aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/RCU/whatisRCU.txt
blob: e41a7fecf0d3c33354aef84985cb564619ab6c77 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
Please note that the "What is RCU?" LWN series is an excellent place
to start learning about RCU:

1.	What is RCU, Fundamentally?  http://lwn.net/Articles/262464/
2.	What is RCU? Part 2: Usage   http://lwn.net/Articles/263130/
3.	RCU part 3: the RCU API      http://lwn.net/Articles/264090/


What is RCU?

RCU is a synchronization mechanism that was added to the Linux kernel
during the 2.5 development effort that is optimized for read-mostly
situations.  Although RCU is actually quite simple once you understand it,
getting there can sometimes be a challenge.  Part of the problem is that
most of the past descriptions of RCU have been written with the mistaken
assumption that there is "one true way" to describe RCU.  Instead,
the experience has been that different people must take different paths
to arrive at an understanding of RCU.  This document provides several
different paths, as follows:

1.	RCU OVERVIEW
2.	WHAT IS RCU'S CORE API?
3.	WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
4.	WHAT IF MY UPDATING THREAD CANNOT BLOCK?
5.	WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
6.	ANALOGY WITH READER-WRITER LOCKING
7.	FULL LIST OF RCU APIs
8.	ANSWERS TO QUICK QUIZZES

People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
some point.  People who prefer to start with an API that they can then
experiment with should focus on Section 2.  People who prefer to start
with example uses should focus on Sections 3 and 4.  People who need to
understand the RCU implementation should focus on Section 5, then dive
into the kernel source code.  People who reason best by analogy should
focus on Section 6.  Section 7 serves as an index to the docbook API
documentation, and Section 8 is the traditional answer key.

So, start with the section that makes the most sense to you and your
preferred method of learning.  If you need to know everything about
everything, feel free to read the whole thing -- but if you are really
that type of person, you have perused the source code and will therefore
never need this document anyway.  ;-)


1.  RCU OVERVIEW

The basic idea behind RCU is to split updates into "removal" and
"reclamation" phases.  The removal phase removes references to data items
within a data structure (possibly by replacing them with references to
new versions of these data items), and can run concurrently with readers.
The reason that it is safe to run the removal phase concurrently with
readers is the semantics of modern CPUs guarantee that readers will see
either the old or the new version of the data structure rather than a
partially updated reference.  The reclamation phase does the work of reclaiming
(e.g., freeing) the data items removed from the data structure during the
removal phase.  Because reclaiming data items can disrupt any readers
concurrently referencing those data items, the reclamation phase must
not start until readers no longer hold references to those data items.

Splitting the update into removal and reclamation phases permits the
updater to perform the removal phase immediately, and to defer the
reclamation phase until all readers active during the removal phase have
completed, either by blocking until they finish or by registering a
callback that is invoked after they finish.  Only readers that are active
during the removal phase need be considered, because any reader starting
after the removal phase will be unable to gain a reference to the removed
data items, and therefore cannot be disrupted by the reclamation phase.

So the typical RCU update sequence goes something like the following:

a.	Remove pointers to a data structure, so that subsequent
	readers cannot gain a reference to it.

b.	Wait for all previous readers to complete their RCU read-side
	critical sections.

c.	At this point, there cannot be any readers who hold references
	to the data structure, so it now may safely be reclaimed
	(e.g., kfree()d).

Step (b) above is the key idea underlying RCU's deferred destruction.
The ability to wait until all readers are done allows RCU readers to
use much lighter-weight synchronization, in some cases, absolutely no
synchronization at all.  In contrast, in more conventional lock-based
schemes, readers must use heavy-weight synchronization in order to
prevent an updater from deleting the data structure out from under them.
This is because lock-based updaters typically update data items in place,
and must therefore exclude readers.  In contrast, RCU-based updaters
typically take advantage of the fact that writes to single aligned
pointers are atomic on modern CPUs, allowing atomic insertion, removal,
and replacement of data items in a linked structure without disrupting
readers.  Concurrent RCU readers can then continue accessing the old
versions, and can dispense with the atomic operations, memory barriers,
and communications cache misses that are so expensive on present-day
SMP computer systems, even in absence of lock contention.

In the three-step procedure shown above, the updater is performing both
the removal and the reclamation step, but it is often helpful for an
entirely different thread to do the reclamation, as is in fact the case
in the Linux kernel's directory-entry cache (dcache).  Even if the same
thread performs both the update step (step (a) above) and the reclamation
step (step (c) above), it is often helpful to think of them separately.
For example, RCU readers and updaters need not communicate at all,
but RCU provides implicit low-overhead communication between readers
and reclaimers, namely, in step (b) above.

So how the heck can a reclaimer tell when a reader is done, given
that readers are not doing any sort of synchronization operations???
Read on to learn about how RCU's API makes this easy.


2.  WHAT IS RCU'S CORE API?

The core RCU API is quite small:

a.	rcu_read_lock()
b.	rcu_read_unlock()
c.	synchronize_rcu() / call_rcu()
d.	rcu_assign_pointer()
e.	rcu_dereference()

There are many other members of the RCU API, but the rest can be
expressed in terms of these five, though most implementations instead
express synchronize_rcu() in terms of the call_rcu() callback API.

The five core RCU APIs are described below, the other 18 will be enumerated
later.  See the kernel docbook documentation for more info, or look directly
at the function header comments.

rcu_read_lock()

	void rcu_read_lock(void);

	Used by a reader to inform the reclaimer that the reader is
	entering an RCU read-side critical section.  It is illegal
	to block while in an RCU read-side critical section, though
	kernels built with CONFIG_TREE_PREEMPT_RCU can preempt RCU
	read-side critical sections.  Any RCU-protected data structure
	accessed during an RCU read-side critical section is guaranteed to
	remain unreclaimed for the full duration of that critical section.
	Reference counts may be used in conjunction with RCU to maintain
	longer-term references to data structures.

rcu_read_unlock()

	void rcu_read_unlock(void);

	Used by a reader to inform the reclaimer that the reader is
	exiting an RCU read-side critical section.  Note that RCU
	read-side critical sections may be nested and/or overlapping.

synchronize_rcu()

	void synchronize_rcu(void);

	Marks the end of updater code and the beginning of reclaimer
	code.  It does this by blocking until all pre-existing RCU
	read-side critical sections on all CPUs have completed.
	Note that synchronize_rcu() will -not- necessarily wait for
	any subsequent RCU read-side critical sections to complete.
	For example, consider the following sequence of events:

	         CPU 0                  CPU 1                 CPU 2
	     ----------------- ------------------------- ---------------
	 1.  rcu_read_lock()
	 2.                    enters synchronize_rcu()
	 3.                                               rcu_read_lock()
	 4.  rcu_read_unlock()
	 5.                     exits synchronize_rcu()
	 6.                                              rcu_read_unlock()

	To reiterate, synchronize_rcu() waits only for ongoing RCU
	read-side critical sections to complete, not necessarily for
	any that begin after synchronize_rcu() is invoked.

	Of course, synchronize_rcu() does not necessarily return
	-immediately- after the last pre-existing RCU read-side critical
	section completes.  For one thing, there might well be scheduling
	delays.  For another thing, many RCU implementations process
	requests in batches in order to improve efficiencies, which can
	further delay synchronize_rcu().

	Since synchronize_rcu() is the API that must figure out when
	readers are done, its implementation is key to RCU.  For RCU
	to be useful in all but the most read-intensive situations,
	synchronize_rcu()'s overhead must also be quite small.

	The call_rcu() API is a callback form of synchronize_rcu(),
	and is described in more detail in a later section.  Instead of
	blocking, it registers a function and argument which are invoked
	after all ongoing RCU read-side critical sections have completed.
	This callback variant is particularly useful in situations where
	it is illegal to block or where update-side performance is
	critically important.

	However, the call_rcu() API should not be used lightly, as use
	of the synchronize_rcu() API generally results in simpler code.
	In addition, the synchronize_rcu() API has the nice property
	of automatically limiting update rate should grace periods
	be delayed.  This property results in system resilience in face
	of denial-of-service attacks.  Code using call_rcu() should limit
	update rate in order to gain this same sort of resilience.  See
	checklist.txt for some approaches to limiting the update rate.

rcu_assign_pointer()

	typeof(p) rcu_assign_pointer(p, typeof(p) v);

	Yes, rcu_assign_pointer() -is- implemented as a macro, though it
	would be cool to be able to declare a function in this manner.
	(Compiler experts will no doubt disagree.)

	The updater uses this function to assign a new value to an
	RCU-protected pointer, in order to safely communicate the change
	in value from the updater to the reader.  This function returns
	the new value, and also executes any memory-barrier instructions
	required for a given CPU architecture.

	Perhaps just as important, it serves to document (1) which
	pointers are protected by RCU and (2) the point at which a
	given structure becomes accessible to other CPUs.  That said,
	rcu_assign_pointer() is most frequently used indirectly, via
	the _rcu list-manipulation primitives such as list_add_rcu().

rcu_dereference()

	typeof(p) rcu_dereference(p);

	Like rcu_assign_pointer(), rcu_dereference() must be implemented
	as a macro.

	The reader uses rcu_dereference() to fetch an RCU-protected
	pointer, which returns a value that may then be safely
	dereferenced.  Note that rcu_deference() does not actually
	dereference the pointer, instead, it protects the pointer for
	later dereferencing.  It also executes any needed memory-barrier
	instructions for a given CPU architecture.  Currently, only Alpha
	needs memory barriers within rcu_dereference() -- on other CPUs,
	it compiles to nothing, not even a compiler directive.

	Common coding practice uses rcu_dereference() to copy an
	RCU-protected pointer to a local variable, then dereferences
	this local variable, for example as follows:

		p = rcu_dereference(head.next);
		return p->data;

	However, in this case, one could just as easily combine these
	into one statement:

		return rcu_dereference(head.next)->data;

	If you are going to be fetching multiple fields from the
	RCU-protected structure, using the local variable is of
	course preferred.  Repeated rcu_dereference() calls look
	ugly and incur unnecessary overhead on Alpha CPUs.

	Note that the value returned by rcu_dereference() is valid
	only within the enclosing RCU read-side critical section.
	For example, the following is -not- legal:

		rcu_read_lock();
		p = rcu_dereference(head.next);
		rcu_read_unlock();
		x = p->address;
		rcu_read_lock();
		y = p->data;
		rcu_read_unlock();

	Holding a reference from one RCU read-side critical section
	to another is just as illegal as holding a reference from
	one lock-based critical section to another!  Similarly,
	using a reference outside of the critical section in which
	it was acquired is just as illegal as doing so with normal
	locking.

	As with rcu_assign_pointer(), an important function of
	rcu_dereference() is to document which pointers are protected by
	RCU, in particular, flagging a pointer that is subject to changing
	at any time, including immediately after the rcu_dereference().
	And, again like rcu_assign_pointer(), rcu_dereference() is
	typically used indirectly, via the _rcu list-manipulation
	primitives, such as list_for_each_entry_rcu().

The following diagram shows how each API communicates among the
reader, updater, and reclaimer.


	    rcu_assign_pointer()
	    			    +--------+
	    +---------------------->| reader |---------+
	    |                       +--------+         |
	    |                           |              |
	    |                           |              | Protect:
	    |                           |              | rcu_read_lock()
	    |                           |              | rcu_read_unlock()
	    |        rcu_dereference()  |              |
       +---------+                      |              |
       | updater |<---------------------+              |
       +---------+                                     V
	    |                                    +-----------+
	    +----------------------------------->| reclaimer |
	    				         +-----------+
	      Defer:
	      synchronize_rcu() & call_rcu()


The RCU infrastructure observes the time sequence of rcu_read_lock(),
rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
order to determine when (1) synchronize_rcu() invocations may return
to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
implementations of the RCU infrastructure make heavy use of batching in
order to amortize their overhead over many uses of the corresponding APIs.

There are no fewer than three RCU mechanisms in the Linux kernel; the
diagram above shows the first one, which is by far the most commonly used.
The rcu_dereference() and rcu_assign_pointer() primitives are used for
all three mechanisms, but different defer and protect primitives are
used as follows:

	Defer			Protect

a.	synchronize_rcu()	rcu_read_lock() / rcu_read_unlock()
	call_rcu()

b.	call_rcu_bh()		rcu_read_lock_bh() / rcu_read_unlock_bh()

c.	synchronize_sched()	preempt_disable() / preempt_enable()
				local_irq_save() / local_irq_restore()
				hardirq enter / hardirq exit
				NMI enter / NMI exit

These three mechanisms are used as follows:

a.	RCU applied to normal data structures.

b.	RCU applied to networking data structures that may be subjected
	to remote denial-of-service attacks.

c.	RCU applied to scheduler and interrupt/NMI-handler tasks.

Again, most uses will be of (a).  The (b) and (c) cases are important
for specialized uses, but are relatively uncommon.


3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?

This section shows a simple use of the core RCU API to protect a
global pointer to a dynamically allocated structure.  More-typical
uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.

	struct foo {
		int a;
		char b;
		long c;
	};
	DEFINE_SPINLOCK(foo_mutex);

	struct foo *gbl_foo;

	/*
	 * Create a new struct foo that is the same as the one currently
	 * pointed to by gbl_foo, except that field "a" is replaced
	 * with "new_a".  Points gbl_foo to the new structure, and
	 * frees up the old structure after a grace period.
	 *
	 * Uses rcu_assign_pointer() to ensure that concurrent readers
	 * see the initialized version of the new structure.
	 *
	 * Uses synchronize_rcu() to ensure that any readers that might
	 * have references to the old structure complete before freeing
	 * the old structure.
	 */
	void foo_update_a(int new_a)
	{
		struct foo *new_fp;
		struct foo *old_fp;

		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
		spin_lock(&foo_mutex);
		old_fp = gbl_foo;
		*new_fp = *old_fp;
		new_fp->a = new_a;
		rcu_assign_pointer(gbl_foo, new_fp);
		spin_unlock(&foo_mutex);
		synchronize_rcu();
		kfree(old_fp);
	}

	/*
	 * Return the value of field "a" of the current gbl_foo
	 * structure.  Use rcu_read_lock() and rcu_read_unlock()
	 * to ensure that the structure does not get deleted out
	 * from under us, and use rcu_dereference() to ensure that
	 * we see the initialized version of the structure (important
	 * for DEC Alpha and for people reading the code).
	 */
	int foo_get_a(void)
	{
		int retval;

		rcu_read_lock();
		retval = rcu_dereference(gbl_foo)->a;
		rcu_read_unlock();
		return retval;
	}

So, to sum up:

o	Use rcu_read_lock() and rcu_read_unlock() to guard RCU
	read-side critical sections.

o	Within an RCU read-side critical section, use rcu_dereference()
	to dereference RCU-protected pointers.

o	Use some solid scheme (such as locks or semaphores) to
	keep concurrent updates from interfering with each other.

o	Use rcu_assign_pointer() to update an RCU-protected pointer.
	This primitive protects concurrent readers from the updater,
	-not- concurrent updates from each other!  You therefore still
	need to use locking (or something similar) to keep concurrent
	rcu_assign_pointer() primitives from interfering with each other.

o	Use synchronize_rcu() -after- removing a data element from an
	RCU-protected data structure, but -before- reclaiming/freeing
	the data element, in order to wait for the completion of all
	RCU read-side critical sections that might be referencing that
	data item.

See checklist.txt for additional rules to follow when using RCU.
And again, more-typical uses of RCU may be found in listRCU.txt,
arrayRCU.txt, and NMI-RCU.txt.


4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?

In the example above, foo_update_a() blocks until a grace period elapses.
This is quite simple, but in some cases one cannot afford to wait so
long -- there might be other high-priority work to be done.

In such cases, one uses call_rcu() rather than synchronize_rcu().
The call_rcu() API is as follows:

	void call_rcu(struct rcu_head * head,
		      void (*func)(struct rcu_head *head));

This function invokes func(head) after a grace period has elapsed.
This invocation might happen from either softirq or process context,
so the function is not permitted to block.  The foo struct needs to
have an rcu_head structure added, perhaps as follows:

	struct foo {
		int a;
		char b;
		long c;
		struct rcu_head rcu;
	};

The foo_update_a() function might then be written as follows:

	/*
	 * Create a new struct foo that is the same as the one currently
	 * pointed to by gbl_foo, except that field "a" is replaced
	 * with "new_a".  Points gbl_foo to the new structure, and
	 * frees up the old structure after a grace period.
	 *
	 * Uses rcu_assign_pointer() to ensure that concurrent readers
	 * see the initialized version of the new structure.
	 *
	 * Uses call_rcu() to ensure that any readers that might have
	 * references to the old structure complete before freeing the
	 * old structure.
	 */
	void foo_update_a(int new_a)
	{
		struct foo *new_fp;
		struct foo *old_fp;

		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
		spin_lock(&foo_mutex);
		old_fp = gbl_foo;
		*new_fp = *old_fp;
		new_fp->a = new_a;
		rcu_assign_pointer(gbl_foo, new_fp);
		spin_unlock(&foo_mutex);
		call_rcu(&old_fp->rcu, foo_reclaim);
	}

The foo_reclaim() function might appear as follows:

	void foo_reclaim(struct rcu_head *rp)
	{
		struct foo *fp = container_of(rp, struct foo, rcu);

		kfree(fp);
	}

The container_of() primitive is a macro that, given a pointer into a
struct, the type of the struct, and the pointed-to field within the
struct, returns a pointer to the beginning of the struct.

The use of call_rcu() permits the caller of foo_update_a() to
immediately regain control, without needing to worry further about the
old version of the newly updated element.  It also clearly shows the
RCU distinction between updater, namely foo_update_a(), and reclaimer,
namely foo_reclaim().

The summary of advice is the same as for the previous section, except
that we are now using call_rcu() rather than synchronize_rcu():

o	Use call_rcu() -after- removing a data element from an
	RCU-protected data structure in order to register a callback
	function that will be invoked after the completion of all RCU
	read-side critical sections that might be referencing that
	data item.

Again, see checklist.txt for additional rules governing the use of RCU.


5.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?

One of the nice things about RCU is that it has extremely simple "toy"
implementations that are a good first step towards understanding the
production-quality implementations in the Linux kernel.  This section
presents two such "toy" implementations of RCU, one that is implemented
in terms of familiar locking primitives, and another that more closely
resembles "classic" RCU.  Both are way too simple for real-world use,
lacking both functionality and performance.  However, they are useful
in getting a feel for how RCU works.  See kernel/rcupdate.c for a
production-quality implementation, and see:

	http://www.rdrop.com/users/paulmck/RCU

for papers describing the Linux kernel RCU implementation.  The OLS'01
and OLS'02 papers are a good introduction, and the dissertation provides
more details on the current implementation as of early 2004.


5A.  "TOY" IMPLEMENTATION #1: LOCKING

This section presents a "toy" RCU implementation that is based on
familiar locking primitives.  Its overhead makes it a non-starter for
real-life use, as does its lack of scalability.  It is also unsuitable
for realtime use, since it allows scheduling latency to "bleed" from
one read-side critical section to another.

However, it is probably the easiest implementation to relate to, so is
a good starting point.

It is extremely simple:

	static DEFINE_RWLOCK(rcu_gp_mutex);

	void rcu_read_lock(void)
	{
		read_lock(&rcu_gp_mutex);
	}

	void rcu_read_unlock(void)
	{
		read_unlock(&rcu_gp_mutex);
	}

	void synchronize_rcu(void)
	{
		write_lock(&rcu_gp_mutex);
		write_unlock(&rcu_gp_mutex);
	}

[You can ignore rcu_assign_pointer() and rcu_dereference() without
missing much.  But here they are anyway.  And whatever you do, don't
forget about them when submitting patches making use of RCU!]

	#define rcu_assign_pointer(p, v)	({ \
							smp_wmb(); \
							(p) = (v); \
						})

	#define rcu_dereference(p)     ({ \
					typeof(p) _________p1 = p; \
					smp_read_barrier_depends(); \
					(_________p1); \
					})


The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
and release a global reader-writer lock.  The synchronize_rcu()
primitive write-acquires this same lock, then immediately releases
it.  This means that once synchronize_rcu() exits, all RCU read-side
critical sections that were in progress before synchronize_rcu() was
called are guaranteed to have completed -- there is no way that
synchronize_rcu() would have been able to write-acquire the lock
otherwise.

It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired.  Note also that rcu_read_lock() is immune
from deadlock (an important property of RCU).  The reason for this is
that the only thing that can block rcu_read_lock() is a synchronize_rcu().
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
so there can be no deadlock cycle.

Quick Quiz #1:	Why is this argument naive?  How could a deadlock
		occur when using this algorithm in a real-world Linux
		kernel?  How could this deadlock be avoided?


5B.  "TOY" EXAMPLE #2: CLASSIC RCU

This section presents a "toy" RCU implementation that is based on
"classic RCU".  It is also short on performance (but only for updates) and
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
are the same as those shown in the preceding section, so they are omitted.

	void rcu_read_lock(void) { }

	void rcu_read_unlock(void) { }

	void synchronize_rcu(void)
	{
		int cpu;

		for_each_possible_cpu(cpu)
			run_on(cpu);
	}

Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
This is the great strength of classic RCU in a non-preemptive kernel:
read-side overhead is precisely zero, at least on non-Alpha CPUs.
And there is absolutely no way that rcu_read_lock() can possibly
participate in a deadlock cycle!

The implementation of synchronize_rcu() simply schedules itself on each
CPU in turn.  The run_on() primitive can be implemented straightforwardly
in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
"toy" implementation would restore the affinity upon completion rather
than just leaving all tasks running on the last CPU, but when I said
"toy", I meant -toy-!

So how the heck is this supposed to work???

Remember that it is illegal to block while in an RCU read-side critical
section.  Therefore, if a given CPU executes a context switch, we know
that it must have completed all preceding RCU read-side critical sections.
Once -all- CPUs have executed a context switch, then -all- preceding
RCU read-side critical sections will have completed.

So, suppose that we remove a data item from its structure and then invoke
synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
that there are no RCU read-side critical sections holding a reference
to that data item, so we can safely reclaim it.

Quick Quiz #2:	Give an example where Classic RCU's read-side
		overhead is -negative-.

Quick Quiz #3:  If it is illegal to block in an RCU read-side
		critical section, what the heck do you do in
		PREEMPT_RT, where normal spinlocks can block???


6.  ANALOGY WITH READER-WRITER LOCKING

Although RCU can be used in many different ways, a very common use of
RCU is analogous to reader-writer locking.  The following unified
diff shows how closely related RCU and reader-writer locking can be.

	@@ -13,15 +14,15 @@
		struct list_head *lp;
		struct el *p;

	-	read_lock();
	-	list_for_each_entry(p, head, lp) {
	+	rcu_read_lock();
	+	list_for_each_entry_rcu(p, head, lp) {
			if (p->key == key) {
				*result = p->data;
	-			read_unlock();
	+			rcu_read_unlock();
				return 1;
			}
		}
	-	read_unlock();
	+	rcu_read_unlock();
		return 0;
	 }

	@@ -29,15 +30,16 @@
	 {
		struct el *p;

	-	write_lock(&listmutex);
	+	spin_lock(&listmutex);
		list_for_each_entry(p, head, lp) {
			if (p->key == key) {
	-			list_del(&p->list);
	-			write_unlock(&listmutex);
	+			list_del_rcu(&p->list);
	+			spin_unlock(&listmutex);
	+			synchronize_rcu();
				kfree(p);
				return 1;
			}
		}
	-	write_unlock(&listmutex);
	+	spin_unlock(&listmutex);
		return 0;
	 }

Or, for those who prefer a side-by-side listing:

 1 struct el {                          1 struct el {
 2   struct list_head list;             2   struct list_head list;
 3   long key;                          3   long key;
 4   spinlock_t mutex;                  4   spinlock_t mutex;
 5   int data;                          5   int data;
 6   /* Other data fields */            6   /* Other data fields */
 7 };                                   7 };
 8 spinlock_t listmutex;                8 spinlock_t listmutex;
 9 struct el head;                      9 struct el head;

 1 int search(long key, int *result)    1 int search(long key, int *result)
 2 {                                    2 {
 3   struct list_head *lp;              3   struct list_head *lp;
 4   struct el *p;                      4   struct el *p;
 5                                      5
 6   read_lock();                       6   rcu_read_lock();
 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
 8     if (p->key == key) {             8     if (p->key == key) {
 9       *result = p->data;             9       *result = p->data;
10       read_unlock();                10       rcu_read_unlock();
11       return 1;                     11       return 1;
12     }                               12     }
13   }                                 13   }
14   read_unlock();                    14   rcu_read_unlock();
15   return 0;                         15   return 0;
16 }                                   16 }

 1 int delete(long key)                 1 int delete(long key)
 2 {                                    2 {
 3   struct el *p;                      3   struct el *p;
 4                                      4
 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
 7     if (p->key == key) {             7     if (p->key == key) {
 8       list_del(&p->list);            8       list_del_rcu(&p->list);
 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
                                       10       synchronize_rcu();
10       kfree(p);                     11       kfree(p);
11       return 1;                     12       return 1;
12     }                               13     }
13   }                                 14   }
14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
15   return 0;                         16   return 0;
16 }                                   17 }

Either way, the differences are quite small.  Read-side locking moves
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
a reader-writer lock to a simple spinlock, and a synchronize_rcu()
precedes the kfree().

However, there is one potential catch: the read-side and update-side
critical sections can now run concurrently.  In many cases, this will
not be a problem, but it is necessary to check carefully regardless.
For example, if multiple independent list updates must be seen as
a single atomic update, converting to RCU will require special care.

Also, the presence of synchronize_rcu() means that the RCU version of
delete() can now block.  If this is a problem, there is a callback-based
mechanism that never blocks, namely call_rcu(), that can be used in
place of synchronize_rcu().


7.  FULL LIST OF RCU APIs

The RCU APIs are documented in docbook-format header comments in the
Linux-kernel source code, but it helps to have a full list of the
APIs, since there does not appear to be a way to categorize them
in docbook.  Here is the list, by category.

RCU pointer/list traversal:

	rcu_dereference
	list_for_each_entry_rcu
	hlist_for_each_entry_rcu
	hlist_nulls_for_each_entry_rcu

	list_for_each_continue_rcu	(to be deprecated in favor of new
					 list_for_each_entry_continue_rcu)

RCU pointer/list update:

	rcu_assign_pointer
	list_add_rcu
	list_add_tail_rcu
	list_del_rcu
	list_replace_rcu
	hlist_del_rcu
	hlist_add_after_rcu
	hlist_add_before_rcu
	hlist_add_head_rcu
	hlist_replace_rcu
	list_splice_init_rcu()

RCU:	Critical sections	Grace period		Barrier

	rcu_read_lock		synchronize_net		rcu_barrier
	rcu_read_unlock		synchronize_rcu
				synchronize_rcu_expedited
				call_rcu


bh:	Critical sections	Grace period		Barrier

	rcu_read_lock_bh	call_rcu_bh		rcu_barrier_bh
	rcu_read_unlock_bh	synchronize_rcu_bh
				synchronize_rcu_bh_expedited


sched:	Critical sections	Grace period		Barrier

	rcu_read_lock_sched	synchronize_sched	rcu_barrier_sched
	rcu_read_unlock_sched	call_rcu_sched
	[preempt_disable]	synchronize_sched_expedited
	[and friends]


SRCU:	Critical sections	Grace period		Barrier

	srcu_read_lock		synchronize_srcu	N/A
	srcu_read_unlock

SRCU:	Initialization/cleanup
	init_srcu_struct
	cleanup_srcu_struct

See the comment headers in the source code (or the docbook generated
from them) for more information.


8.  ANSWERS TO QUICK QUIZZES

Quick Quiz #1:	Why is this argument naive?  How could a deadlock
		occur when using this algorithm in a real-world Linux
		kernel?  [Referring to the lock-based "toy" RCU
		algorithm.]

Answer:		Consider the following sequence of events:

		1.	CPU 0 acquires some unrelated lock, call it
			"problematic_lock", disabling irq via
			spin_lock_irqsave().

		2.	CPU 1 enters synchronize_rcu(), write-acquiring
			rcu_gp_mutex.

		3.	CPU 0 enters rcu_read_lock(), but must wait
			because CPU 1 holds rcu_gp_mutex.

		4.	CPU 1 is interrupted, and the irq handler
			attempts to acquire problematic_lock.

		The system is now deadlocked.

		One way to avoid this deadlock is to use an approach like
		that of CONFIG_PREEMPT_RT, where all normal spinlocks
		become blocking locks, and all irq handlers execute in
		the context of special tasks.  In this case, in step 4
		above, the irq handler would block, allowing CPU 1 to
		release rcu_gp_mutex, avoiding the deadlock.

		Even in the absence of deadlock, this RCU implementation
		allows latency to "bleed" from readers to other
		readers through synchronize_rcu().  To see this,
		consider task A in an RCU read-side critical section
		(thus read-holding rcu_gp_mutex), task B blocked
		attempting to write-acquire rcu_gp_mutex, and
		task C blocked in rcu_read_lock() attempting to
		read_acquire rcu_gp_mutex.  Task A's RCU read-side
		latency is holding up task C, albeit indirectly via
		task B.

		Realtime RCU implementations therefore use a counter-based
		approach where tasks in RCU read-side critical sections
		cannot be blocked by tasks executing synchronize_rcu().

Quick Quiz #2:	Give an example where Classic RCU's read-side
		overhead is -negative-.

Answer:		Imagine a single-CPU system with a non-CONFIG_PREEMPT
		kernel where a routing table is used by process-context
		code, but can be updated by irq-context code (for example,
		by an "ICMP REDIRECT" packet).	The usual way of handling
		this would be to have the process-context code disable
		interrupts while searching the routing table.  Use of
		RCU allows such interrupt-disabling to be dispensed with.
		Thus, without RCU, you pay the cost of disabling interrupts,
		and with RCU you don't.

		One can argue that the overhead of RCU in this
		case is negative with respect to the single-CPU
		interrupt-disabling approach.  Others might argue that
		the overhead of RCU is merely zero, and that replacing
		the positive overhead of the interrupt-disabling scheme
		with the zero-overhead RCU scheme does not constitute
		negative overhead.

		In real life, of course, things are more complex.  But
		even the theoretical possibility of negative overhead for
		a synchronization primitive is a bit unexpected.  ;-)

Quick Quiz #3:  If it is illegal to block in an RCU read-side
		critical section, what the heck do you do in
		PREEMPT_RT, where normal spinlocks can block???

Answer:		Just as PREEMPT_RT permits preemption of spinlock
		critical sections, it permits preemption of RCU
		read-side critical sections.  It also permits
		spinlocks blocking while in RCU read-side critical
		sections.

		Why the apparent inconsistency?  Because it is it
		possible to use priority boosting to keep the RCU
		grace periods short if need be (for example, if running
		short of memory).  In contrast, if blocking waiting
		for (say) network reception, there is no way to know
		what should be boosted.  Especially given that the
		process we need to boost might well be a human being
		who just went out for a pizza or something.  And although
		a computer-operated cattle prod might arouse serious
		interest, it might also provoke serious objections.
		Besides, how does the computer know what pizza parlor
		the human being went to???


ACKNOWLEDGEMENTS

My thanks to the people who helped make this human-readable, including
Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.


For more information, see http://www.rdrop.com/users/paulmck/RCU.