About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / RCU / whatisRCU.txt




Custom Search

Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 EST.

1	Please note that the "What is RCU?" LWN series is an excellent place
2	to start learning about RCU:
3	
4	1.	What is RCU, Fundamentally?  http://lwn.net/Articles/262464/
5	2.	What is RCU? Part 2: Usage   http://lwn.net/Articles/263130/
6	3.	RCU part 3: the RCU API      http://lwn.net/Articles/264090/
7	4.	The RCU API, 2010 Edition    http://lwn.net/Articles/418853/
8	
9	
10	What is RCU?
11	
12	RCU is a synchronization mechanism that was added to the Linux kernel
13	during the 2.5 development effort that is optimized for read-mostly
14	situations.  Although RCU is actually quite simple once you understand it,
15	getting there can sometimes be a challenge.  Part of the problem is that
16	most of the past descriptions of RCU have been written with the mistaken
17	assumption that there is "one true way" to describe RCU.  Instead,
18	the experience has been that different people must take different paths
19	to arrive at an understanding of RCU.  This document provides several
20	different paths, as follows:
21	
22	1.	RCU OVERVIEW
23	2.	WHAT IS RCU'S CORE API?
24	3.	WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
25	4.	WHAT IF MY UPDATING THREAD CANNOT BLOCK?
26	5.	WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
27	6.	ANALOGY WITH READER-WRITER LOCKING
28	7.	FULL LIST OF RCU APIs
29	8.	ANSWERS TO QUICK QUIZZES
30	
31	People who prefer starting with a conceptual overview should focus on
32	Section 1, though most readers will profit by reading this section at
33	some point.  People who prefer to start with an API that they can then
34	experiment with should focus on Section 2.  People who prefer to start
35	with example uses should focus on Sections 3 and 4.  People who need to
36	understand the RCU implementation should focus on Section 5, then dive
37	into the kernel source code.  People who reason best by analogy should
38	focus on Section 6.  Section 7 serves as an index to the docbook API
39	documentation, and Section 8 is the traditional answer key.
40	
41	So, start with the section that makes the most sense to you and your
42	preferred method of learning.  If you need to know everything about
43	everything, feel free to read the whole thing -- but if you are really
44	that type of person, you have perused the source code and will therefore
45	never need this document anyway.  ;-)
46	
47	
48	1.  RCU OVERVIEW
49	
50	The basic idea behind RCU is to split updates into "removal" and
51	"reclamation" phases.  The removal phase removes references to data items
52	within a data structure (possibly by replacing them with references to
53	new versions of these data items), and can run concurrently with readers.
54	The reason that it is safe to run the removal phase concurrently with
55	readers is the semantics of modern CPUs guarantee that readers will see
56	either the old or the new version of the data structure rather than a
57	partially updated reference.  The reclamation phase does the work of reclaiming
58	(e.g., freeing) the data items removed from the data structure during the
59	removal phase.  Because reclaiming data items can disrupt any readers
60	concurrently referencing those data items, the reclamation phase must
61	not start until readers no longer hold references to those data items.
62	
63	Splitting the update into removal and reclamation phases permits the
64	updater to perform the removal phase immediately, and to defer the
65	reclamation phase until all readers active during the removal phase have
66	completed, either by blocking until they finish or by registering a
67	callback that is invoked after they finish.  Only readers that are active
68	during the removal phase need be considered, because any reader starting
69	after the removal phase will be unable to gain a reference to the removed
70	data items, and therefore cannot be disrupted by the reclamation phase.
71	
72	So the typical RCU update sequence goes something like the following:
73	
74	a.	Remove pointers to a data structure, so that subsequent
75		readers cannot gain a reference to it.
76	
77	b.	Wait for all previous readers to complete their RCU read-side
78		critical sections.
79	
80	c.	At this point, there cannot be any readers who hold references
81		to the data structure, so it now may safely be reclaimed
82		(e.g., kfree()d).
83	
84	Step (b) above is the key idea underlying RCU's deferred destruction.
85	The ability to wait until all readers are done allows RCU readers to
86	use much lighter-weight synchronization, in some cases, absolutely no
87	synchronization at all.  In contrast, in more conventional lock-based
88	schemes, readers must use heavy-weight synchronization in order to
89	prevent an updater from deleting the data structure out from under them.
90	This is because lock-based updaters typically update data items in place,
91	and must therefore exclude readers.  In contrast, RCU-based updaters
92	typically take advantage of the fact that writes to single aligned
93	pointers are atomic on modern CPUs, allowing atomic insertion, removal,
94	and replacement of data items in a linked structure without disrupting
95	readers.  Concurrent RCU readers can then continue accessing the old
96	versions, and can dispense with the atomic operations, memory barriers,
97	and communications cache misses that are so expensive on present-day
98	SMP computer systems, even in absence of lock contention.
99	
100	In the three-step procedure shown above, the updater is performing both
101	the removal and the reclamation step, but it is often helpful for an
102	entirely different thread to do the reclamation, as is in fact the case
103	in the Linux kernel's directory-entry cache (dcache).  Even if the same
104	thread performs both the update step (step (a) above) and the reclamation
105	step (step (c) above), it is often helpful to think of them separately.
106	For example, RCU readers and updaters need not communicate at all,
107	but RCU provides implicit low-overhead communication between readers
108	and reclaimers, namely, in step (b) above.
109	
110	So how the heck can a reclaimer tell when a reader is done, given
111	that readers are not doing any sort of synchronization operations???
112	Read on to learn about how RCU's API makes this easy.
113	
114	
115	2.  WHAT IS RCU'S CORE API?
116	
117	The core RCU API is quite small:
118	
119	a.	rcu_read_lock()
120	b.	rcu_read_unlock()
121	c.	synchronize_rcu() / call_rcu()
122	d.	rcu_assign_pointer()
123	e.	rcu_dereference()
124	
125	There are many other members of the RCU API, but the rest can be
126	expressed in terms of these five, though most implementations instead
127	express synchronize_rcu() in terms of the call_rcu() callback API.
128	
129	The five core RCU APIs are described below, the other 18 will be enumerated
130	later.  See the kernel docbook documentation for more info, or look directly
131	at the function header comments.
132	
133	rcu_read_lock()
134	
135		void rcu_read_lock(void);
136	
137		Used by a reader to inform the reclaimer that the reader is
138		entering an RCU read-side critical section.  It is illegal
139		to block while in an RCU read-side critical section, though
140		kernels built with CONFIG_PREEMPT_RCU can preempt RCU
141		read-side critical sections.  Any RCU-protected data structure
142		accessed during an RCU read-side critical section is guaranteed to
143		remain unreclaimed for the full duration of that critical section.
144		Reference counts may be used in conjunction with RCU to maintain
145		longer-term references to data structures.
146	
147	rcu_read_unlock()
148	
149		void rcu_read_unlock(void);
150	
151		Used by a reader to inform the reclaimer that the reader is
152		exiting an RCU read-side critical section.  Note that RCU
153		read-side critical sections may be nested and/or overlapping.
154	
155	synchronize_rcu()
156	
157		void synchronize_rcu(void);
158	
159		Marks the end of updater code and the beginning of reclaimer
160		code.  It does this by blocking until all pre-existing RCU
161		read-side critical sections on all CPUs have completed.
162		Note that synchronize_rcu() will -not- necessarily wait for
163		any subsequent RCU read-side critical sections to complete.
164		For example, consider the following sequence of events:
165	
166		         CPU 0                  CPU 1                 CPU 2
167		     ----------------- ------------------------- ---------------
168		 1.  rcu_read_lock()
169		 2.                    enters synchronize_rcu()
170		 3.                                               rcu_read_lock()
171		 4.  rcu_read_unlock()
172		 5.                     exits synchronize_rcu()
173		 6.                                              rcu_read_unlock()
174	
175		To reiterate, synchronize_rcu() waits only for ongoing RCU
176		read-side critical sections to complete, not necessarily for
177		any that begin after synchronize_rcu() is invoked.
178	
179		Of course, synchronize_rcu() does not necessarily return
180		-immediately- after the last pre-existing RCU read-side critical
181		section completes.  For one thing, there might well be scheduling
182		delays.  For another thing, many RCU implementations process
183		requests in batches in order to improve efficiencies, which can
184		further delay synchronize_rcu().
185	
186		Since synchronize_rcu() is the API that must figure out when
187		readers are done, its implementation is key to RCU.  For RCU
188		to be useful in all but the most read-intensive situations,
189		synchronize_rcu()'s overhead must also be quite small.
190	
191		The call_rcu() API is a callback form of synchronize_rcu(),
192		and is described in more detail in a later section.  Instead of
193		blocking, it registers a function and argument which are invoked
194		after all ongoing RCU read-side critical sections have completed.
195		This callback variant is particularly useful in situations where
196		it is illegal to block or where update-side performance is
197		critically important.
198	
199		However, the call_rcu() API should not be used lightly, as use
200		of the synchronize_rcu() API generally results in simpler code.
201		In addition, the synchronize_rcu() API has the nice property
202		of automatically limiting update rate should grace periods
203		be delayed.  This property results in system resilience in face
204		of denial-of-service attacks.  Code using call_rcu() should limit
205		update rate in order to gain this same sort of resilience.  See
206		checklist.txt for some approaches to limiting the update rate.
207	
208	rcu_assign_pointer()
209	
210		typeof(p) rcu_assign_pointer(p, typeof(p) v);
211	
212		Yes, rcu_assign_pointer() -is- implemented as a macro, though it
213		would be cool to be able to declare a function in this manner.
214		(Compiler experts will no doubt disagree.)
215	
216		The updater uses this function to assign a new value to an
217		RCU-protected pointer, in order to safely communicate the change
218		in value from the updater to the reader.  This function returns
219		the new value, and also executes any memory-barrier instructions
220		required for a given CPU architecture.
221	
222		Perhaps just as important, it serves to document (1) which
223		pointers are protected by RCU and (2) the point at which a
224		given structure becomes accessible to other CPUs.  That said,
225		rcu_assign_pointer() is most frequently used indirectly, via
226		the _rcu list-manipulation primitives such as list_add_rcu().
227	
228	rcu_dereference()
229	
230		typeof(p) rcu_dereference(p);
231	
232		Like rcu_assign_pointer(), rcu_dereference() must be implemented
233		as a macro.
234	
235		The reader uses rcu_dereference() to fetch an RCU-protected
236		pointer, which returns a value that may then be safely
237		dereferenced.  Note that rcu_deference() does not actually
238		dereference the pointer, instead, it protects the pointer for
239		later dereferencing.  It also executes any needed memory-barrier
240		instructions for a given CPU architecture.  Currently, only Alpha
241		needs memory barriers within rcu_dereference() -- on other CPUs,
242		it compiles to nothing, not even a compiler directive.
243	
244		Common coding practice uses rcu_dereference() to copy an
245		RCU-protected pointer to a local variable, then dereferences
246		this local variable, for example as follows:
247	
248			p = rcu_dereference(head.next);
249			return p->data;
250	
251		However, in this case, one could just as easily combine these
252		into one statement:
253	
254			return rcu_dereference(head.next)->data;
255	
256		If you are going to be fetching multiple fields from the
257		RCU-protected structure, using the local variable is of
258		course preferred.  Repeated rcu_dereference() calls look
259		ugly, do not guarantee that the same pointer will be returned
260		if an update happened while in the critical section, and incur
261		unnecessary overhead on Alpha CPUs.
262	
263		Note that the value returned by rcu_dereference() is valid
264		only within the enclosing RCU read-side critical section.
265		For example, the following is -not- legal:
266	
267			rcu_read_lock();
268			p = rcu_dereference(head.next);
269			rcu_read_unlock();
270			x = p->address;	/* BUG!!! */
271			rcu_read_lock();
272			y = p->data;	/* BUG!!! */
273			rcu_read_unlock();
274	
275		Holding a reference from one RCU read-side critical section
276		to another is just as illegal as holding a reference from
277		one lock-based critical section to another!  Similarly,
278		using a reference outside of the critical section in which
279		it was acquired is just as illegal as doing so with normal
280		locking.
281	
282		As with rcu_assign_pointer(), an important function of
283		rcu_dereference() is to document which pointers are protected by
284		RCU, in particular, flagging a pointer that is subject to changing
285		at any time, including immediately after the rcu_dereference().
286		And, again like rcu_assign_pointer(), rcu_dereference() is
287		typically used indirectly, via the _rcu list-manipulation
288		primitives, such as list_for_each_entry_rcu().
289	
290	The following diagram shows how each API communicates among the
291	reader, updater, and reclaimer.
292	
293	
294		    rcu_assign_pointer()
295		    			    +--------+
296		    +---------------------->| reader |---------+
297		    |                       +--------+         |
298		    |                           |              |
299		    |                           |              | Protect:
300		    |                           |              | rcu_read_lock()
301		    |                           |              | rcu_read_unlock()
302		    |        rcu_dereference()  |              |
303	       +---------+                      |              |
304	       | updater |<---------------------+              |
305	       +---------+                                     V
306		    |                                    +-----------+
307		    +----------------------------------->| reclaimer |
308		    				         +-----------+
309		      Defer:
310		      synchronize_rcu() & call_rcu()
311	
312	
313	The RCU infrastructure observes the time sequence of rcu_read_lock(),
314	rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
315	order to determine when (1) synchronize_rcu() invocations may return
316	to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
317	implementations of the RCU infrastructure make heavy use of batching in
318	order to amortize their overhead over many uses of the corresponding APIs.
319	
320	There are no fewer than three RCU mechanisms in the Linux kernel; the
321	diagram above shows the first one, which is by far the most commonly used.
322	The rcu_dereference() and rcu_assign_pointer() primitives are used for
323	all three mechanisms, but different defer and protect primitives are
324	used as follows:
325	
326		Defer			Protect
327	
328	a.	synchronize_rcu()	rcu_read_lock() / rcu_read_unlock()
329		call_rcu()		rcu_dereference()
330	
331	b.	synchronize_rcu_bh()	rcu_read_lock_bh() / rcu_read_unlock_bh()
332		call_rcu_bh()		rcu_dereference_bh()
333	
334	c.	synchronize_sched()	rcu_read_lock_sched() / rcu_read_unlock_sched()
335		call_rcu_sched()	preempt_disable() / preempt_enable()
336					local_irq_save() / local_irq_restore()
337					hardirq enter / hardirq exit
338					NMI enter / NMI exit
339					rcu_dereference_sched()
340	
341	These three mechanisms are used as follows:
342	
343	a.	RCU applied to normal data structures.
344	
345	b.	RCU applied to networking data structures that may be subjected
346		to remote denial-of-service attacks.
347	
348	c.	RCU applied to scheduler and interrupt/NMI-handler tasks.
349	
350	Again, most uses will be of (a).  The (b) and (c) cases are important
351	for specialized uses, but are relatively uncommon.
352	
353	
354	3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
355	
356	This section shows a simple use of the core RCU API to protect a
357	global pointer to a dynamically allocated structure.  More-typical
358	uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
359	
360		struct foo {
361			int a;
362			char b;
363			long c;
364		};
365		DEFINE_SPINLOCK(foo_mutex);
366	
367		struct foo __rcu *gbl_foo;
368	
369		/*
370		 * Create a new struct foo that is the same as the one currently
371		 * pointed to by gbl_foo, except that field "a" is replaced
372		 * with "new_a".  Points gbl_foo to the new structure, and
373		 * frees up the old structure after a grace period.
374		 *
375		 * Uses rcu_assign_pointer() to ensure that concurrent readers
376		 * see the initialized version of the new structure.
377		 *
378		 * Uses synchronize_rcu() to ensure that any readers that might
379		 * have references to the old structure complete before freeing
380		 * the old structure.
381		 */
382		void foo_update_a(int new_a)
383		{
384			struct foo *new_fp;
385			struct foo *old_fp;
386	
387			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
388			spin_lock(&foo_mutex);
389			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
390			*new_fp = *old_fp;
391			new_fp->a = new_a;
392			rcu_assign_pointer(gbl_foo, new_fp);
393			spin_unlock(&foo_mutex);
394			synchronize_rcu();
395			kfree(old_fp);
396		}
397	
398		/*
399		 * Return the value of field "a" of the current gbl_foo
400		 * structure.  Use rcu_read_lock() and rcu_read_unlock()
401		 * to ensure that the structure does not get deleted out
402		 * from under us, and use rcu_dereference() to ensure that
403		 * we see the initialized version of the structure (important
404		 * for DEC Alpha and for people reading the code).
405		 */
406		int foo_get_a(void)
407		{
408			int retval;
409	
410			rcu_read_lock();
411			retval = rcu_dereference(gbl_foo)->a;
412			rcu_read_unlock();
413			return retval;
414		}
415	
416	So, to sum up:
417	
418	o	Use rcu_read_lock() and rcu_read_unlock() to guard RCU
419		read-side critical sections.
420	
421	o	Within an RCU read-side critical section, use rcu_dereference()
422		to dereference RCU-protected pointers.
423	
424	o	Use some solid scheme (such as locks or semaphores) to
425		keep concurrent updates from interfering with each other.
426	
427	o	Use rcu_assign_pointer() to update an RCU-protected pointer.
428		This primitive protects concurrent readers from the updater,
429		-not- concurrent updates from each other!  You therefore still
430		need to use locking (or something similar) to keep concurrent
431		rcu_assign_pointer() primitives from interfering with each other.
432	
433	o	Use synchronize_rcu() -after- removing a data element from an
434		RCU-protected data structure, but -before- reclaiming/freeing
435		the data element, in order to wait for the completion of all
436		RCU read-side critical sections that might be referencing that
437		data item.
438	
439	See checklist.txt for additional rules to follow when using RCU.
440	And again, more-typical uses of RCU may be found in listRCU.txt,
441	arrayRCU.txt, and NMI-RCU.txt.
442	
443	
444	4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?
445	
446	In the example above, foo_update_a() blocks until a grace period elapses.
447	This is quite simple, but in some cases one cannot afford to wait so
448	long -- there might be other high-priority work to be done.
449	
450	In such cases, one uses call_rcu() rather than synchronize_rcu().
451	The call_rcu() API is as follows:
452	
453		void call_rcu(struct rcu_head * head,
454			      void (*func)(struct rcu_head *head));
455	
456	This function invokes func(head) after a grace period has elapsed.
457	This invocation might happen from either softirq or process context,
458	so the function is not permitted to block.  The foo struct needs to
459	have an rcu_head structure added, perhaps as follows:
460	
461		struct foo {
462			int a;
463			char b;
464			long c;
465			struct rcu_head rcu;
466		};
467	
468	The foo_update_a() function might then be written as follows:
469	
470		/*
471		 * Create a new struct foo that is the same as the one currently
472		 * pointed to by gbl_foo, except that field "a" is replaced
473		 * with "new_a".  Points gbl_foo to the new structure, and
474		 * frees up the old structure after a grace period.
475		 *
476		 * Uses rcu_assign_pointer() to ensure that concurrent readers
477		 * see the initialized version of the new structure.
478		 *
479		 * Uses call_rcu() to ensure that any readers that might have
480		 * references to the old structure complete before freeing the
481		 * old structure.
482		 */
483		void foo_update_a(int new_a)
484		{
485			struct foo *new_fp;
486			struct foo *old_fp;
487	
488			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
489			spin_lock(&foo_mutex);
490			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
491			*new_fp = *old_fp;
492			new_fp->a = new_a;
493			rcu_assign_pointer(gbl_foo, new_fp);
494			spin_unlock(&foo_mutex);
495			call_rcu(&old_fp->rcu, foo_reclaim);
496		}
497	
498	The foo_reclaim() function might appear as follows:
499	
500		void foo_reclaim(struct rcu_head *rp)
501		{
502			struct foo *fp = container_of(rp, struct foo, rcu);
503	
504			foo_cleanup(fp->a);
505	
506			kfree(fp);
507		}
508	
509	The container_of() primitive is a macro that, given a pointer into a
510	struct, the type of the struct, and the pointed-to field within the
511	struct, returns a pointer to the beginning of the struct.
512	
513	The use of call_rcu() permits the caller of foo_update_a() to
514	immediately regain control, without needing to worry further about the
515	old version of the newly updated element.  It also clearly shows the
516	RCU distinction between updater, namely foo_update_a(), and reclaimer,
517	namely foo_reclaim().
518	
519	The summary of advice is the same as for the previous section, except
520	that we are now using call_rcu() rather than synchronize_rcu():
521	
522	o	Use call_rcu() -after- removing a data element from an
523		RCU-protected data structure in order to register a callback
524		function that will be invoked after the completion of all RCU
525		read-side critical sections that might be referencing that
526		data item.
527	
528	If the callback for call_rcu() is not doing anything more than calling
529	kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
530	to avoid having to write your own callback:
531	
532		kfree_rcu(old_fp, rcu);
533	
534	Again, see checklist.txt for additional rules governing the use of RCU.
535	
536	
537	5.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
538	
539	One of the nice things about RCU is that it has extremely simple "toy"
540	implementations that are a good first step towards understanding the
541	production-quality implementations in the Linux kernel.  This section
542	presents two such "toy" implementations of RCU, one that is implemented
543	in terms of familiar locking primitives, and another that more closely
544	resembles "classic" RCU.  Both are way too simple for real-world use,
545	lacking both functionality and performance.  However, they are useful
546	in getting a feel for how RCU works.  See kernel/rcupdate.c for a
547	production-quality implementation, and see:
548	
549		http://www.rdrop.com/users/paulmck/RCU
550	
551	for papers describing the Linux kernel RCU implementation.  The OLS'01
552	and OLS'02 papers are a good introduction, and the dissertation provides
553	more details on the current implementation as of early 2004.
554	
555	
556	5A.  "TOY" IMPLEMENTATION #1: LOCKING
557	
558	This section presents a "toy" RCU implementation that is based on
559	familiar locking primitives.  Its overhead makes it a non-starter for
560	real-life use, as does its lack of scalability.  It is also unsuitable
561	for realtime use, since it allows scheduling latency to "bleed" from
562	one read-side critical section to another.
563	
564	However, it is probably the easiest implementation to relate to, so is
565	a good starting point.
566	
567	It is extremely simple:
568	
569		static DEFINE_RWLOCK(rcu_gp_mutex);
570	
571		void rcu_read_lock(void)
572		{
573			read_lock(&rcu_gp_mutex);
574		}
575	
576		void rcu_read_unlock(void)
577		{
578			read_unlock(&rcu_gp_mutex);
579		}
580	
581		void synchronize_rcu(void)
582		{
583			write_lock(&rcu_gp_mutex);
584			write_unlock(&rcu_gp_mutex);
585		}
586	
587	[You can ignore rcu_assign_pointer() and rcu_dereference() without
588	missing much.  But here they are anyway.  And whatever you do, don't
589	forget about them when submitting patches making use of RCU!]
590	
591		#define rcu_assign_pointer(p, v)	({ \
592								smp_wmb(); \
593								(p) = (v); \
594							})
595	
596		#define rcu_dereference(p)     ({ \
597						typeof(p) _________p1 = p; \
598						smp_read_barrier_depends(); \
599						(_________p1); \
600						})
601	
602	
603	The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
604	and release a global reader-writer lock.  The synchronize_rcu()
605	primitive write-acquires this same lock, then immediately releases
606	it.  This means that once synchronize_rcu() exits, all RCU read-side
607	critical sections that were in progress before synchronize_rcu() was
608	called are guaranteed to have completed -- there is no way that
609	synchronize_rcu() would have been able to write-acquire the lock
610	otherwise.
611	
612	It is possible to nest rcu_read_lock(), since reader-writer locks may
613	be recursively acquired.  Note also that rcu_read_lock() is immune
614	from deadlock (an important property of RCU).  The reason for this is
615	that the only thing that can block rcu_read_lock() is a synchronize_rcu().
616	But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
617	so there can be no deadlock cycle.
618	
619	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
620			occur when using this algorithm in a real-world Linux
621			kernel?  How could this deadlock be avoided?
622	
623	
624	5B.  "TOY" EXAMPLE #2: CLASSIC RCU
625	
626	This section presents a "toy" RCU implementation that is based on
627	"classic RCU".  It is also short on performance (but only for updates) and
628	on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
629	kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
630	are the same as those shown in the preceding section, so they are omitted.
631	
632		void rcu_read_lock(void) { }
633	
634		void rcu_read_unlock(void) { }
635	
636		void synchronize_rcu(void)
637		{
638			int cpu;
639	
640			for_each_possible_cpu(cpu)
641				run_on(cpu);
642		}
643	
644	Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
645	This is the great strength of classic RCU in a non-preemptive kernel:
646	read-side overhead is precisely zero, at least on non-Alpha CPUs.
647	And there is absolutely no way that rcu_read_lock() can possibly
648	participate in a deadlock cycle!
649	
650	The implementation of synchronize_rcu() simply schedules itself on each
651	CPU in turn.  The run_on() primitive can be implemented straightforwardly
652	in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
653	"toy" implementation would restore the affinity upon completion rather
654	than just leaving all tasks running on the last CPU, but when I said
655	"toy", I meant -toy-!
656	
657	So how the heck is this supposed to work???
658	
659	Remember that it is illegal to block while in an RCU read-side critical
660	section.  Therefore, if a given CPU executes a context switch, we know
661	that it must have completed all preceding RCU read-side critical sections.
662	Once -all- CPUs have executed a context switch, then -all- preceding
663	RCU read-side critical sections will have completed.
664	
665	So, suppose that we remove a data item from its structure and then invoke
666	synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
667	that there are no RCU read-side critical sections holding a reference
668	to that data item, so we can safely reclaim it.
669	
670	Quick Quiz #2:	Give an example where Classic RCU's read-side
671			overhead is -negative-.
672	
673	Quick Quiz #3:  If it is illegal to block in an RCU read-side
674			critical section, what the heck do you do in
675			PREEMPT_RT, where normal spinlocks can block???
676	
677	
678	6.  ANALOGY WITH READER-WRITER LOCKING
679	
680	Although RCU can be used in many different ways, a very common use of
681	RCU is analogous to reader-writer locking.  The following unified
682	diff shows how closely related RCU and reader-writer locking can be.
683	
684		@@ -5,5 +5,5 @@ struct el {
685		 	int data;
686		 	/* Other data fields */
687		 };
688		-rwlock_t listmutex;
689		+spinlock_t listmutex;
690		 struct el head;
691	
692		@@ -13,15 +14,15 @@
693			struct list_head *lp;
694			struct el *p;
695	
696		-	read_lock(&listmutex);
697		-	list_for_each_entry(p, head, lp) {
698		+	rcu_read_lock();
699		+	list_for_each_entry_rcu(p, head, lp) {
700				if (p->key == key) {
701					*result = p->data;
702		-			read_unlock(&listmutex);
703		+			rcu_read_unlock();
704					return 1;
705				}
706			}
707		-	read_unlock(&listmutex);
708		+	rcu_read_unlock();
709			return 0;
710		 }
711	
712		@@ -29,15 +30,16 @@
713		 {
714			struct el *p;
715	
716		-	write_lock(&listmutex);
717		+	spin_lock(&listmutex);
718			list_for_each_entry(p, head, lp) {
719				if (p->key == key) {
720		-			list_del(&p->list);
721		-			write_unlock(&listmutex);
722		+			list_del_rcu(&p->list);
723		+			spin_unlock(&listmutex);
724		+			synchronize_rcu();
725					kfree(p);
726					return 1;
727				}
728			}
729		-	write_unlock(&listmutex);
730		+	spin_unlock(&listmutex);
731			return 0;
732		 }
733	
734	Or, for those who prefer a side-by-side listing:
735	
736	 1 struct el {                          1 struct el {
737	 2   struct list_head list;             2   struct list_head list;
738	 3   long key;                          3   long key;
739	 4   spinlock_t mutex;                  4   spinlock_t mutex;
740	 5   int data;                          5   int data;
741	 6   /* Other data fields */            6   /* Other data fields */
742	 7 };                                   7 };
743	 8 rwlock_t listmutex;                  8 spinlock_t listmutex;
744	 9 struct el head;                      9 struct el head;
745	
746	 1 int search(long key, int *result)    1 int search(long key, int *result)
747	 2 {                                    2 {
748	 3   struct list_head *lp;              3   struct list_head *lp;
749	 4   struct el *p;                      4   struct el *p;
750	 5                                      5
751	 6   read_lock(&listmutex);             6   rcu_read_lock();
752	 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
753	 8     if (p->key == key) {             8     if (p->key == key) {
754	 9       *result = p->data;             9       *result = p->data;
755	10       read_unlock(&listmutex);      10       rcu_read_unlock();
756	11       return 1;                     11       return 1;
757	12     }                               12     }
758	13   }                                 13   }
759	14   read_unlock(&listmutex);          14   rcu_read_unlock();
760	15   return 0;                         15   return 0;
761	16 }                                   16 }
762	
763	 1 int delete(long key)                 1 int delete(long key)
764	 2 {                                    2 {
765	 3   struct el *p;                      3   struct el *p;
766	 4                                      4
767	 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
768	 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
769	 7     if (p->key == key) {             7     if (p->key == key) {
770	 8       list_del(&p->list);            8       list_del_rcu(&p->list);
771	 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
772	                                       10       synchronize_rcu();
773	10       kfree(p);                     11       kfree(p);
774	11       return 1;                     12       return 1;
775	12     }                               13     }
776	13   }                                 14   }
777	14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
778	15   return 0;                         16   return 0;
779	16 }                                   17 }
780	
781	Either way, the differences are quite small.  Read-side locking moves
782	to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
783	a reader-writer lock to a simple spinlock, and a synchronize_rcu()
784	precedes the kfree().
785	
786	However, there is one potential catch: the read-side and update-side
787	critical sections can now run concurrently.  In many cases, this will
788	not be a problem, but it is necessary to check carefully regardless.
789	For example, if multiple independent list updates must be seen as
790	a single atomic update, converting to RCU will require special care.
791	
792	Also, the presence of synchronize_rcu() means that the RCU version of
793	delete() can now block.  If this is a problem, there is a callback-based
794	mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
795	be used in place of synchronize_rcu().
796	
797	
798	7.  FULL LIST OF RCU APIs
799	
800	The RCU APIs are documented in docbook-format header comments in the
801	Linux-kernel source code, but it helps to have a full list of the
802	APIs, since there does not appear to be a way to categorize them
803	in docbook.  Here is the list, by category.
804	
805	RCU list traversal:
806	
807		list_entry_rcu
808		list_first_entry_rcu
809		list_next_rcu
810		list_for_each_entry_rcu
811		list_for_each_entry_continue_rcu
812		hlist_first_rcu
813		hlist_next_rcu
814		hlist_pprev_rcu
815		hlist_for_each_entry_rcu
816		hlist_for_each_entry_rcu_bh
817		hlist_for_each_entry_continue_rcu
818		hlist_for_each_entry_continue_rcu_bh
819		hlist_nulls_first_rcu
820		hlist_nulls_for_each_entry_rcu
821		hlist_bl_first_rcu
822		hlist_bl_for_each_entry_rcu
823	
824	RCU pointer/list update:
825	
826		rcu_assign_pointer
827		list_add_rcu
828		list_add_tail_rcu
829		list_del_rcu
830		list_replace_rcu
831		hlist_add_behind_rcu
832		hlist_add_before_rcu
833		hlist_add_head_rcu
834		hlist_del_rcu
835		hlist_del_init_rcu
836		hlist_replace_rcu
837		list_splice_init_rcu()
838		hlist_nulls_del_init_rcu
839		hlist_nulls_del_rcu
840		hlist_nulls_add_head_rcu
841		hlist_bl_add_head_rcu
842		hlist_bl_del_init_rcu
843		hlist_bl_del_rcu
844		hlist_bl_set_first_rcu
845	
846	RCU:	Critical sections	Grace period		Barrier
847	
848		rcu_read_lock		synchronize_net		rcu_barrier
849		rcu_read_unlock		synchronize_rcu
850		rcu_dereference		synchronize_rcu_expedited
851		rcu_read_lock_held	call_rcu
852		rcu_dereference_check	kfree_rcu
853		rcu_dereference_protected
854	
855	bh:	Critical sections	Grace period		Barrier
856	
857		rcu_read_lock_bh	call_rcu_bh		rcu_barrier_bh
858		rcu_read_unlock_bh	synchronize_rcu_bh
859		rcu_dereference_bh	synchronize_rcu_bh_expedited
860		rcu_dereference_bh_check
861		rcu_dereference_bh_protected
862		rcu_read_lock_bh_held
863	
864	sched:	Critical sections	Grace period		Barrier
865	
866		rcu_read_lock_sched	synchronize_sched	rcu_barrier_sched
867		rcu_read_unlock_sched	call_rcu_sched
868		[preempt_disable]	synchronize_sched_expedited
869		[and friends]
870		rcu_read_lock_sched_notrace
871		rcu_read_unlock_sched_notrace
872		rcu_dereference_sched
873		rcu_dereference_sched_check
874		rcu_dereference_sched_protected
875		rcu_read_lock_sched_held
876	
877	
878	SRCU:	Critical sections	Grace period		Barrier
879	
880		srcu_read_lock		synchronize_srcu	srcu_barrier
881		srcu_read_unlock	call_srcu
882		srcu_dereference	synchronize_srcu_expedited
883		srcu_dereference_check
884		srcu_read_lock_held
885	
886	SRCU:	Initialization/cleanup
887		init_srcu_struct
888		cleanup_srcu_struct
889	
890	All:  lockdep-checked RCU-protected pointer access
891	
892		rcu_access_pointer
893		rcu_dereference_raw
894		RCU_LOCKDEP_WARN
895		rcu_sleep_check
896		RCU_NONIDLE
897	
898	See the comment headers in the source code (or the docbook generated
899	from them) for more information.
900	
901	However, given that there are no fewer than four families of RCU APIs
902	in the Linux kernel, how do you choose which one to use?  The following
903	list can be helpful:
904	
905	a.	Will readers need to block?  If so, you need SRCU.
906	
907	b.	What about the -rt patchset?  If readers would need to block
908		in an non-rt kernel, you need SRCU.  If readers would block
909		in a -rt kernel, but not in a non-rt kernel, SRCU is not
910		necessary.
911	
912	c.	Do you need to treat NMI handlers, hardirq handlers,
913		and code segments with preemption disabled (whether
914		via preempt_disable(), local_irq_save(), local_bh_disable(),
915		or some other mechanism) as if they were explicit RCU readers?
916		If so, RCU-sched is the only choice that will work for you.
917	
918	d.	Do you need RCU grace periods to complete even in the face
919		of softirq monopolization of one or more of the CPUs?  For
920		example, is your code subject to network-based denial-of-service
921		attacks?  If so, you need RCU-bh.
922	
923	e.	Is your workload too update-intensive for normal use of
924		RCU, but inappropriate for other synchronization mechanisms?
925		If so, consider SLAB_DESTROY_BY_RCU.  But please be careful!
926	
927	f.	Do you need read-side critical sections that are respected
928		even though they are in the middle of the idle loop, during
929		user-mode execution, or on an offlined CPU?  If so, SRCU is the
930		only choice that will work for you.
931	
932	g.	Otherwise, use RCU.
933	
934	Of course, this all assumes that you have determined that RCU is in fact
935	the right tool for your job.
936	
937	
938	8.  ANSWERS TO QUICK QUIZZES
939	
940	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
941			occur when using this algorithm in a real-world Linux
942			kernel?  [Referring to the lock-based "toy" RCU
943			algorithm.]
944	
945	Answer:		Consider the following sequence of events:
946	
947			1.	CPU 0 acquires some unrelated lock, call it
948				"problematic_lock", disabling irq via
949				spin_lock_irqsave().
950	
951			2.	CPU 1 enters synchronize_rcu(), write-acquiring
952				rcu_gp_mutex.
953	
954			3.	CPU 0 enters rcu_read_lock(), but must wait
955				because CPU 1 holds rcu_gp_mutex.
956	
957			4.	CPU 1 is interrupted, and the irq handler
958				attempts to acquire problematic_lock.
959	
960			The system is now deadlocked.
961	
962			One way to avoid this deadlock is to use an approach like
963			that of CONFIG_PREEMPT_RT, where all normal spinlocks
964			become blocking locks, and all irq handlers execute in
965			the context of special tasks.  In this case, in step 4
966			above, the irq handler would block, allowing CPU 1 to
967			release rcu_gp_mutex, avoiding the deadlock.
968	
969			Even in the absence of deadlock, this RCU implementation
970			allows latency to "bleed" from readers to other
971			readers through synchronize_rcu().  To see this,
972			consider task A in an RCU read-side critical section
973			(thus read-holding rcu_gp_mutex), task B blocked
974			attempting to write-acquire rcu_gp_mutex, and
975			task C blocked in rcu_read_lock() attempting to
976			read_acquire rcu_gp_mutex.  Task A's RCU read-side
977			latency is holding up task C, albeit indirectly via
978			task B.
979	
980			Realtime RCU implementations therefore use a counter-based
981			approach where tasks in RCU read-side critical sections
982			cannot be blocked by tasks executing synchronize_rcu().
983	
984	Quick Quiz #2:	Give an example where Classic RCU's read-side
985			overhead is -negative-.
986	
987	Answer:		Imagine a single-CPU system with a non-CONFIG_PREEMPT
988			kernel where a routing table is used by process-context
989			code, but can be updated by irq-context code (for example,
990			by an "ICMP REDIRECT" packet).	The usual way of handling
991			this would be to have the process-context code disable
992			interrupts while searching the routing table.  Use of
993			RCU allows such interrupt-disabling to be dispensed with.
994			Thus, without RCU, you pay the cost of disabling interrupts,
995			and with RCU you don't.
996	
997			One can argue that the overhead of RCU in this
998			case is negative with respect to the single-CPU
999			interrupt-disabling approach.  Others might argue that
1000			the overhead of RCU is merely zero, and that replacing
1001			the positive overhead of the interrupt-disabling scheme
1002			with the zero-overhead RCU scheme does not constitute
1003			negative overhead.
1004	
1005			In real life, of course, things are more complex.  But
1006			even the theoretical possibility of negative overhead for
1007			a synchronization primitive is a bit unexpected.  ;-)
1008	
1009	Quick Quiz #3:  If it is illegal to block in an RCU read-side
1010			critical section, what the heck do you do in
1011			PREEMPT_RT, where normal spinlocks can block???
1012	
1013	Answer:		Just as PREEMPT_RT permits preemption of spinlock
1014			critical sections, it permits preemption of RCU
1015			read-side critical sections.  It also permits
1016			spinlocks blocking while in RCU read-side critical
1017			sections.
1018	
1019			Why the apparent inconsistency?  Because it is it
1020			possible to use priority boosting to keep the RCU
1021			grace periods short if need be (for example, if running
1022			short of memory).  In contrast, if blocking waiting
1023			for (say) network reception, there is no way to know
1024			what should be boosted.  Especially given that the
1025			process we need to boost might well be a human being
1026			who just went out for a pizza or something.  And although
1027			a computer-operated cattle prod might arouse serious
1028			interest, it might also provoke serious objections.
1029			Besides, how does the computer know what pizza parlor
1030			the human being went to???
1031	
1032	
1033	ACKNOWLEDGEMENTS
1034	
1035	My thanks to the people who helped make this human-readable, including
1036	Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1037	
1038	
1039	For more information, see http://www.rdrop.com/users/paulmck/RCU.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.