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Documentation / RCU / whatisRCU.txt

Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.

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