1.. _whatisrcu_doc:
2
3What is RCU?  --  "Read, Copy, Update"
4======================================
5
6Please note that the "What is RCU?" LWN series is an excellent place
7to start learning about RCU:
8
9| 1.	What is RCU, Fundamentally?  http://lwn.net/Articles/262464/
10| 2.	What is RCU? Part 2: Usage   http://lwn.net/Articles/263130/
11| 3.	RCU part 3: the RCU API      http://lwn.net/Articles/264090/
12| 4.	The RCU API, 2010 Edition    http://lwn.net/Articles/418853/
13| 	2010 Big API Table           http://lwn.net/Articles/419086/
14| 5.	The RCU API, 2014 Edition    http://lwn.net/Articles/609904/
15|	2014 Big API Table           http://lwn.net/Articles/609973/
16
17
18What is RCU?
19
20RCU is a synchronization mechanism that was added to the Linux kernel
21during the 2.5 development effort that is optimized for read-mostly
22situations.  Although RCU is actually quite simple once you understand it,
23getting there can sometimes be a challenge.  Part of the problem is that
24most of the past descriptions of RCU have been written with the mistaken
25assumption that there is "one true way" to describe RCU.  Instead,
26the experience has been that different people must take different paths
27to arrive at an understanding of RCU.  This document provides several
28different paths, as follows:
29
30:ref:`1.	RCU OVERVIEW <1_whatisRCU>`
31
32:ref:`2.	WHAT IS RCU'S CORE API? <2_whatisRCU>`
33
34:ref:`3.	WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>`
35
36:ref:`4.	WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>`
37
38:ref:`5.	WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>`
39
40:ref:`6.	ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
41
42:ref:`7.	FULL LIST OF RCU APIs <7_whatisRCU>`
43
44:ref:`8.	ANSWERS TO QUICK QUIZZES <8_whatisRCU>`
45
46People who prefer starting with a conceptual overview should focus on
47Section 1, though most readers will profit by reading this section at
48some point.  People who prefer to start with an API that they can then
49experiment with should focus on Section 2.  People who prefer to start
50with example uses should focus on Sections 3 and 4.  People who need to
51understand the RCU implementation should focus on Section 5, then dive
52into the kernel source code.  People who reason best by analogy should
53focus on Section 6.  Section 7 serves as an index to the docbook API
54documentation, and Section 8 is the traditional answer key.
55
56So, start with the section that makes the most sense to you and your
57preferred method of learning.  If you need to know everything about
58everything, feel free to read the whole thing -- but if you are really
59that type of person, you have perused the source code and will therefore
60never need this document anyway.  ;-)
61
62.. _1_whatisRCU:
63
641.  RCU OVERVIEW
65----------------
66
67The basic idea behind RCU is to split updates into "removal" and
68"reclamation" phases.  The removal phase removes references to data items
69within a data structure (possibly by replacing them with references to
70new versions of these data items), and can run concurrently with readers.
71The reason that it is safe to run the removal phase concurrently with
72readers is the semantics of modern CPUs guarantee that readers will see
73either the old or the new version of the data structure rather than a
74partially updated reference.  The reclamation phase does the work of reclaiming
75(e.g., freeing) the data items removed from the data structure during the
76removal phase.  Because reclaiming data items can disrupt any readers
77concurrently referencing those data items, the reclamation phase must
78not start until readers no longer hold references to those data items.
79
80Splitting the update into removal and reclamation phases permits the
81updater to perform the removal phase immediately, and to defer the
82reclamation phase until all readers active during the removal phase have
83completed, either by blocking until they finish or by registering a
84callback that is invoked after they finish.  Only readers that are active
85during the removal phase need be considered, because any reader starting
86after the removal phase will be unable to gain a reference to the removed
87data items, and therefore cannot be disrupted by the reclamation phase.
88
89So the typical RCU update sequence goes something like the following:
90
91a.	Remove pointers to a data structure, so that subsequent
92	readers cannot gain a reference to it.
93
94b.	Wait for all previous readers to complete their RCU read-side
95	critical sections.
96
97c.	At this point, there cannot be any readers who hold references
98	to the data structure, so it now may safely be reclaimed
99	(e.g., kfree()d).
100
101Step (b) above is the key idea underlying RCU's deferred destruction.
102The ability to wait until all readers are done allows RCU readers to
103use much lighter-weight synchronization, in some cases, absolutely no
104synchronization at all.  In contrast, in more conventional lock-based
105schemes, readers must use heavy-weight synchronization in order to
106prevent an updater from deleting the data structure out from under them.
107This is because lock-based updaters typically update data items in place,
108and must therefore exclude readers.  In contrast, RCU-based updaters
109typically take advantage of the fact that writes to single aligned
110pointers are atomic on modern CPUs, allowing atomic insertion, removal,
111and replacement of data items in a linked structure without disrupting
112readers.  Concurrent RCU readers can then continue accessing the old
113versions, and can dispense with the atomic operations, memory barriers,
114and communications cache misses that are so expensive on present-day
115SMP computer systems, even in absence of lock contention.
116
117In the three-step procedure shown above, the updater is performing both
118the removal and the reclamation step, but it is often helpful for an
119entirely different thread to do the reclamation, as is in fact the case
120in the Linux kernel's directory-entry cache (dcache).  Even if the same
121thread performs both the update step (step (a) above) and the reclamation
122step (step (c) above), it is often helpful to think of them separately.
123For example, RCU readers and updaters need not communicate at all,
124but RCU provides implicit low-overhead communication between readers
125and reclaimers, namely, in step (b) above.
126
127So how the heck can a reclaimer tell when a reader is done, given
128that readers are not doing any sort of synchronization operations???
129Read on to learn about how RCU's API makes this easy.
130
131.. _2_whatisRCU:
132
1332.  WHAT IS RCU'S CORE API?
134---------------------------
135
136The core RCU API is quite small:
137
138a.	rcu_read_lock()
139b.	rcu_read_unlock()
140c.	synchronize_rcu() / call_rcu()
141d.	rcu_assign_pointer()
142e.	rcu_dereference()
143
144There are many other members of the RCU API, but the rest can be
145expressed in terms of these five, though most implementations instead
146express synchronize_rcu() in terms of the call_rcu() callback API.
147
148The five core RCU APIs are described below, the other 18 will be enumerated
149later.  See the kernel docbook documentation for more info, or look directly
150at the function header comments.
151
152rcu_read_lock()
153^^^^^^^^^^^^^^^
154	void rcu_read_lock(void);
155
156	Used by a reader to inform the reclaimer that the reader is
157	entering an RCU read-side critical section.  It is illegal
158	to block while in an RCU read-side critical section, though
159	kernels built with CONFIG_PREEMPT_RCU can preempt RCU
160	read-side critical sections.  Any RCU-protected data structure
161	accessed during an RCU read-side critical section is guaranteed to
162	remain unreclaimed for the full duration of that critical section.
163	Reference counts may be used in conjunction with RCU to maintain
164	longer-term references to data structures.
165
166rcu_read_unlock()
167^^^^^^^^^^^^^^^^^
168	void rcu_read_unlock(void);
169
170	Used by a reader to inform the reclaimer that the reader is
171	exiting an RCU read-side critical section.  Note that RCU
172	read-side critical sections may be nested and/or overlapping.
173
174synchronize_rcu()
175^^^^^^^^^^^^^^^^^
176	void synchronize_rcu(void);
177
178	Marks the end of updater code and the beginning of reclaimer
179	code.  It does this by blocking until all pre-existing RCU
180	read-side critical sections on all CPUs have completed.
181	Note that synchronize_rcu() will **not** necessarily wait for
182	any subsequent RCU read-side critical sections to complete.
183	For example, consider the following sequence of events::
184
185	         CPU 0                  CPU 1                 CPU 2
186	     ----------------- ------------------------- ---------------
187	 1.  rcu_read_lock()
188	 2.                    enters synchronize_rcu()
189	 3.                                               rcu_read_lock()
190	 4.  rcu_read_unlock()
191	 5.                     exits synchronize_rcu()
192	 6.                                              rcu_read_unlock()
193
194	To reiterate, synchronize_rcu() waits only for ongoing RCU
195	read-side critical sections to complete, not necessarily for
196	any that begin after synchronize_rcu() is invoked.
197
198	Of course, synchronize_rcu() does not necessarily return
199	**immediately** after the last pre-existing RCU read-side critical
200	section completes.  For one thing, there might well be scheduling
201	delays.  For another thing, many RCU implementations process
202	requests in batches in order to improve efficiencies, which can
203	further delay synchronize_rcu().
204
205	Since synchronize_rcu() is the API that must figure out when
206	readers are done, its implementation is key to RCU.  For RCU
207	to be useful in all but the most read-intensive situations,
208	synchronize_rcu()'s overhead must also be quite small.
209
210	The call_rcu() API is a callback form of synchronize_rcu(),
211	and is described in more detail in a later section.  Instead of
212	blocking, it registers a function and argument which are invoked
213	after all ongoing RCU read-side critical sections have completed.
214	This callback variant is particularly useful in situations where
215	it is illegal to block or where update-side performance is
216	critically important.
217
218	However, the call_rcu() API should not be used lightly, as use
219	of the synchronize_rcu() API generally results in simpler code.
220	In addition, the synchronize_rcu() API has the nice property
221	of automatically limiting update rate should grace periods
222	be delayed.  This property results in system resilience in face
223	of denial-of-service attacks.  Code using call_rcu() should limit
224	update rate in order to gain this same sort of resilience.  See
225	checklist.txt for some approaches to limiting the update rate.
226
227rcu_assign_pointer()
228^^^^^^^^^^^^^^^^^^^^
229	void rcu_assign_pointer(p, typeof(p) v);
230
231	Yes, rcu_assign_pointer() **is** implemented as a macro, though it
232	would be cool to be able to declare a function in this manner.
233	(Compiler experts will no doubt disagree.)
234
235	The updater uses this function to assign a new value to an
236	RCU-protected pointer, in order to safely communicate the change
237	in value from the updater to the reader.  This macro does not
238	evaluate to an rvalue, but it does execute any memory-barrier
239	instructions required for a given CPU architecture.
240
241	Perhaps just as important, it serves to document (1) which
242	pointers are protected by RCU and (2) the point at which a
243	given structure becomes accessible to other CPUs.  That said,
244	rcu_assign_pointer() is most frequently used indirectly, via
245	the _rcu list-manipulation primitives such as list_add_rcu().
246
247rcu_dereference()
248^^^^^^^^^^^^^^^^^
249	typeof(p) rcu_dereference(p);
250
251	Like rcu_assign_pointer(), rcu_dereference() must be implemented
252	as a macro.
253
254	The reader uses rcu_dereference() to fetch an RCU-protected
255	pointer, which returns a value that may then be safely
256	dereferenced.  Note that rcu_dereference() does not actually
257	dereference the pointer, instead, it protects the pointer for
258	later dereferencing.  It also executes any needed memory-barrier
259	instructions for a given CPU architecture.  Currently, only Alpha
260	needs memory barriers within rcu_dereference() -- on other CPUs,
261	it compiles to nothing, not even a compiler directive.
262
263	Common coding practice uses rcu_dereference() to copy an
264	RCU-protected pointer to a local variable, then dereferences
265	this local variable, for example as follows::
266
267		p = rcu_dereference(head.next);
268		return p->data;
269
270	However, in this case, one could just as easily combine these
271	into one statement::
272
273		return rcu_dereference(head.next)->data;
274
275	If you are going to be fetching multiple fields from the
276	RCU-protected structure, using the local variable is of
277	course preferred.  Repeated rcu_dereference() calls look
278	ugly, do not guarantee that the same pointer will be returned
279	if an update happened while in the critical section, and incur
280	unnecessary overhead on Alpha CPUs.
281
282	Note that the value returned by rcu_dereference() is valid
283	only within the enclosing RCU read-side critical section [1]_.
284	For example, the following is **not** legal::
285
286		rcu_read_lock();
287		p = rcu_dereference(head.next);
288		rcu_read_unlock();
289		x = p->address;	/* BUG!!! */
290		rcu_read_lock();
291		y = p->data;	/* BUG!!! */
292		rcu_read_unlock();
293
294	Holding a reference from one RCU read-side critical section
295	to another is just as illegal as holding a reference from
296	one lock-based critical section to another!  Similarly,
297	using a reference outside of the critical section in which
298	it was acquired is just as illegal as doing so with normal
299	locking.
300
301	As with rcu_assign_pointer(), an important function of
302	rcu_dereference() is to document which pointers are protected by
303	RCU, in particular, flagging a pointer that is subject to changing
304	at any time, including immediately after the rcu_dereference().
305	And, again like rcu_assign_pointer(), rcu_dereference() is
306	typically used indirectly, via the _rcu list-manipulation
307	primitives, such as list_for_each_entry_rcu() [2]_.
308
309.. 	[1] The variant rcu_dereference_protected() can be used outside
310	of an RCU read-side critical section as long as the usage is
311	protected by locks acquired by the update-side code.  This variant
312	avoids the lockdep warning that would happen when using (for
313	example) rcu_dereference() without rcu_read_lock() protection.
314	Using rcu_dereference_protected() also has the advantage
315	of permitting compiler optimizations that rcu_dereference()
316	must prohibit.	The rcu_dereference_protected() variant takes
317	a lockdep expression to indicate which locks must be acquired
318	by the caller. If the indicated protection is not provided,
319	a lockdep splat is emitted.  See Documentation/RCU/Design/Requirements/Requirements.rst
320	and the API's code comments for more details and example usage.
321
322.. 	[2] If the list_for_each_entry_rcu() instance might be used by
323	update-side code as well as by RCU readers, then an additional
324	lockdep expression can be added to its list of arguments.
325	For example, given an additional "lock_is_held(&mylock)" argument,
326	the RCU lockdep code would complain only if this instance was
327	invoked outside of an RCU read-side critical section and without
328	the protection of mylock.
329
330The following diagram shows how each API communicates among the
331reader, updater, and reclaimer.
332::
333
334
335	    rcu_assign_pointer()
336	                            +--------+
337	    +---------------------->| reader |---------+
338	    |                       +--------+         |
339	    |                           |              |
340	    |                           |              | Protect:
341	    |                           |              | rcu_read_lock()
342	    |                           |              | rcu_read_unlock()
343	    |        rcu_dereference()  |              |
344	    +---------+                 |              |
345	    | updater |<----------------+              |
346	    +---------+                                V
347	    |                                    +-----------+
348	    +----------------------------------->| reclaimer |
349	                                         +-----------+
350	      Defer:
351	      synchronize_rcu() & call_rcu()
352
353
354The RCU infrastructure observes the time sequence of rcu_read_lock(),
355rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
356order to determine when (1) synchronize_rcu() invocations may return
357to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
358implementations of the RCU infrastructure make heavy use of batching in
359order to amortize their overhead over many uses of the corresponding APIs.
360
361There are at least three flavors of RCU usage in the Linux kernel. The diagram
362above shows the most common one. On the updater side, the rcu_assign_pointer(),
363synchronize_rcu() and call_rcu() primitives used are the same for all three
364flavors. However for protection (on the reader side), the primitives used vary
365depending on the flavor:
366
367a.	rcu_read_lock() / rcu_read_unlock()
368	rcu_dereference()
369
370b.	rcu_read_lock_bh() / rcu_read_unlock_bh()
371	local_bh_disable() / local_bh_enable()
372	rcu_dereference_bh()
373
374c.	rcu_read_lock_sched() / rcu_read_unlock_sched()
375	preempt_disable() / preempt_enable()
376	local_irq_save() / local_irq_restore()
377	hardirq enter / hardirq exit
378	NMI enter / NMI exit
379	rcu_dereference_sched()
380
381These three flavors are used as follows:
382
383a.	RCU applied to normal data structures.
384
385b.	RCU applied to networking data structures that may be subjected
386	to remote denial-of-service attacks.
387
388c.	RCU applied to scheduler and interrupt/NMI-handler tasks.
389
390Again, most uses will be of (a).  The (b) and (c) cases are important
391for specialized uses, but are relatively uncommon.
392
393.. _3_whatisRCU:
394
3953.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
396-----------------------------------------------
397
398This section shows a simple use of the core RCU API to protect a
399global pointer to a dynamically allocated structure.  More-typical
400uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`,
401:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`.
402::
403
404	struct foo {
405		int a;
406		char b;
407		long c;
408	};
409	DEFINE_SPINLOCK(foo_mutex);
410
411	struct foo __rcu *gbl_foo;
412
413	/*
414	 * Create a new struct foo that is the same as the one currently
415	 * pointed to by gbl_foo, except that field "a" is replaced
416	 * with "new_a".  Points gbl_foo to the new structure, and
417	 * frees up the old structure after a grace period.
418	 *
419	 * Uses rcu_assign_pointer() to ensure that concurrent readers
420	 * see the initialized version of the new structure.
421	 *
422	 * Uses synchronize_rcu() to ensure that any readers that might
423	 * have references to the old structure complete before freeing
424	 * the old structure.
425	 */
426	void foo_update_a(int new_a)
427	{
428		struct foo *new_fp;
429		struct foo *old_fp;
430
431		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
432		spin_lock(&foo_mutex);
433		old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
434		*new_fp = *old_fp;
435		new_fp->a = new_a;
436		rcu_assign_pointer(gbl_foo, new_fp);
437		spin_unlock(&foo_mutex);
438		synchronize_rcu();
439		kfree(old_fp);
440	}
441
442	/*
443	 * Return the value of field "a" of the current gbl_foo
444	 * structure.  Use rcu_read_lock() and rcu_read_unlock()
445	 * to ensure that the structure does not get deleted out
446	 * from under us, and use rcu_dereference() to ensure that
447	 * we see the initialized version of the structure (important
448	 * for DEC Alpha and for people reading the code).
449	 */
450	int foo_get_a(void)
451	{
452		int retval;
453
454		rcu_read_lock();
455		retval = rcu_dereference(gbl_foo)->a;
456		rcu_read_unlock();
457		return retval;
458	}
459
460So, to sum up:
461
462-	Use rcu_read_lock() and rcu_read_unlock() to guard RCU
463	read-side critical sections.
464
465-	Within an RCU read-side critical section, use rcu_dereference()
466	to dereference RCU-protected pointers.
467
468-	Use some solid scheme (such as locks or semaphores) to
469	keep concurrent updates from interfering with each other.
470
471-	Use rcu_assign_pointer() to update an RCU-protected pointer.
472	This primitive protects concurrent readers from the updater,
473	**not** concurrent updates from each other!  You therefore still
474	need to use locking (or something similar) to keep concurrent
475	rcu_assign_pointer() primitives from interfering with each other.
476
477-	Use synchronize_rcu() **after** removing a data element from an
478	RCU-protected data structure, but **before** reclaiming/freeing
479	the data element, in order to wait for the completion of all
480	RCU read-side critical sections that might be referencing that
481	data item.
482
483See checklist.txt for additional rules to follow when using RCU.
484And again, more-typical uses of RCU may be found in :ref:`listRCU.rst
485<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst
486<NMI_rcu_doc>`.
487
488.. _4_whatisRCU:
489
4904.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?
491--------------------------------------------
492
493In the example above, foo_update_a() blocks until a grace period elapses.
494This is quite simple, but in some cases one cannot afford to wait so
495long -- there might be other high-priority work to be done.
496
497In such cases, one uses call_rcu() rather than synchronize_rcu().
498The call_rcu() API is as follows::
499
500	void call_rcu(struct rcu_head *head, rcu_callback_t func);
501
502This function invokes func(head) after a grace period has elapsed.
503This invocation might happen from either softirq or process context,
504so the function is not permitted to block.  The foo struct needs to
505have an rcu_head structure added, perhaps as follows::
506
507	struct foo {
508		int a;
509		char b;
510		long c;
511		struct rcu_head rcu;
512	};
513
514The foo_update_a() function might then be written as follows::
515
516	/*
517	 * Create a new struct foo that is the same as the one currently
518	 * pointed to by gbl_foo, except that field "a" is replaced
519	 * with "new_a".  Points gbl_foo to the new structure, and
520	 * frees up the old structure after a grace period.
521	 *
522	 * Uses rcu_assign_pointer() to ensure that concurrent readers
523	 * see the initialized version of the new structure.
524	 *
525	 * Uses call_rcu() to ensure that any readers that might have
526	 * references to the old structure complete before freeing the
527	 * old structure.
528	 */
529	void foo_update_a(int new_a)
530	{
531		struct foo *new_fp;
532		struct foo *old_fp;
533
534		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
535		spin_lock(&foo_mutex);
536		old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
537		*new_fp = *old_fp;
538		new_fp->a = new_a;
539		rcu_assign_pointer(gbl_foo, new_fp);
540		spin_unlock(&foo_mutex);
541		call_rcu(&old_fp->rcu, foo_reclaim);
542	}
543
544The foo_reclaim() function might appear as follows::
545
546	void foo_reclaim(struct rcu_head *rp)
547	{
548		struct foo *fp = container_of(rp, struct foo, rcu);
549
550		foo_cleanup(fp->a);
551
552		kfree(fp);
553	}
554
555The container_of() primitive is a macro that, given a pointer into a
556struct, the type of the struct, and the pointed-to field within the
557struct, returns a pointer to the beginning of the struct.
558
559The use of call_rcu() permits the caller of foo_update_a() to
560immediately regain control, without needing to worry further about the
561old version of the newly updated element.  It also clearly shows the
562RCU distinction between updater, namely foo_update_a(), and reclaimer,
563namely foo_reclaim().
564
565The summary of advice is the same as for the previous section, except
566that we are now using call_rcu() rather than synchronize_rcu():
567
568-	Use call_rcu() **after** removing a data element from an
569	RCU-protected data structure in order to register a callback
570	function that will be invoked after the completion of all RCU
571	read-side critical sections that might be referencing that
572	data item.
573
574If the callback for call_rcu() is not doing anything more than calling
575kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
576to avoid having to write your own callback::
577
578	kfree_rcu(old_fp, rcu);
579
580Again, see checklist.txt for additional rules governing the use of RCU.
581
582.. _5_whatisRCU:
583
5845.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
585------------------------------------------------
586
587One of the nice things about RCU is that it has extremely simple "toy"
588implementations that are a good first step towards understanding the
589production-quality implementations in the Linux kernel.  This section
590presents two such "toy" implementations of RCU, one that is implemented
591in terms of familiar locking primitives, and another that more closely
592resembles "classic" RCU.  Both are way too simple for real-world use,
593lacking both functionality and performance.  However, they are useful
594in getting a feel for how RCU works.  See kernel/rcu/update.c for a
595production-quality implementation, and see:
596
597	http://www.rdrop.com/users/paulmck/RCU
598
599for papers describing the Linux kernel RCU implementation.  The OLS'01
600and OLS'02 papers are a good introduction, and the dissertation provides
601more details on the current implementation as of early 2004.
602
603
6045A.  "TOY" IMPLEMENTATION #1: LOCKING
605^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
606This section presents a "toy" RCU implementation that is based on
607familiar locking primitives.  Its overhead makes it a non-starter for
608real-life use, as does its lack of scalability.  It is also unsuitable
609for realtime use, since it allows scheduling latency to "bleed" from
610one read-side critical section to another.  It also assumes recursive
611reader-writer locks:  If you try this with non-recursive locks, and
612you allow nested rcu_read_lock() calls, you can deadlock.
613
614However, it is probably the easiest implementation to relate to, so is
615a good starting point.
616
617It is extremely simple::
618
619	static DEFINE_RWLOCK(rcu_gp_mutex);
620
621	void rcu_read_lock(void)
622	{
623		read_lock(&rcu_gp_mutex);
624	}
625
626	void rcu_read_unlock(void)
627	{
628		read_unlock(&rcu_gp_mutex);
629	}
630
631	void synchronize_rcu(void)
632	{
633		write_lock(&rcu_gp_mutex);
634		smp_mb__after_spinlock();
635		write_unlock(&rcu_gp_mutex);
636	}
637
638[You can ignore rcu_assign_pointer() and rcu_dereference() without missing
639much.  But here are simplified versions anyway.  And whatever you do,
640don't forget about them when submitting patches making use of RCU!]::
641
642	#define rcu_assign_pointer(p, v) \
643	({ \
644		smp_store_release(&(p), (v)); \
645	})
646
647	#define rcu_dereference(p) \
648	({ \
649		typeof(p) _________p1 = READ_ONCE(p); \
650		(_________p1); \
651	})
652
653
654The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
655and release a global reader-writer lock.  The synchronize_rcu()
656primitive write-acquires this same lock, then releases it.  This means
657that once synchronize_rcu() exits, all RCU read-side critical sections
658that were in progress before synchronize_rcu() was called are guaranteed
659to have completed -- there is no way that synchronize_rcu() would have
660been able to write-acquire the lock otherwise.  The smp_mb__after_spinlock()
661promotes synchronize_rcu() to a full memory barrier in compliance with
662the "Memory-Barrier Guarantees" listed in:
663
664	Documentation/RCU/Design/Requirements/Requirements.rst
665
666It is possible to nest rcu_read_lock(), since reader-writer locks may
667be recursively acquired.  Note also that rcu_read_lock() is immune
668from deadlock (an important property of RCU).  The reason for this is
669that the only thing that can block rcu_read_lock() is a synchronize_rcu().
670But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
671so there can be no deadlock cycle.
672
673.. _quiz_1:
674
675Quick Quiz #1:
676		Why is this argument naive?  How could a deadlock
677		occur when using this algorithm in a real-world Linux
678		kernel?  How could this deadlock be avoided?
679
680:ref:`Answers to Quick Quiz <8_whatisRCU>`
681
6825B.  "TOY" EXAMPLE #2: CLASSIC RCU
683^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
684This section presents a "toy" RCU implementation that is based on
685"classic RCU".  It is also short on performance (but only for updates) and
686on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
687kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
688are the same as those shown in the preceding section, so they are omitted.
689::
690
691	void rcu_read_lock(void) { }
692
693	void rcu_read_unlock(void) { }
694
695	void synchronize_rcu(void)
696	{
697		int cpu;
698
699		for_each_possible_cpu(cpu)
700			run_on(cpu);
701	}
702
703Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
704This is the great strength of classic RCU in a non-preemptive kernel:
705read-side overhead is precisely zero, at least on non-Alpha CPUs.
706And there is absolutely no way that rcu_read_lock() can possibly
707participate in a deadlock cycle!
708
709The implementation of synchronize_rcu() simply schedules itself on each
710CPU in turn.  The run_on() primitive can be implemented straightforwardly
711in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
712"toy" implementation would restore the affinity upon completion rather
713than just leaving all tasks running on the last CPU, but when I said
714"toy", I meant **toy**!
715
716So how the heck is this supposed to work???
717
718Remember that it is illegal to block while in an RCU read-side critical
719section.  Therefore, if a given CPU executes a context switch, we know
720that it must have completed all preceding RCU read-side critical sections.
721Once **all** CPUs have executed a context switch, then **all** preceding
722RCU read-side critical sections will have completed.
723
724So, suppose that we remove a data item from its structure and then invoke
725synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
726that there are no RCU read-side critical sections holding a reference
727to that data item, so we can safely reclaim it.
728
729.. _quiz_2:
730
731Quick Quiz #2:
732		Give an example where Classic RCU's read-side
733		overhead is **negative**.
734
735:ref:`Answers to Quick Quiz <8_whatisRCU>`
736
737.. _quiz_3:
738
739Quick Quiz #3:
740		If it is illegal to block in an RCU read-side
741		critical section, what the heck do you do in
742		PREEMPT_RT, where normal spinlocks can block???
743
744:ref:`Answers to Quick Quiz <8_whatisRCU>`
745
746.. _6_whatisRCU:
747
7486.  ANALOGY WITH READER-WRITER LOCKING
749--------------------------------------
750
751Although RCU can be used in many different ways, a very common use of
752RCU is analogous to reader-writer locking.  The following unified
753diff shows how closely related RCU and reader-writer locking can be.
754::
755
756	@@ -5,5 +5,5 @@ struct el {
757	 	int data;
758	 	/* Other data fields */
759	 };
760	-rwlock_t listmutex;
761	+spinlock_t listmutex;
762	 struct el head;
763
764	@@ -13,15 +14,15 @@
765		struct list_head *lp;
766		struct el *p;
767
768	-	read_lock(&listmutex);
769	-	list_for_each_entry(p, head, lp) {
770	+	rcu_read_lock();
771	+	list_for_each_entry_rcu(p, head, lp) {
772			if (p->key == key) {
773				*result = p->data;
774	-			read_unlock(&listmutex);
775	+			rcu_read_unlock();
776				return 1;
777			}
778		}
779	-	read_unlock(&listmutex);
780	+	rcu_read_unlock();
781		return 0;
782	 }
783
784	@@ -29,15 +30,16 @@
785	 {
786		struct el *p;
787
788	-	write_lock(&listmutex);
789	+	spin_lock(&listmutex);
790		list_for_each_entry(p, head, lp) {
791			if (p->key == key) {
792	-			list_del(&p->list);
793	-			write_unlock(&listmutex);
794	+			list_del_rcu(&p->list);
795	+			spin_unlock(&listmutex);
796	+			synchronize_rcu();
797				kfree(p);
798				return 1;
799			}
800		}
801	-	write_unlock(&listmutex);
802	+	spin_unlock(&listmutex);
803		return 0;
804	 }
805
806Or, for those who prefer a side-by-side listing::
807
808 1 struct el {                          1 struct el {
809 2   struct list_head list;             2   struct list_head list;
810 3   long key;                          3   long key;
811 4   spinlock_t mutex;                  4   spinlock_t mutex;
812 5   int data;                          5   int data;
813 6   /* Other data fields */            6   /* Other data fields */
814 7 };                                   7 };
815 8 rwlock_t listmutex;                  8 spinlock_t listmutex;
816 9 struct el head;                      9 struct el head;
817
818::
819
820  1 int search(long key, int *result)    1 int search(long key, int *result)
821  2 {                                    2 {
822  3   struct list_head *lp;              3   struct list_head *lp;
823  4   struct el *p;                      4   struct el *p;
824  5                                      5
825  6   read_lock(&listmutex);             6   rcu_read_lock();
826  7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
827  8     if (p->key == key) {             8     if (p->key == key) {
828  9       *result = p->data;             9       *result = p->data;
829 10       read_unlock(&listmutex);      10       rcu_read_unlock();
830 11       return 1;                     11       return 1;
831 12     }                               12     }
832 13   }                                 13   }
833 14   read_unlock(&listmutex);          14   rcu_read_unlock();
834 15   return 0;                         15   return 0;
835 16 }                                   16 }
836
837::
838
839  1 int delete(long key)                 1 int delete(long key)
840  2 {                                    2 {
841  3   struct el *p;                      3   struct el *p;
842  4                                      4
843  5   write_lock(&listmutex);            5   spin_lock(&listmutex);
844  6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
845  7     if (p->key == key) {             7     if (p->key == key) {
846  8       list_del(&p->list);            8       list_del_rcu(&p->list);
847  9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
848                                        10       synchronize_rcu();
849 10       kfree(p);                     11       kfree(p);
850 11       return 1;                     12       return 1;
851 12     }                               13     }
852 13   }                                 14   }
853 14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
854 15   return 0;                         16   return 0;
855 16 }                                   17 }
856
857Either way, the differences are quite small.  Read-side locking moves
858to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
859a reader-writer lock to a simple spinlock, and a synchronize_rcu()
860precedes the kfree().
861
862However, there is one potential catch: the read-side and update-side
863critical sections can now run concurrently.  In many cases, this will
864not be a problem, but it is necessary to check carefully regardless.
865For example, if multiple independent list updates must be seen as
866a single atomic update, converting to RCU will require special care.
867
868Also, the presence of synchronize_rcu() means that the RCU version of
869delete() can now block.  If this is a problem, there is a callback-based
870mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
871be used in place of synchronize_rcu().
872
873.. _7_whatisRCU:
874
8757.  FULL LIST OF RCU APIs
876-------------------------
877
878The RCU APIs are documented in docbook-format header comments in the
879Linux-kernel source code, but it helps to have a full list of the
880APIs, since there does not appear to be a way to categorize them
881in docbook.  Here is the list, by category.
882
883RCU list traversal::
884
885	list_entry_rcu
886	list_entry_lockless
887	list_first_entry_rcu
888	list_next_rcu
889	list_for_each_entry_rcu
890	list_for_each_entry_continue_rcu
891	list_for_each_entry_from_rcu
892	list_first_or_null_rcu
893	list_next_or_null_rcu
894	hlist_first_rcu
895	hlist_next_rcu
896	hlist_pprev_rcu
897	hlist_for_each_entry_rcu
898	hlist_for_each_entry_rcu_bh
899	hlist_for_each_entry_from_rcu
900	hlist_for_each_entry_continue_rcu
901	hlist_for_each_entry_continue_rcu_bh
902	hlist_nulls_first_rcu
903	hlist_nulls_for_each_entry_rcu
904	hlist_bl_first_rcu
905	hlist_bl_for_each_entry_rcu
906
907RCU pointer/list update::
908
909	rcu_assign_pointer
910	list_add_rcu
911	list_add_tail_rcu
912	list_del_rcu
913	list_replace_rcu
914	hlist_add_behind_rcu
915	hlist_add_before_rcu
916	hlist_add_head_rcu
917	hlist_add_tail_rcu
918	hlist_del_rcu
919	hlist_del_init_rcu
920	hlist_replace_rcu
921	list_splice_init_rcu
922	list_splice_tail_init_rcu
923	hlist_nulls_del_init_rcu
924	hlist_nulls_del_rcu
925	hlist_nulls_add_head_rcu
926	hlist_bl_add_head_rcu
927	hlist_bl_del_init_rcu
928	hlist_bl_del_rcu
929	hlist_bl_set_first_rcu
930
931RCU::
932
933	Critical sections	Grace period		Barrier
934
935	rcu_read_lock		synchronize_net		rcu_barrier
936	rcu_read_unlock		synchronize_rcu
937	rcu_dereference		synchronize_rcu_expedited
938	rcu_read_lock_held	call_rcu
939	rcu_dereference_check	kfree_rcu
940	rcu_dereference_protected
941
942bh::
943
944	Critical sections	Grace period		Barrier
945
946	rcu_read_lock_bh	call_rcu		rcu_barrier
947	rcu_read_unlock_bh	synchronize_rcu
948	[local_bh_disable]	synchronize_rcu_expedited
949	[and friends]
950	rcu_dereference_bh
951	rcu_dereference_bh_check
952	rcu_dereference_bh_protected
953	rcu_read_lock_bh_held
954
955sched::
956
957	Critical sections	Grace period		Barrier
958
959	rcu_read_lock_sched	call_rcu		rcu_barrier
960	rcu_read_unlock_sched	synchronize_rcu
961	[preempt_disable]	synchronize_rcu_expedited
962	[and friends]
963	rcu_read_lock_sched_notrace
964	rcu_read_unlock_sched_notrace
965	rcu_dereference_sched
966	rcu_dereference_sched_check
967	rcu_dereference_sched_protected
968	rcu_read_lock_sched_held
969
970
971SRCU::
972
973	Critical sections	Grace period		Barrier
974
975	srcu_read_lock		call_srcu		srcu_barrier
976	srcu_read_unlock	synchronize_srcu
977	srcu_dereference	synchronize_srcu_expedited
978	srcu_dereference_check
979	srcu_read_lock_held
980
981SRCU: Initialization/cleanup::
982
983	DEFINE_SRCU
984	DEFINE_STATIC_SRCU
985	init_srcu_struct
986	cleanup_srcu_struct
987
988All: lockdep-checked RCU-protected pointer access::
989
990	rcu_access_pointer
991	rcu_dereference_raw
992	RCU_LOCKDEP_WARN
993	rcu_sleep_check
994	RCU_NONIDLE
995
996See the comment headers in the source code (or the docbook generated
997from them) for more information.
998
999However, given that there are no fewer than four families of RCU APIs
1000in the Linux kernel, how do you choose which one to use?  The following
1001list can be helpful:
1002
1003a.	Will readers need to block?  If so, you need SRCU.
1004
1005b.	What about the -rt patchset?  If readers would need to block
1006	in an non-rt kernel, you need SRCU.  If readers would block
1007	in a -rt kernel, but not in a non-rt kernel, SRCU is not
1008	necessary.  (The -rt patchset turns spinlocks into sleeplocks,
1009	hence this distinction.)
1010
1011c.	Do you need to treat NMI handlers, hardirq handlers,
1012	and code segments with preemption disabled (whether
1013	via preempt_disable(), local_irq_save(), local_bh_disable(),
1014	or some other mechanism) as if they were explicit RCU readers?
1015	If so, RCU-sched is the only choice that will work for you.
1016
1017d.	Do you need RCU grace periods to complete even in the face
1018	of softirq monopolization of one or more of the CPUs?  For
1019	example, is your code subject to network-based denial-of-service
1020	attacks?  If so, you should disable softirq across your readers,
1021	for example, by using rcu_read_lock_bh().
1022
1023e.	Is your workload too update-intensive for normal use of
1024	RCU, but inappropriate for other synchronization mechanisms?
1025	If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
1026	named SLAB_DESTROY_BY_RCU).  But please be careful!
1027
1028f.	Do you need read-side critical sections that are respected
1029	even though they are in the middle of the idle loop, during
1030	user-mode execution, or on an offlined CPU?  If so, SRCU is the
1031	only choice that will work for you.
1032
1033g.	Otherwise, use RCU.
1034
1035Of course, this all assumes that you have determined that RCU is in fact
1036the right tool for your job.
1037
1038.. _8_whatisRCU:
1039
10408.  ANSWERS TO QUICK QUIZZES
1041----------------------------
1042
1043Quick Quiz #1:
1044		Why is this argument naive?  How could a deadlock
1045		occur when using this algorithm in a real-world Linux
1046		kernel?  [Referring to the lock-based "toy" RCU
1047		algorithm.]
1048
1049Answer:
1050		Consider the following sequence of events:
1051
1052		1.	CPU 0 acquires some unrelated lock, call it
1053			"problematic_lock", disabling irq via
1054			spin_lock_irqsave().
1055
1056		2.	CPU 1 enters synchronize_rcu(), write-acquiring
1057			rcu_gp_mutex.
1058
1059		3.	CPU 0 enters rcu_read_lock(), but must wait
1060			because CPU 1 holds rcu_gp_mutex.
1061
1062		4.	CPU 1 is interrupted, and the irq handler
1063			attempts to acquire problematic_lock.
1064
1065		The system is now deadlocked.
1066
1067		One way to avoid this deadlock is to use an approach like
1068		that of CONFIG_PREEMPT_RT, where all normal spinlocks
1069		become blocking locks, and all irq handlers execute in
1070		the context of special tasks.  In this case, in step 4
1071		above, the irq handler would block, allowing CPU 1 to
1072		release rcu_gp_mutex, avoiding the deadlock.
1073
1074		Even in the absence of deadlock, this RCU implementation
1075		allows latency to "bleed" from readers to other
1076		readers through synchronize_rcu().  To see this,
1077		consider task A in an RCU read-side critical section
1078		(thus read-holding rcu_gp_mutex), task B blocked
1079		attempting to write-acquire rcu_gp_mutex, and
1080		task C blocked in rcu_read_lock() attempting to
1081		read_acquire rcu_gp_mutex.  Task A's RCU read-side
1082		latency is holding up task C, albeit indirectly via
1083		task B.
1084
1085		Realtime RCU implementations therefore use a counter-based
1086		approach where tasks in RCU read-side critical sections
1087		cannot be blocked by tasks executing synchronize_rcu().
1088
1089:ref:`Back to Quick Quiz #1 <quiz_1>`
1090
1091Quick Quiz #2:
1092		Give an example where Classic RCU's read-side
1093		overhead is **negative**.
1094
1095Answer:
1096		Imagine a single-CPU system with a non-CONFIG_PREEMPT
1097		kernel where a routing table is used by process-context
1098		code, but can be updated by irq-context code (for example,
1099		by an "ICMP REDIRECT" packet).	The usual way of handling
1100		this would be to have the process-context code disable
1101		interrupts while searching the routing table.  Use of
1102		RCU allows such interrupt-disabling to be dispensed with.
1103		Thus, without RCU, you pay the cost of disabling interrupts,
1104		and with RCU you don't.
1105
1106		One can argue that the overhead of RCU in this
1107		case is negative with respect to the single-CPU
1108		interrupt-disabling approach.  Others might argue that
1109		the overhead of RCU is merely zero, and that replacing
1110		the positive overhead of the interrupt-disabling scheme
1111		with the zero-overhead RCU scheme does not constitute
1112		negative overhead.
1113
1114		In real life, of course, things are more complex.  But
1115		even the theoretical possibility of negative overhead for
1116		a synchronization primitive is a bit unexpected.  ;-)
1117
1118:ref:`Back to Quick Quiz #2 <quiz_2>`
1119
1120Quick Quiz #3:
1121		If it is illegal to block in an RCU read-side
1122		critical section, what the heck do you do in
1123		PREEMPT_RT, where normal spinlocks can block???
1124
1125Answer:
1126		Just as PREEMPT_RT permits preemption of spinlock
1127		critical sections, it permits preemption of RCU
1128		read-side critical sections.  It also permits
1129		spinlocks blocking while in RCU read-side critical
1130		sections.
1131
1132		Why the apparent inconsistency?  Because it is
1133		possible to use priority boosting to keep the RCU
1134		grace periods short if need be (for example, if running
1135		short of memory).  In contrast, if blocking waiting
1136		for (say) network reception, there is no way to know
1137		what should be boosted.  Especially given that the
1138		process we need to boost might well be a human being
1139		who just went out for a pizza or something.  And although
1140		a computer-operated cattle prod might arouse serious
1141		interest, it might also provoke serious objections.
1142		Besides, how does the computer know what pizza parlor
1143		the human being went to???
1144
1145:ref:`Back to Quick Quiz #3 <quiz_3>`
1146
1147ACKNOWLEDGEMENTS
1148
1149My thanks to the people who helped make this human-readable, including
1150Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1151
1152
1153For more information, see http://www.rdrop.com/users/paulmck/RCU.
1154