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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 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		2010 Big API Table           http://lwn.net/Articles/419086/
9	5.	The RCU API, 2014 Edition    http://lwn.net/Articles/609904/
10		2014 Big API Table           http://lwn.net/Articles/609973/
11	
12	
13	What is RCU?
14	
15	RCU is a synchronization mechanism that was added to the Linux kernel
16	during the 2.5 development effort that is optimized for read-mostly
17	situations.  Although RCU is actually quite simple once you understand it,
18	getting there can sometimes be a challenge.  Part of the problem is that
19	most of the past descriptions of RCU have been written with the mistaken
20	assumption that there is "one true way" to describe RCU.  Instead,
21	the experience has been that different people must take different paths
22	to arrive at an understanding of RCU.  This document provides several
23	different paths, as follows:
24	
25	1.	RCU OVERVIEW
26	2.	WHAT IS RCU'S CORE API?
27	3.	WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
28	4.	WHAT IF MY UPDATING THREAD CANNOT BLOCK?
29	5.	WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
30	6.	ANALOGY WITH READER-WRITER LOCKING
31	7.	FULL LIST OF RCU APIs
32	8.	ANSWERS TO QUICK QUIZZES
33	
34	People who prefer starting with a conceptual overview should focus on
35	Section 1, though most readers will profit by reading this section at
36	some point.  People who prefer to start with an API that they can then
37	experiment with should focus on Section 2.  People who prefer to start
38	with example uses should focus on Sections 3 and 4.  People who need to
39	understand the RCU implementation should focus on Section 5, then dive
40	into the kernel source code.  People who reason best by analogy should
41	focus on Section 6.  Section 7 serves as an index to the docbook API
42	documentation, and Section 8 is the traditional answer key.
43	
44	So, start with the section that makes the most sense to you and your
45	preferred method of learning.  If you need to know everything about
46	everything, feel free to read the whole thing -- but if you are really
47	that type of person, you have perused the source code and will therefore
48	never need this document anyway.  ;-)
49	
50	
51	1.  RCU OVERVIEW
52	
53	The basic idea behind RCU is to split updates into "removal" and
54	"reclamation" phases.  The removal phase removes references to data items
55	within a data structure (possibly by replacing them with references to
56	new versions of these data items), and can run concurrently with readers.
57	The reason that it is safe to run the removal phase concurrently with
58	readers is the semantics of modern CPUs guarantee that readers will see
59	either the old or the new version of the data structure rather than a
60	partially updated reference.  The reclamation phase does the work of reclaiming
61	(e.g., freeing) the data items removed from the data structure during the
62	removal phase.  Because reclaiming data items can disrupt any readers
63	concurrently referencing those data items, the reclamation phase must
64	not start until readers no longer hold references to those data items.
65	
66	Splitting the update into removal and reclamation phases permits the
67	updater to perform the removal phase immediately, and to defer the
68	reclamation phase until all readers active during the removal phase have
69	completed, either by blocking until they finish or by registering a
70	callback that is invoked after they finish.  Only readers that are active
71	during the removal phase need be considered, because any reader starting
72	after the removal phase will be unable to gain a reference to the removed
73	data items, and therefore cannot be disrupted by the reclamation phase.
74	
75	So the typical RCU update sequence goes something like the following:
76	
77	a.	Remove pointers to a data structure, so that subsequent
78		readers cannot gain a reference to it.
79	
80	b.	Wait for all previous readers to complete their RCU read-side
81		critical sections.
82	
83	c.	At this point, there cannot be any readers who hold references
84		to the data structure, so it now may safely be reclaimed
85		(e.g., kfree()d).
86	
87	Step (b) above is the key idea underlying RCU's deferred destruction.
88	The ability to wait until all readers are done allows RCU readers to
89	use much lighter-weight synchronization, in some cases, absolutely no
90	synchronization at all.  In contrast, in more conventional lock-based
91	schemes, readers must use heavy-weight synchronization in order to
92	prevent an updater from deleting the data structure out from under them.
93	This is because lock-based updaters typically update data items in place,
94	and must therefore exclude readers.  In contrast, RCU-based updaters
95	typically take advantage of the fact that writes to single aligned
96	pointers are atomic on modern CPUs, allowing atomic insertion, removal,
97	and replacement of data items in a linked structure without disrupting
98	readers.  Concurrent RCU readers can then continue accessing the old
99	versions, and can dispense with the atomic operations, memory barriers,
100	and communications cache misses that are so expensive on present-day
101	SMP computer systems, even in absence of lock contention.
102	
103	In the three-step procedure shown above, the updater is performing both
104	the removal and the reclamation step, but it is often helpful for an
105	entirely different thread to do the reclamation, as is in fact the case
106	in the Linux kernel's directory-entry cache (dcache).  Even if the same
107	thread performs both the update step (step (a) above) and the reclamation
108	step (step (c) above), it is often helpful to think of them separately.
109	For example, RCU readers and updaters need not communicate at all,
110	but RCU provides implicit low-overhead communication between readers
111	and reclaimers, namely, in step (b) above.
112	
113	So how the heck can a reclaimer tell when a reader is done, given
114	that readers are not doing any sort of synchronization operations???
115	Read on to learn about how RCU's API makes this easy.
116	
117	
118	2.  WHAT IS RCU'S CORE API?
119	
120	The core RCU API is quite small:
121	
122	a.	rcu_read_lock()
123	b.	rcu_read_unlock()
124	c.	synchronize_rcu() / call_rcu()
125	d.	rcu_assign_pointer()
126	e.	rcu_dereference()
127	
128	There are many other members of the RCU API, but the rest can be
129	expressed in terms of these five, though most implementations instead
130	express synchronize_rcu() in terms of the call_rcu() callback API.
131	
132	The five core RCU APIs are described below, the other 18 will be enumerated
133	later.  See the kernel docbook documentation for more info, or look directly
134	at the function header comments.
135	
136	rcu_read_lock()
137	
138		void rcu_read_lock(void);
139	
140		Used by a reader to inform the reclaimer that the reader is
141		entering an RCU read-side critical section.  It is illegal
142		to block while in an RCU read-side critical section, though
143		kernels built with CONFIG_PREEMPT_RCU can preempt RCU
144		read-side critical sections.  Any RCU-protected data structure
145		accessed during an RCU read-side critical section is guaranteed to
146		remain unreclaimed for the full duration of that critical section.
147		Reference counts may be used in conjunction with RCU to maintain
148		longer-term references to data structures.
149	
150	rcu_read_unlock()
151	
152		void rcu_read_unlock(void);
153	
154		Used by a reader to inform the reclaimer that the reader is
155		exiting an RCU read-side critical section.  Note that RCU
156		read-side critical sections may be nested and/or overlapping.
157	
158	synchronize_rcu()
159	
160		void synchronize_rcu(void);
161	
162		Marks the end of updater code and the beginning of reclaimer
163		code.  It does this by blocking until all pre-existing RCU
164		read-side critical sections on all CPUs have completed.
165		Note that synchronize_rcu() will -not- necessarily wait for
166		any subsequent RCU read-side critical sections to complete.
167		For example, consider the following sequence of events:
168	
169		         CPU 0                  CPU 1                 CPU 2
170		     ----------------- ------------------------- ---------------
171		 1.  rcu_read_lock()
172		 2.                    enters synchronize_rcu()
173		 3.                                               rcu_read_lock()
174		 4.  rcu_read_unlock()
175		 5.                     exits synchronize_rcu()
176		 6.                                              rcu_read_unlock()
177	
178		To reiterate, synchronize_rcu() waits only for ongoing RCU
179		read-side critical sections to complete, not necessarily for
180		any that begin after synchronize_rcu() is invoked.
181	
182		Of course, synchronize_rcu() does not necessarily return
183		-immediately- after the last pre-existing RCU read-side critical
184		section completes.  For one thing, there might well be scheduling
185		delays.  For another thing, many RCU implementations process
186		requests in batches in order to improve efficiencies, which can
187		further delay synchronize_rcu().
188	
189		Since synchronize_rcu() is the API that must figure out when
190		readers are done, its implementation is key to RCU.  For RCU
191		to be useful in all but the most read-intensive situations,
192		synchronize_rcu()'s overhead must also be quite small.
193	
194		The call_rcu() API is a callback form of synchronize_rcu(),
195		and is described in more detail in a later section.  Instead of
196		blocking, it registers a function and argument which are invoked
197		after all ongoing RCU read-side critical sections have completed.
198		This callback variant is particularly useful in situations where
199		it is illegal to block or where update-side performance is
200		critically important.
201	
202		However, the call_rcu() API should not be used lightly, as use
203		of the synchronize_rcu() API generally results in simpler code.
204		In addition, the synchronize_rcu() API has the nice property
205		of automatically limiting update rate should grace periods
206		be delayed.  This property results in system resilience in face
207		of denial-of-service attacks.  Code using call_rcu() should limit
208		update rate in order to gain this same sort of resilience.  See
209		checklist.txt for some approaches to limiting the update rate.
210	
211	rcu_assign_pointer()
212	
213		typeof(p) rcu_assign_pointer(p, typeof(p) v);
214	
215		Yes, rcu_assign_pointer() -is- implemented as a macro, though it
216		would be cool to be able to declare a function in this manner.
217		(Compiler experts will no doubt disagree.)
218	
219		The updater uses this function to assign a new value to an
220		RCU-protected pointer, in order to safely communicate the change
221		in value from the updater to the reader.  This function returns
222		the new value, and also executes any memory-barrier instructions
223		required for a given CPU architecture.
224	
225		Perhaps just as important, it serves to document (1) which
226		pointers are protected by RCU and (2) the point at which a
227		given structure becomes accessible to other CPUs.  That said,
228		rcu_assign_pointer() is most frequently used indirectly, via
229		the _rcu list-manipulation primitives such as list_add_rcu().
230	
231	rcu_dereference()
232	
233		typeof(p) rcu_dereference(p);
234	
235		Like rcu_assign_pointer(), rcu_dereference() must be implemented
236		as a macro.
237	
238		The reader uses rcu_dereference() to fetch an RCU-protected
239		pointer, which returns a value that may then be safely
240		dereferenced.  Note that rcu_deference() does not actually
241		dereference the pointer, instead, it protects the pointer for
242		later dereferencing.  It also executes any needed memory-barrier
243		instructions for a given CPU architecture.  Currently, only Alpha
244		needs memory barriers within rcu_dereference() -- on other CPUs,
245		it compiles to nothing, not even a compiler directive.
246	
247		Common coding practice uses rcu_dereference() to copy an
248		RCU-protected pointer to a local variable, then dereferences
249		this local variable, for example as follows:
250	
251			p = rcu_dereference(head.next);
252			return p->data;
253	
254		However, in this case, one could just as easily combine these
255		into one statement:
256	
257			return rcu_dereference(head.next)->data;
258	
259		If you are going to be fetching multiple fields from the
260		RCU-protected structure, using the local variable is of
261		course preferred.  Repeated rcu_dereference() calls look
262		ugly, do not guarantee that the same pointer will be returned
263		if an update happened while in the critical section, and incur
264		unnecessary overhead on Alpha CPUs.
265	
266		Note that the value returned by rcu_dereference() is valid
267		only within the enclosing RCU read-side critical section.
268		For example, the following is -not- legal:
269	
270			rcu_read_lock();
271			p = rcu_dereference(head.next);
272			rcu_read_unlock();
273			x = p->address;	/* BUG!!! */
274			rcu_read_lock();
275			y = p->data;	/* BUG!!! */
276			rcu_read_unlock();
277	
278		Holding a reference from one RCU read-side critical section
279		to another is just as illegal as holding a reference from
280		one lock-based critical section to another!  Similarly,
281		using a reference outside of the critical section in which
282		it was acquired is just as illegal as doing so with normal
283		locking.
284	
285		As with rcu_assign_pointer(), an important function of
286		rcu_dereference() is to document which pointers are protected by
287		RCU, in particular, flagging a pointer that is subject to changing
288		at any time, including immediately after the rcu_dereference().
289		And, again like rcu_assign_pointer(), rcu_dereference() is
290		typically used indirectly, via the _rcu list-manipulation
291		primitives, such as list_for_each_entry_rcu().
292	
293	The following diagram shows how each API communicates among the
294	reader, updater, and reclaimer.
295	
296	
297		    rcu_assign_pointer()
298		    			    +--------+
299		    +---------------------->| reader |---------+
300		    |                       +--------+         |
301		    |                           |              |
302		    |                           |              | Protect:
303		    |                           |              | rcu_read_lock()
304		    |                           |              | rcu_read_unlock()
305		    |        rcu_dereference()  |              |
306	       +---------+                      |              |
307	       | updater |<---------------------+              |
308	       +---------+                                     V
309		    |                                    +-----------+
310		    +----------------------------------->| reclaimer |
311		    				         +-----------+
312		      Defer:
313		      synchronize_rcu() & call_rcu()
314	
315	
316	The RCU infrastructure observes the time sequence of rcu_read_lock(),
317	rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
318	order to determine when (1) synchronize_rcu() invocations may return
319	to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
320	implementations of the RCU infrastructure make heavy use of batching in
321	order to amortize their overhead over many uses of the corresponding APIs.
322	
323	There are no fewer than three RCU mechanisms in the Linux kernel; the
324	diagram above shows the first one, which is by far the most commonly used.
325	The rcu_dereference() and rcu_assign_pointer() primitives are used for
326	all three mechanisms, but different defer and protect primitives are
327	used as follows:
328	
329		Defer			Protect
330	
331	a.	synchronize_rcu()	rcu_read_lock() / rcu_read_unlock()
332		call_rcu()		rcu_dereference()
333	
334	b.	synchronize_rcu_bh()	rcu_read_lock_bh() / rcu_read_unlock_bh()
335		call_rcu_bh()		rcu_dereference_bh()
336	
337	c.	synchronize_sched()	rcu_read_lock_sched() / rcu_read_unlock_sched()
338		call_rcu_sched()	preempt_disable() / preempt_enable()
339					local_irq_save() / local_irq_restore()
340					hardirq enter / hardirq exit
341					NMI enter / NMI exit
342					rcu_dereference_sched()
343	
344	These three mechanisms are used as follows:
345	
346	a.	RCU applied to normal data structures.
347	
348	b.	RCU applied to networking data structures that may be subjected
349		to remote denial-of-service attacks.
350	
351	c.	RCU applied to scheduler and interrupt/NMI-handler tasks.
352	
353	Again, most uses will be of (a).  The (b) and (c) cases are important
354	for specialized uses, but are relatively uncommon.
355	
356	
357	3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
358	
359	This section shows a simple use of the core RCU API to protect a
360	global pointer to a dynamically allocated structure.  More-typical
361	uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
362	
363		struct foo {
364			int a;
365			char b;
366			long c;
367		};
368		DEFINE_SPINLOCK(foo_mutex);
369	
370		struct foo __rcu *gbl_foo;
371	
372		/*
373		 * Create a new struct foo that is the same as the one currently
374		 * pointed to by gbl_foo, except that field "a" is replaced
375		 * with "new_a".  Points gbl_foo to the new structure, and
376		 * frees up the old structure after a grace period.
377		 *
378		 * Uses rcu_assign_pointer() to ensure that concurrent readers
379		 * see the initialized version of the new structure.
380		 *
381		 * Uses synchronize_rcu() to ensure that any readers that might
382		 * have references to the old structure complete before freeing
383		 * the old structure.
384		 */
385		void foo_update_a(int new_a)
386		{
387			struct foo *new_fp;
388			struct foo *old_fp;
389	
390			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
391			spin_lock(&foo_mutex);
392			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
393			*new_fp = *old_fp;
394			new_fp->a = new_a;
395			rcu_assign_pointer(gbl_foo, new_fp);
396			spin_unlock(&foo_mutex);
397			synchronize_rcu();
398			kfree(old_fp);
399		}
400	
401		/*
402		 * Return the value of field "a" of the current gbl_foo
403		 * structure.  Use rcu_read_lock() and rcu_read_unlock()
404		 * to ensure that the structure does not get deleted out
405		 * from under us, and use rcu_dereference() to ensure that
406		 * we see the initialized version of the structure (important
407		 * for DEC Alpha and for people reading the code).
408		 */
409		int foo_get_a(void)
410		{
411			int retval;
412	
413			rcu_read_lock();
414			retval = rcu_dereference(gbl_foo)->a;
415			rcu_read_unlock();
416			return retval;
417		}
418	
419	So, to sum up:
420	
421	o	Use rcu_read_lock() and rcu_read_unlock() to guard RCU
422		read-side critical sections.
423	
424	o	Within an RCU read-side critical section, use rcu_dereference()
425		to dereference RCU-protected pointers.
426	
427	o	Use some solid scheme (such as locks or semaphores) to
428		keep concurrent updates from interfering with each other.
429	
430	o	Use rcu_assign_pointer() to update an RCU-protected pointer.
431		This primitive protects concurrent readers from the updater,
432		-not- concurrent updates from each other!  You therefore still
433		need to use locking (or something similar) to keep concurrent
434		rcu_assign_pointer() primitives from interfering with each other.
435	
436	o	Use synchronize_rcu() -after- removing a data element from an
437		RCU-protected data structure, but -before- reclaiming/freeing
438		the data element, in order to wait for the completion of all
439		RCU read-side critical sections that might be referencing that
440		data item.
441	
442	See checklist.txt for additional rules to follow when using RCU.
443	And again, more-typical uses of RCU may be found in listRCU.txt,
444	arrayRCU.txt, and NMI-RCU.txt.
445	
446	
447	4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?
448	
449	In the example above, foo_update_a() blocks until a grace period elapses.
450	This is quite simple, but in some cases one cannot afford to wait so
451	long -- there might be other high-priority work to be done.
452	
453	In such cases, one uses call_rcu() rather than synchronize_rcu().
454	The call_rcu() API is as follows:
455	
456		void call_rcu(struct rcu_head * head,
457			      void (*func)(struct rcu_head *head));
458	
459	This function invokes func(head) after a grace period has elapsed.
460	This invocation might happen from either softirq or process context,
461	so the function is not permitted to block.  The foo struct needs to
462	have an rcu_head structure added, perhaps as follows:
463	
464		struct foo {
465			int a;
466			char b;
467			long c;
468			struct rcu_head rcu;
469		};
470	
471	The foo_update_a() function might then be written as follows:
472	
473		/*
474		 * Create a new struct foo that is the same as the one currently
475		 * pointed to by gbl_foo, except that field "a" is replaced
476		 * with "new_a".  Points gbl_foo to the new structure, and
477		 * frees up the old structure after a grace period.
478		 *
479		 * Uses rcu_assign_pointer() to ensure that concurrent readers
480		 * see the initialized version of the new structure.
481		 *
482		 * Uses call_rcu() to ensure that any readers that might have
483		 * references to the old structure complete before freeing the
484		 * old structure.
485		 */
486		void foo_update_a(int new_a)
487		{
488			struct foo *new_fp;
489			struct foo *old_fp;
490	
491			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
492			spin_lock(&foo_mutex);
493			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
494			*new_fp = *old_fp;
495			new_fp->a = new_a;
496			rcu_assign_pointer(gbl_foo, new_fp);
497			spin_unlock(&foo_mutex);
498			call_rcu(&old_fp->rcu, foo_reclaim);
499		}
500	
501	The foo_reclaim() function might appear as follows:
502	
503		void foo_reclaim(struct rcu_head *rp)
504		{
505			struct foo *fp = container_of(rp, struct foo, rcu);
506	
507			foo_cleanup(fp->a);
508	
509			kfree(fp);
510		}
511	
512	The container_of() primitive is a macro that, given a pointer into a
513	struct, the type of the struct, and the pointed-to field within the
514	struct, returns a pointer to the beginning of the struct.
515	
516	The use of call_rcu() permits the caller of foo_update_a() to
517	immediately regain control, without needing to worry further about the
518	old version of the newly updated element.  It also clearly shows the
519	RCU distinction between updater, namely foo_update_a(), and reclaimer,
520	namely foo_reclaim().
521	
522	The summary of advice is the same as for the previous section, except
523	that we are now using call_rcu() rather than synchronize_rcu():
524	
525	o	Use call_rcu() -after- removing a data element from an
526		RCU-protected data structure in order to register a callback
527		function that will be invoked after the completion of all RCU
528		read-side critical sections that might be referencing that
529		data item.
530	
531	If the callback for call_rcu() is not doing anything more than calling
532	kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
533	to avoid having to write your own callback:
534	
535		kfree_rcu(old_fp, rcu);
536	
537	Again, see checklist.txt for additional rules governing the use of RCU.
538	
539	
540	5.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
541	
542	One of the nice things about RCU is that it has extremely simple "toy"
543	implementations that are a good first step towards understanding the
544	production-quality implementations in the Linux kernel.  This section
545	presents two such "toy" implementations of RCU, one that is implemented
546	in terms of familiar locking primitives, and another that more closely
547	resembles "classic" RCU.  Both are way too simple for real-world use,
548	lacking both functionality and performance.  However, they are useful
549	in getting a feel for how RCU works.  See kernel/rcupdate.c for a
550	production-quality implementation, and see:
551	
552		http://www.rdrop.com/users/paulmck/RCU
553	
554	for papers describing the Linux kernel RCU implementation.  The OLS'01
555	and OLS'02 papers are a good introduction, and the dissertation provides
556	more details on the current implementation as of early 2004.
557	
558	
559	5A.  "TOY" IMPLEMENTATION #1: LOCKING
560	
561	This section presents a "toy" RCU implementation that is based on
562	familiar locking primitives.  Its overhead makes it a non-starter for
563	real-life use, as does its lack of scalability.  It is also unsuitable
564	for realtime use, since it allows scheduling latency to "bleed" from
565	one read-side critical section to another.
566	
567	However, it is probably the easiest implementation to relate to, so is
568	a good starting point.
569	
570	It is extremely simple:
571	
572		static DEFINE_RWLOCK(rcu_gp_mutex);
573	
574		void rcu_read_lock(void)
575		{
576			read_lock(&rcu_gp_mutex);
577		}
578	
579		void rcu_read_unlock(void)
580		{
581			read_unlock(&rcu_gp_mutex);
582		}
583	
584		void synchronize_rcu(void)
585		{
586			write_lock(&rcu_gp_mutex);
587			write_unlock(&rcu_gp_mutex);
588		}
589	
590	[You can ignore rcu_assign_pointer() and rcu_dereference() without
591	missing much.  But here they are anyway.  And whatever you do, don't
592	forget about them when submitting patches making use of RCU!]
593	
594		#define rcu_assign_pointer(p, v)	({ \
595								smp_wmb(); \
596								(p) = (v); \
597							})
598	
599		#define rcu_dereference(p)     ({ \
600						typeof(p) _________p1 = p; \
601						smp_read_barrier_depends(); \
602						(_________p1); \
603						})
604	
605	
606	The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
607	and release a global reader-writer lock.  The synchronize_rcu()
608	primitive write-acquires this same lock, then immediately releases
609	it.  This means that once synchronize_rcu() exits, all RCU read-side
610	critical sections that were in progress before synchronize_rcu() was
611	called are guaranteed to have completed -- there is no way that
612	synchronize_rcu() would have been able to write-acquire the lock
613	otherwise.
614	
615	It is possible to nest rcu_read_lock(), since reader-writer locks may
616	be recursively acquired.  Note also that rcu_read_lock() is immune
617	from deadlock (an important property of RCU).  The reason for this is
618	that the only thing that can block rcu_read_lock() is a synchronize_rcu().
619	But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
620	so there can be no deadlock cycle.
621	
622	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
623			occur when using this algorithm in a real-world Linux
624			kernel?  How could this deadlock be avoided?
625	
626	
627	5B.  "TOY" EXAMPLE #2: CLASSIC RCU
628	
629	This section presents a "toy" RCU implementation that is based on
630	"classic RCU".  It is also short on performance (but only for updates) and
631	on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
632	kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
633	are the same as those shown in the preceding section, so they are omitted.
634	
635		void rcu_read_lock(void) { }
636	
637		void rcu_read_unlock(void) { }
638	
639		void synchronize_rcu(void)
640		{
641			int cpu;
642	
643			for_each_possible_cpu(cpu)
644				run_on(cpu);
645		}
646	
647	Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
648	This is the great strength of classic RCU in a non-preemptive kernel:
649	read-side overhead is precisely zero, at least on non-Alpha CPUs.
650	And there is absolutely no way that rcu_read_lock() can possibly
651	participate in a deadlock cycle!
652	
653	The implementation of synchronize_rcu() simply schedules itself on each
654	CPU in turn.  The run_on() primitive can be implemented straightforwardly
655	in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
656	"toy" implementation would restore the affinity upon completion rather
657	than just leaving all tasks running on the last CPU, but when I said
658	"toy", I meant -toy-!
659	
660	So how the heck is this supposed to work???
661	
662	Remember that it is illegal to block while in an RCU read-side critical
663	section.  Therefore, if a given CPU executes a context switch, we know
664	that it must have completed all preceding RCU read-side critical sections.
665	Once -all- CPUs have executed a context switch, then -all- preceding
666	RCU read-side critical sections will have completed.
667	
668	So, suppose that we remove a data item from its structure and then invoke
669	synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
670	that there are no RCU read-side critical sections holding a reference
671	to that data item, so we can safely reclaim it.
672	
673	Quick Quiz #2:	Give an example where Classic RCU's read-side
674			overhead is -negative-.
675	
676	Quick Quiz #3:  If it is illegal to block in an RCU read-side
677			critical section, what the heck do you do in
678			PREEMPT_RT, where normal spinlocks can block???
679	
680	
681	6.  ANALOGY WITH READER-WRITER LOCKING
682	
683	Although RCU can be used in many different ways, a very common use of
684	RCU is analogous to reader-writer locking.  The following unified
685	diff shows how closely related RCU and reader-writer locking can be.
686	
687		@@ -5,5 +5,5 @@ struct el {
688		 	int data;
689		 	/* Other data fields */
690		 };
691		-rwlock_t listmutex;
692		+spinlock_t listmutex;
693		 struct el head;
694	
695		@@ -13,15 +14,15 @@
696			struct list_head *lp;
697			struct el *p;
698	
699		-	read_lock(&listmutex);
700		-	list_for_each_entry(p, head, lp) {
701		+	rcu_read_lock();
702		+	list_for_each_entry_rcu(p, head, lp) {
703				if (p->key == key) {
704					*result = p->data;
705		-			read_unlock(&listmutex);
706		+			rcu_read_unlock();
707					return 1;
708				}
709			}
710		-	read_unlock(&listmutex);
711		+	rcu_read_unlock();
712			return 0;
713		 }
714	
715		@@ -29,15 +30,16 @@
716		 {
717			struct el *p;
718	
719		-	write_lock(&listmutex);
720		+	spin_lock(&listmutex);
721			list_for_each_entry(p, head, lp) {
722				if (p->key == key) {
723		-			list_del(&p->list);
724		-			write_unlock(&listmutex);
725		+			list_del_rcu(&p->list);
726		+			spin_unlock(&listmutex);
727		+			synchronize_rcu();
728					kfree(p);
729					return 1;
730				}
731			}
732		-	write_unlock(&listmutex);
733		+	spin_unlock(&listmutex);
734			return 0;
735		 }
736	
737	Or, for those who prefer a side-by-side listing:
738	
739	 1 struct el {                          1 struct el {
740	 2   struct list_head list;             2   struct list_head list;
741	 3   long key;                          3   long key;
742	 4   spinlock_t mutex;                  4   spinlock_t mutex;
743	 5   int data;                          5   int data;
744	 6   /* Other data fields */            6   /* Other data fields */
745	 7 };                                   7 };
746	 8 rwlock_t listmutex;                  8 spinlock_t listmutex;
747	 9 struct el head;                      9 struct el head;
748	
749	 1 int search(long key, int *result)    1 int search(long key, int *result)
750	 2 {                                    2 {
751	 3   struct list_head *lp;              3   struct list_head *lp;
752	 4   struct el *p;                      4   struct el *p;
753	 5                                      5
754	 6   read_lock(&listmutex);             6   rcu_read_lock();
755	 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
756	 8     if (p->key == key) {             8     if (p->key == key) {
757	 9       *result = p->data;             9       *result = p->data;
758	10       read_unlock(&listmutex);      10       rcu_read_unlock();
759	11       return 1;                     11       return 1;
760	12     }                               12     }
761	13   }                                 13   }
762	14   read_unlock(&listmutex);          14   rcu_read_unlock();
763	15   return 0;                         15   return 0;
764	16 }                                   16 }
765	
766	 1 int delete(long key)                 1 int delete(long key)
767	 2 {                                    2 {
768	 3   struct el *p;                      3   struct el *p;
769	 4                                      4
770	 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
771	 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
772	 7     if (p->key == key) {             7     if (p->key == key) {
773	 8       list_del(&p->list);            8       list_del_rcu(&p->list);
774	 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
775	                                       10       synchronize_rcu();
776	10       kfree(p);                     11       kfree(p);
777	11       return 1;                     12       return 1;
778	12     }                               13     }
779	13   }                                 14   }
780	14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
781	15   return 0;                         16   return 0;
782	16 }                                   17 }
783	
784	Either way, the differences are quite small.  Read-side locking moves
785	to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
786	a reader-writer lock to a simple spinlock, and a synchronize_rcu()
787	precedes the kfree().
788	
789	However, there is one potential catch: the read-side and update-side
790	critical sections can now run concurrently.  In many cases, this will
791	not be a problem, but it is necessary to check carefully regardless.
792	For example, if multiple independent list updates must be seen as
793	a single atomic update, converting to RCU will require special care.
794	
795	Also, the presence of synchronize_rcu() means that the RCU version of
796	delete() can now block.  If this is a problem, there is a callback-based
797	mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
798	be used in place of synchronize_rcu().
799	
800	
801	7.  FULL LIST OF RCU APIs
802	
803	The RCU APIs are documented in docbook-format header comments in the
804	Linux-kernel source code, but it helps to have a full list of the
805	APIs, since there does not appear to be a way to categorize them
806	in docbook.  Here is the list, by category.
807	
808	RCU list traversal:
809	
810		list_entry_rcu
811		list_first_entry_rcu
812		list_next_rcu
813		list_for_each_entry_rcu
814		list_for_each_entry_continue_rcu
815		hlist_first_rcu
816		hlist_next_rcu
817		hlist_pprev_rcu
818		hlist_for_each_entry_rcu
819		hlist_for_each_entry_rcu_bh
820		hlist_for_each_entry_continue_rcu
821		hlist_for_each_entry_continue_rcu_bh
822		hlist_nulls_first_rcu
823		hlist_nulls_for_each_entry_rcu
824		hlist_bl_first_rcu
825		hlist_bl_for_each_entry_rcu
826	
827	RCU pointer/list update:
828	
829		rcu_assign_pointer
830		list_add_rcu
831		list_add_tail_rcu
832		list_del_rcu
833		list_replace_rcu
834		hlist_add_behind_rcu
835		hlist_add_before_rcu
836		hlist_add_head_rcu
837		hlist_del_rcu
838		hlist_del_init_rcu
839		hlist_replace_rcu
840		list_splice_init_rcu()
841		hlist_nulls_del_init_rcu
842		hlist_nulls_del_rcu
843		hlist_nulls_add_head_rcu
844		hlist_bl_add_head_rcu
845		hlist_bl_del_init_rcu
846		hlist_bl_del_rcu
847		hlist_bl_set_first_rcu
848	
849	RCU:	Critical sections	Grace period		Barrier
850	
851		rcu_read_lock		synchronize_net		rcu_barrier
852		rcu_read_unlock		synchronize_rcu
853		rcu_dereference		synchronize_rcu_expedited
854		rcu_read_lock_held	call_rcu
855		rcu_dereference_check	kfree_rcu
856		rcu_dereference_protected
857	
858	bh:	Critical sections	Grace period		Barrier
859	
860		rcu_read_lock_bh	call_rcu_bh		rcu_barrier_bh
861		rcu_read_unlock_bh	synchronize_rcu_bh
862		rcu_dereference_bh	synchronize_rcu_bh_expedited
863		rcu_dereference_bh_check
864		rcu_dereference_bh_protected
865		rcu_read_lock_bh_held
866	
867	sched:	Critical sections	Grace period		Barrier
868	
869		rcu_read_lock_sched	synchronize_sched	rcu_barrier_sched
870		rcu_read_unlock_sched	call_rcu_sched
871		[preempt_disable]	synchronize_sched_expedited
872		[and friends]
873		rcu_read_lock_sched_notrace
874		rcu_read_unlock_sched_notrace
875		rcu_dereference_sched
876		rcu_dereference_sched_check
877		rcu_dereference_sched_protected
878		rcu_read_lock_sched_held
879	
880	
881	SRCU:	Critical sections	Grace period		Barrier
882	
883		srcu_read_lock		synchronize_srcu	srcu_barrier
884		srcu_read_unlock	call_srcu
885		srcu_dereference	synchronize_srcu_expedited
886		srcu_dereference_check
887		srcu_read_lock_held
888	
889	SRCU:	Initialization/cleanup
890		init_srcu_struct
891		cleanup_srcu_struct
892	
893	All:  lockdep-checked RCU-protected pointer access
894	
895		rcu_access_pointer
896		rcu_dereference_raw
897		RCU_LOCKDEP_WARN
898		rcu_sleep_check
899		RCU_NONIDLE
900	
901	See the comment headers in the source code (or the docbook generated
902	from them) for more information.
903	
904	However, given that there are no fewer than four families of RCU APIs
905	in the Linux kernel, how do you choose which one to use?  The following
906	list can be helpful:
907	
908	a.	Will readers need to block?  If so, you need SRCU.
909	
910	b.	What about the -rt patchset?  If readers would need to block
911		in an non-rt kernel, you need SRCU.  If readers would block
912		in a -rt kernel, but not in a non-rt kernel, SRCU is not
913		necessary.
914	
915	c.	Do you need to treat NMI handlers, hardirq handlers,
916		and code segments with preemption disabled (whether
917		via preempt_disable(), local_irq_save(), local_bh_disable(),
918		or some other mechanism) as if they were explicit RCU readers?
919		If so, RCU-sched is the only choice that will work for you.
920	
921	d.	Do you need RCU grace periods to complete even in the face
922		of softirq monopolization of one or more of the CPUs?  For
923		example, is your code subject to network-based denial-of-service
924		attacks?  If so, you need RCU-bh.
925	
926	e.	Is your workload too update-intensive for normal use of
927		RCU, but inappropriate for other synchronization mechanisms?
928		If so, consider SLAB_DESTROY_BY_RCU.  But please be careful!
929	
930	f.	Do you need read-side critical sections that are respected
931		even though they are in the middle of the idle loop, during
932		user-mode execution, or on an offlined CPU?  If so, SRCU is the
933		only choice that will work for you.
934	
935	g.	Otherwise, use RCU.
936	
937	Of course, this all assumes that you have determined that RCU is in fact
938	the right tool for your job.
939	
940	
941	8.  ANSWERS TO QUICK QUIZZES
942	
943	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
944			occur when using this algorithm in a real-world Linux
945			kernel?  [Referring to the lock-based "toy" RCU
946			algorithm.]
947	
948	Answer:		Consider the following sequence of events:
949	
950			1.	CPU 0 acquires some unrelated lock, call it
951				"problematic_lock", disabling irq via
952				spin_lock_irqsave().
953	
954			2.	CPU 1 enters synchronize_rcu(), write-acquiring
955				rcu_gp_mutex.
956	
957			3.	CPU 0 enters rcu_read_lock(), but must wait
958				because CPU 1 holds rcu_gp_mutex.
959	
960			4.	CPU 1 is interrupted, and the irq handler
961				attempts to acquire problematic_lock.
962	
963			The system is now deadlocked.
964	
965			One way to avoid this deadlock is to use an approach like
966			that of CONFIG_PREEMPT_RT, where all normal spinlocks
967			become blocking locks, and all irq handlers execute in
968			the context of special tasks.  In this case, in step 4
969			above, the irq handler would block, allowing CPU 1 to
970			release rcu_gp_mutex, avoiding the deadlock.
971	
972			Even in the absence of deadlock, this RCU implementation
973			allows latency to "bleed" from readers to other
974			readers through synchronize_rcu().  To see this,
975			consider task A in an RCU read-side critical section
976			(thus read-holding rcu_gp_mutex), task B blocked
977			attempting to write-acquire rcu_gp_mutex, and
978			task C blocked in rcu_read_lock() attempting to
979			read_acquire rcu_gp_mutex.  Task A's RCU read-side
980			latency is holding up task C, albeit indirectly via
981			task B.
982	
983			Realtime RCU implementations therefore use a counter-based
984			approach where tasks in RCU read-side critical sections
985			cannot be blocked by tasks executing synchronize_rcu().
986	
987	Quick Quiz #2:	Give an example where Classic RCU's read-side
988			overhead is -negative-.
989	
990	Answer:		Imagine a single-CPU system with a non-CONFIG_PREEMPT
991			kernel where a routing table is used by process-context
992			code, but can be updated by irq-context code (for example,
993			by an "ICMP REDIRECT" packet).	The usual way of handling
994			this would be to have the process-context code disable
995			interrupts while searching the routing table.  Use of
996			RCU allows such interrupt-disabling to be dispensed with.
997			Thus, without RCU, you pay the cost of disabling interrupts,
998			and with RCU you don't.
999	
1000			One can argue that the overhead of RCU in this
1001			case is negative with respect to the single-CPU
1002			interrupt-disabling approach.  Others might argue that
1003			the overhead of RCU is merely zero, and that replacing
1004			the positive overhead of the interrupt-disabling scheme
1005			with the zero-overhead RCU scheme does not constitute
1006			negative overhead.
1007	
1008			In real life, of course, things are more complex.  But
1009			even the theoretical possibility of negative overhead for
1010			a synchronization primitive is a bit unexpected.  ;-)
1011	
1012	Quick Quiz #3:  If it is illegal to block in an RCU read-side
1013			critical section, what the heck do you do in
1014			PREEMPT_RT, where normal spinlocks can block???
1015	
1016	Answer:		Just as PREEMPT_RT permits preemption of spinlock
1017			critical sections, it permits preemption of RCU
1018			read-side critical sections.  It also permits
1019			spinlocks blocking while in RCU read-side critical
1020			sections.
1021	
1022			Why the apparent inconsistency?  Because it is it
1023			possible to use priority boosting to keep the RCU
1024			grace periods short if need be (for example, if running
1025			short of memory).  In contrast, if blocking waiting
1026			for (say) network reception, there is no way to know
1027			what should be boosted.  Especially given that the
1028			process we need to boost might well be a human being
1029			who just went out for a pizza or something.  And although
1030			a computer-operated cattle prod might arouse serious
1031			interest, it might also provoke serious objections.
1032			Besides, how does the computer know what pizza parlor
1033			the human being went to???
1034	
1035	
1036	ACKNOWLEDGEMENTS
1037	
1038	My thanks to the people who helped make this human-readable, including
1039	Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1040	
1041	
1042	For more information, see http://www.rdrop.com/users/paulmck/RCU.
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