
2398 The softirq-disable (AKA “bottom-half”,
2399 hence the “_bh” abbreviations)
2400 flavor of RCU, or <i>RCU-bh</i>, was developed by
2401 Dipankar Sarma to provide a flavor of RCU that could withstand the
2402 network-based denial-of-service attacks researched by Robert
2404 These attacks placed so much networking load on the system
2405 that some of the CPUs never exited softirq execution,
2406 which in turn prevented those CPUs from ever executing a context switch,
2407 which, in the RCU implementation of that time, prevented grace periods
2409 The result was an out-of-memory condition and a system hang.
2412 The solution was the creation of RCU-bh, which does
2413 <tt>local_bh_disable()</tt>
2414 across its read-side critical sections, and which uses the transition
2415 from one type of softirq processing to another as a quiescent state
2416 in addition to context switch, idle, user mode, and offline.
2417 This means that RCU-bh grace periods can complete even when some of
2418 the CPUs execute in softirq indefinitely, thus allowing algorithms
2419 based on RCU-bh to withstand network-based denial-of-service attacks.
2423 <tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt>
2424 disable and re-enable softirq handlers, any attempt to start a softirq
2426 RCU-bh read-side critical section will be deferred.
2427 In this case, <tt>rcu_read_unlock_bh()</tt>
2428 will invoke softirq processing, which can take considerable time.
2429 One can of course argue that this softirq overhead should be associated
2430 with the code following the RCU-bh read-side critical section rather
2431 than <tt>rcu_read_unlock_bh()</tt>, but the fact
2432 is that most profiling tools cannot be expected to make this sort
2433 of fine distinction.
2434 For example, suppose that a three-millisecond-long RCU-bh read-side
2435 critical section executes during a time of heavy networking load.
2436 There will very likely be an attempt to invoke at least one softirq
2437 handler during that three milliseconds, but any such invocation will
2438 be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>.
2439 This can of course make it appear at first glance as if
2440 <tt>rcu_read_unlock_bh()</tt> was executing very slowly.
2444 <a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a>
2446 <tt>rcu_read_lock_bh()</tt>,
2447 <tt>rcu_read_unlock_bh()</tt>,
2448 <tt>rcu_dereference_bh()</tt>,
2449 <tt>rcu_dereference_bh_check()</tt>,
2450 <tt>synchronize_rcu_bh()</tt>,
2451 <tt>synchronize_rcu_bh_expedited()</tt>,
2452 <tt>call_rcu_bh()</tt>,
2453 <tt>rcu_barrier_bh()</tt>, and
2454 <tt>rcu_read_lock_bh_held()</tt>.
2456 <h3><a name="Sched Flavor">Sched Flavor</a></h3>
2459 Before preemptible RCU, waiting for an RCU grace period had the
2460 side effect of also waiting for all pre-existing interrupt
2462 However, there are legitimate preemptible-RCU implementations that
2463 do not have this property, given that any point in the code outside
2464 of an RCU read-side critical section can be a quiescent state.
2465 Therefore, <i>RCU-sched</i> was created, which follows “classic”
2466 RCU in that an RCU-sched grace period waits for for pre-existing
2467 interrupt and NMI handlers.
2468 In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched
2469 APIs have identical implementations, while kernels built with
2470 <tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each.
2473 Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels,
2474 <tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
2475 disable and re-enable preemption, respectively.
2476 This means that if there was a preemption attempt during the
2477 RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt>
2478 will enter the scheduler, with all the latency and overhead entailed.
2479 Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look
2480 as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly.
2481 However, the highest-priority task won't be preempted, so that task
2482 will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations.
2486 <a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a>
2488 <tt>rcu_read_lock_sched()</tt>,
2489 <tt>rcu_read_unlock_sched()</tt>,
2490 <tt>rcu_read_lock_sched_notrace()</tt>,
2491 <tt>rcu_read_unlock_sched_notrace()</tt>,
2492 <tt>rcu_dereference_sched()</tt>,
2493 <tt>rcu_dereference_sched_check()</tt>,
2494 <tt>synchronize_sched()</tt>,
2495 <tt>synchronize_rcu_sched_expedited()</tt>,
2496 <tt>call_rcu_sched()</tt>,
2497 <tt>rcu_barrier_sched()</tt>, and
2498 <tt>rcu_read_lock_sched_held()</tt>.
2499 However, anything that disables preemption also marks an RCU-sched
2500 read-side critical section, including
2501 <tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>,
2502 <tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>,
2505 <h3><a name="Sleepable RCU">Sleepable RCU</a></h3>
2508 For well over a decade, someone saying “I need to block within
2509 an RCU read-side critical section” was a reliable indication
2510 that this someone did not understand RCU.
2511 After all, if you are always blocking in an RCU read-side critical
2512 section, you can probably afford to use a higher-overhead synchronization
2514 However, that changed with the advent of the Linux kernel's notifiers,
2515 whose RCU read-side critical
2516 sections almost never sleep, but sometimes need to.
2517 This resulted in the introduction of
2518 <a href="https://lwn.net/Articles/202847/">sleepable RCU</a>,
2522 SRCU allows different domains to be defined, with each such domain
2523 defined by an instance of an <tt>srcu_struct</tt> structure.
2524 A pointer to this structure must be passed in to each SRCU function,
2525 for example, <tt>synchronize_srcu(&ss)</tt>, where
2526 <tt>ss</tt> is the <tt>srcu_struct</tt> structure.
2527 The key benefit of these domains is that a slow SRCU reader in one
2528 domain does not delay an SRCU grace period in some other domain.
2529 That said, one consequence of these domains is that read-side code
2530 must pass a “cookie” from <tt>srcu_read_lock()</tt>
2531 to <tt>srcu_read_unlock()</tt>, for example, as follows:
2537 3 idx = srcu_read_lock(&ss);
2539 5 srcu_read_unlock(&ss, idx);
2544 As noted above, it is legal to block within SRCU read-side critical sections,
2545 however, with great power comes great responsibility.
2546 If you block forever in one of a given domain's SRCU read-side critical
2547 sections, then that domain's grace periods will also be blocked forever.
2548 Of course, one good way to block forever is to deadlock, which can
2549 happen if any operation in a given domain's SRCU read-side critical
2550 section can block waiting, either directly or indirectly, for that domain's
2551 grace period to elapse.
2552 For example, this results in a self-deadlock:
2558 3 idx = srcu_read_lock(&ss);
2560 5 synchronize_srcu(&ss);
2561 6 srcu_read_unlock(&ss, idx);
2566 However, if line 5 acquired a mutex that was held across
2567 a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>,
2568 deadlock would still be possible.
2569 Furthermore, if line 5 acquired a mutex that was held across
2570 a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>,
2571 and if an <tt>ss1</tt>-domain SRCU read-side critical section
2572 acquired another mutex that was held across as <tt>ss</tt>-domain
2573 <tt>synchronize_srcu()</tt>,
2574 deadlock would again be possible.
2575 Such a deadlock cycle could extend across an arbitrarily large number
2576 of different SRCU domains.
2577 Again, with great power comes great responsibility.
2580 Unlike the other RCU flavors, SRCU read-side critical sections can
2581 run on idle and even offline CPUs.
2582 This ability requires that <tt>srcu_read_lock()</tt> and
2583 <tt>srcu_read_unlock()</tt> contain memory barriers, which means
2584 that SRCU readers will run a bit slower than would RCU readers.
2585 It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt>
2586 API, which, in combination with <tt>srcu_read_unlock()</tt>,
2587 guarantees a full memory barrier.
2591 <a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a>
2593 <tt>srcu_read_lock()</tt>,
2594 <tt>srcu_read_unlock()</tt>,
2595 <tt>srcu_dereference()</tt>,
2596 <tt>srcu_dereference_check()</tt>,
2597 <tt>synchronize_srcu()</tt>,
2598 <tt>synchronize_srcu_expedited()</tt>,
2599 <tt>call_srcu()</tt>,
2600 <tt>srcu_barrier()</tt>, and
2601 <tt>srcu_read_lock_held()</tt>.
2603 <tt>DEFINE_SRCU()</tt>,
2604 <tt>DEFINE_STATIC_SRCU()</tt>, and
2605 <tt>init_srcu_struct()</tt>
2606 APIs for defining and initializing <tt>srcu_struct</tt> structures.
2608 <h3><a name="Tasks RCU">Tasks RCU</a></h3>
2611 Some forms of tracing use “tramopolines” to handle the
2612 binary rewriting required to install different types of probes.
2613 It would be good to be able to free old trampolines, which sounds
2614 like a job for some form of RCU.
2615 However, because it is necessary to be able to install a trace
2616 anywhere in the code, it is not possible to use read-side markers
2617 such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
2618 In addition, it does not work to have these markers in the trampoline
2619 itself, because there would need to be instructions following
2620 <tt>rcu_read_unlock()</tt>.
2621 Although <tt>synchronize_rcu()</tt> would guarantee that execution
2622 reached the <tt>rcu_read_unlock()</tt>, it would not be able to
2623 guarantee that execution had completely left the trampoline.
2626 The solution, in the form of
2627 <a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>,
2629 read-side critical sections that are delimited by voluntary context
2630 switches, that is, calls to <tt>schedule()</tt>,
2631 <tt>cond_resched_rcu_qs()</tt>, and
2632 <tt>synchronize_rcu_tasks()</tt>.
2633 In addition, transitions to and from userspace execution also delimit
2634 tasks-RCU read-side critical sections.
2637 The tasks-RCU API is quite compact, consisting only of
2638 <tt>call_rcu_tasks()</tt>,
2639 <tt>synchronize_rcu_tasks()</tt>, and
2640 <tt>rcu_barrier_tasks()</tt>.
2642 <h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
2645 One of the tricks that RCU uses to attain update-side scalability is
2646 to increase grace-period latency with increasing numbers of CPUs.
2647 If this becomes a serious problem, it will be necessary to rework the
2648 grace-period state machine so as to avoid the need for the additional
2652 Expedited grace periods scan the CPUs, so their latency and overhead
2653 increases with increasing numbers of CPUs.
2654 If this becomes a serious problem on large systems, it will be necessary
2655 to do some redesign to avoid this scalability problem.
2658 RCU disables CPU hotplug in a few places, perhaps most notably in the
2659 expedited grace-period and <tt>rcu_barrier()</tt> operations.
2660 If there is a strong reason to use expedited grace periods in CPU-hotplug
2661 notifiers, it will be necessary to avoid disabling CPU hotplug.
2662 This would introduce some complexity, so there had better be a <i>very</i>
2666 The tradeoff between grace-period latency on the one hand and interruptions
2667 of other CPUs on the other hand may need to be re-examined.
2668 The desire is of course for zero grace-period latency as well as zero
2669 interprocessor interrupts undertaken during an expedited grace period
2671 While this ideal is unlikely to be achievable, it is quite possible that
2672 further improvements can be made.
2675 The multiprocessor implementations of RCU use a combining tree that
2676 groups CPUs so as to reduce lock contention and increase cache locality.
2677 However, this combining tree does not spread its memory across NUMA
2678 nodes nor does it align the CPU groups with hardware features such
2679 as sockets or cores.
2680 Such spreading and alignment is currently believed to be unnecessary
2681 because the hotpath read-side primitives do not access the combining
2682 tree, nor does <tt>call_rcu()</tt> in the common case.
2683 If you believe that your architecture needs such spreading and alignment,
2684 then your architecture should also benefit from the
2685 <tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set
2686 to the number of CPUs in a socket, NUMA node, or whatever.
2687 If the number of CPUs is too large, use a fraction of the number of
2689 If the number of CPUs is a large prime number, well, that certainly
2690 is an “interesting” architectural choice!
2691 More flexible arrangements might be considered, but only if
2692 <tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only
2693 if the inadequacy has been demonstrated by a carefully run and
2694 realistic system-level workload.
2697 Please note that arrangements that require RCU to remap CPU numbers will
2698 require extremely good demonstration of need and full exploration of
2702 There is an embarrassingly large number of flavors of RCU, and this
2703 number has been increasing over time.
2704 Perhaps it will be possible to combine some at some future date.
2707 RCU's various kthreads are reasonably recent additions.
2708 It is quite likely that adjustments will be required to more gracefully
2709 handle extreme loads.
2710 It might also be necessary to be able to relate CPU utilization by
2711 RCU's kthreads and softirq handlers to the code that instigated this
2713 For example, RCU callback overhead might be charged back to the
2714 originating <tt>call_rcu()</tt> instance, though probably not
2715 in production kernels.
2717 <h2><a name="Summary">Summary</a></h2>
2720 This document has presented more than two decade's worth of RCU
2722 Given that the requirements keep changing, this will not be the last
2723 word on this subject, but at least it serves to get an important
2724 subset of the requirements set forth.
2726 <h2><a name="Acknowledgments">Acknowledgments</a></h2>
2728 I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar,
2729 Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and
2730 Andy Lutomirski for their help in rendering
2731 this article human readable, and to Michelle Rankin for her support
2733 Other contributions are acknowledged in the Linux kernel's git archive.
2734 The cartoon is copyright (c) 2013 by Melissa Broussard,
2736 under the terms of the Creative Commons Attribution-Share Alike 3.0
2737 United States license.
projects / linux-2.6-block.git / blob