blob: 8367d393cba2757d19ad6d0fa26667f303900713 [file] [log] [blame]
David Howells108b42b2006-03-31 16:00:29 +01001 ============================
2 LINUX KERNEL MEMORY BARRIERS
3 ============================
4
5By: David Howells <dhowells@redhat.com>
David Howells90fddab2010-03-24 09:43:00 +00006 Paul E. McKenney <paulmck@linux.vnet.ibm.com>
David Howells108b42b2006-03-31 16:00:29 +01007
8Contents:
9
10 (*) Abstract memory access model.
11
12 - Device operations.
13 - Guarantees.
14
15 (*) What are memory barriers?
16
17 - Varieties of memory barrier.
18 - What may not be assumed about memory barriers?
19 - Data dependency barriers.
20 - Control dependencies.
21 - SMP barrier pairing.
22 - Examples of memory barrier sequences.
David Howells670bd952006-06-10 09:54:12 -070023 - Read memory barriers vs load speculation.
Paul E. McKenney241e6662011-02-10 16:54:50 -080024 - Transitivity
David Howells108b42b2006-03-31 16:00:29 +010025
26 (*) Explicit kernel barriers.
27
28 - Compiler barrier.
Jarek Poplawski81fc6322007-05-23 13:58:20 -070029 - CPU memory barriers.
David Howells108b42b2006-03-31 16:00:29 +010030 - MMIO write barrier.
31
32 (*) Implicit kernel memory barriers.
33
34 - Locking functions.
35 - Interrupt disabling functions.
David Howells50fa6102009-04-28 15:01:38 +010036 - Sleep and wake-up functions.
David Howells108b42b2006-03-31 16:00:29 +010037 - Miscellaneous functions.
38
39 (*) Inter-CPU locking barrier effects.
40
41 - Locks vs memory accesses.
42 - Locks vs I/O accesses.
43
44 (*) Where are memory barriers needed?
45
46 - Interprocessor interaction.
47 - Atomic operations.
48 - Accessing devices.
49 - Interrupts.
50
51 (*) Kernel I/O barrier effects.
52
53 (*) Assumed minimum execution ordering model.
54
55 (*) The effects of the cpu cache.
56
57 - Cache coherency.
58 - Cache coherency vs DMA.
59 - Cache coherency vs MMIO.
60
61 (*) The things CPUs get up to.
62
63 - And then there's the Alpha.
64
David Howells90fddab2010-03-24 09:43:00 +000065 (*) Example uses.
66
67 - Circular buffers.
68
David Howells108b42b2006-03-31 16:00:29 +010069 (*) References.
70
71
72============================
73ABSTRACT MEMORY ACCESS MODEL
74============================
75
76Consider the following abstract model of the system:
77
78 : :
79 : :
80 : :
81 +-------+ : +--------+ : +-------+
82 | | : | | : | |
83 | | : | | : | |
84 | CPU 1 |<----->| Memory |<----->| CPU 2 |
85 | | : | | : | |
86 | | : | | : | |
87 +-------+ : +--------+ : +-------+
88 ^ : ^ : ^
89 | : | : |
90 | : | : |
91 | : v : |
92 | : +--------+ : |
93 | : | | : |
94 | : | | : |
95 +---------->| Device |<----------+
96 : | | :
97 : | | :
98 : +--------+ :
99 : :
100
101Each CPU executes a program that generates memory access operations. In the
102abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
103perform the memory operations in any order it likes, provided program causality
104appears to be maintained. Similarly, the compiler may also arrange the
105instructions it emits in any order it likes, provided it doesn't affect the
106apparent operation of the program.
107
108So in the above diagram, the effects of the memory operations performed by a
109CPU are perceived by the rest of the system as the operations cross the
110interface between the CPU and rest of the system (the dotted lines).
111
112
113For example, consider the following sequence of events:
114
115 CPU 1 CPU 2
116 =============== ===============
117 { A == 1; B == 2 }
Alexey Dobriyan615cc2c2014-06-06 14:36:41 -0700118 A = 3; x = B;
119 B = 4; y = A;
David Howells108b42b2006-03-31 16:00:29 +0100120
121The set of accesses as seen by the memory system in the middle can be arranged
122in 24 different combinations:
123
Pranith Kumar8ab8b3e2014-09-02 23:34:29 -0400124 STORE A=3, STORE B=4, y=LOAD A->3, x=LOAD B->4
125 STORE A=3, STORE B=4, x=LOAD B->4, y=LOAD A->3
126 STORE A=3, y=LOAD A->3, STORE B=4, x=LOAD B->4
127 STORE A=3, y=LOAD A->3, x=LOAD B->2, STORE B=4
128 STORE A=3, x=LOAD B->2, STORE B=4, y=LOAD A->3
129 STORE A=3, x=LOAD B->2, y=LOAD A->3, STORE B=4
130 STORE B=4, STORE A=3, y=LOAD A->3, x=LOAD B->4
David Howells108b42b2006-03-31 16:00:29 +0100131 STORE B=4, ...
132 ...
133
134and can thus result in four different combinations of values:
135
Pranith Kumar8ab8b3e2014-09-02 23:34:29 -0400136 x == 2, y == 1
137 x == 2, y == 3
138 x == 4, y == 1
139 x == 4, y == 3
David Howells108b42b2006-03-31 16:00:29 +0100140
141
142Furthermore, the stores committed by a CPU to the memory system may not be
143perceived by the loads made by another CPU in the same order as the stores were
144committed.
145
146
147As a further example, consider this sequence of events:
148
149 CPU 1 CPU 2
150 =============== ===============
151 { A == 1, B == 2, C = 3, P == &A, Q == &C }
152 B = 4; Q = P;
153 P = &B D = *Q;
154
155There is an obvious data dependency here, as the value loaded into D depends on
156the address retrieved from P by CPU 2. At the end of the sequence, any of the
157following results are possible:
158
159 (Q == &A) and (D == 1)
160 (Q == &B) and (D == 2)
161 (Q == &B) and (D == 4)
162
163Note that CPU 2 will never try and load C into D because the CPU will load P
164into Q before issuing the load of *Q.
165
166
167DEVICE OPERATIONS
168-----------------
169
170Some devices present their control interfaces as collections of memory
171locations, but the order in which the control registers are accessed is very
172important. For instance, imagine an ethernet card with a set of internal
173registers that are accessed through an address port register (A) and a data
174port register (D). To read internal register 5, the following code might then
175be used:
176
177 *A = 5;
178 x = *D;
179
180but this might show up as either of the following two sequences:
181
182 STORE *A = 5, x = LOAD *D
183 x = LOAD *D, STORE *A = 5
184
185the second of which will almost certainly result in a malfunction, since it set
186the address _after_ attempting to read the register.
187
188
189GUARANTEES
190----------
191
192There are some minimal guarantees that may be expected of a CPU:
193
194 (*) On any given CPU, dependent memory accesses will be issued in order, with
195 respect to itself. This means that for:
196
Chris Metcalff84cfbb2015-11-23 17:04:17 -0500197 Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
David Howells108b42b2006-03-31 16:00:29 +0100198
199 the CPU will issue the following memory operations:
200
201 Q = LOAD P, D = LOAD *Q
202
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800203 and always in that order. On most systems, smp_read_barrier_depends()
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700204 does nothing, but it is required for DEC Alpha. The READ_ONCE()
Chris Metcalff84cfbb2015-11-23 17:04:17 -0500205 is required to prevent compiler mischief. Please note that you
206 should normally use something like rcu_dereference() instead of
207 open-coding smp_read_barrier_depends().
David Howells108b42b2006-03-31 16:00:29 +0100208
209 (*) Overlapping loads and stores within a particular CPU will appear to be
210 ordered within that CPU. This means that for:
211
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700212 a = READ_ONCE(*X); WRITE_ONCE(*X, b);
David Howells108b42b2006-03-31 16:00:29 +0100213
214 the CPU will only issue the following sequence of memory operations:
215
216 a = LOAD *X, STORE *X = b
217
218 And for:
219
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700220 WRITE_ONCE(*X, c); d = READ_ONCE(*X);
David Howells108b42b2006-03-31 16:00:29 +0100221
222 the CPU will only issue:
223
224 STORE *X = c, d = LOAD *X
225
Matt LaPlantefa00e7e2006-11-30 04:55:36 +0100226 (Loads and stores overlap if they are targeted at overlapping pieces of
David Howells108b42b2006-03-31 16:00:29 +0100227 memory).
228
229And there are a number of things that _must_ or _must_not_ be assumed:
230
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700231 (*) It _must_not_ be assumed that the compiler will do what you want
232 with memory references that are not protected by READ_ONCE() and
233 WRITE_ONCE(). Without them, the compiler is within its rights to
234 do all sorts of "creative" transformations, which are covered in
Paul E. McKenney895f5542016-01-06 14:23:03 -0800235 the COMPILER BARRIER section.
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800236
David Howells108b42b2006-03-31 16:00:29 +0100237 (*) It _must_not_ be assumed that independent loads and stores will be issued
238 in the order given. This means that for:
239
240 X = *A; Y = *B; *D = Z;
241
242 we may get any of the following sequences:
243
244 X = LOAD *A, Y = LOAD *B, STORE *D = Z
245 X = LOAD *A, STORE *D = Z, Y = LOAD *B
246 Y = LOAD *B, X = LOAD *A, STORE *D = Z
247 Y = LOAD *B, STORE *D = Z, X = LOAD *A
248 STORE *D = Z, X = LOAD *A, Y = LOAD *B
249 STORE *D = Z, Y = LOAD *B, X = LOAD *A
250
251 (*) It _must_ be assumed that overlapping memory accesses may be merged or
252 discarded. This means that for:
253
254 X = *A; Y = *(A + 4);
255
256 we may get any one of the following sequences:
257
258 X = LOAD *A; Y = LOAD *(A + 4);
259 Y = LOAD *(A + 4); X = LOAD *A;
260 {X, Y} = LOAD {*A, *(A + 4) };
261
262 And for:
263
Paul E. McKenneyf191eec2012-10-03 10:28:30 -0700264 *A = X; *(A + 4) = Y;
David Howells108b42b2006-03-31 16:00:29 +0100265
Paul E. McKenneyf191eec2012-10-03 10:28:30 -0700266 we may get any of:
David Howells108b42b2006-03-31 16:00:29 +0100267
Paul E. McKenneyf191eec2012-10-03 10:28:30 -0700268 STORE *A = X; STORE *(A + 4) = Y;
269 STORE *(A + 4) = Y; STORE *A = X;
270 STORE {*A, *(A + 4) } = {X, Y};
David Howells108b42b2006-03-31 16:00:29 +0100271
Paul E. McKenney432fbf32014-09-04 17:12:49 -0700272And there are anti-guarantees:
273
274 (*) These guarantees do not apply to bitfields, because compilers often
275 generate code to modify these using non-atomic read-modify-write
276 sequences. Do not attempt to use bitfields to synchronize parallel
277 algorithms.
278
279 (*) Even in cases where bitfields are protected by locks, all fields
280 in a given bitfield must be protected by one lock. If two fields
281 in a given bitfield are protected by different locks, the compiler's
282 non-atomic read-modify-write sequences can cause an update to one
283 field to corrupt the value of an adjacent field.
284
285 (*) These guarantees apply only to properly aligned and sized scalar
286 variables. "Properly sized" currently means variables that are
287 the same size as "char", "short", "int" and "long". "Properly
288 aligned" means the natural alignment, thus no constraints for
289 "char", two-byte alignment for "short", four-byte alignment for
290 "int", and either four-byte or eight-byte alignment for "long",
291 on 32-bit and 64-bit systems, respectively. Note that these
292 guarantees were introduced into the C11 standard, so beware when
293 using older pre-C11 compilers (for example, gcc 4.6). The portion
294 of the standard containing this guarantee is Section 3.14, which
295 defines "memory location" as follows:
296
297 memory location
298 either an object of scalar type, or a maximal sequence
299 of adjacent bit-fields all having nonzero width
300
301 NOTE 1: Two threads of execution can update and access
302 separate memory locations without interfering with
303 each other.
304
305 NOTE 2: A bit-field and an adjacent non-bit-field member
306 are in separate memory locations. The same applies
307 to two bit-fields, if one is declared inside a nested
308 structure declaration and the other is not, or if the two
309 are separated by a zero-length bit-field declaration,
310 or if they are separated by a non-bit-field member
311 declaration. It is not safe to concurrently update two
312 bit-fields in the same structure if all members declared
313 between them are also bit-fields, no matter what the
314 sizes of those intervening bit-fields happen to be.
315
David Howells108b42b2006-03-31 16:00:29 +0100316
317=========================
318WHAT ARE MEMORY BARRIERS?
319=========================
320
321As can be seen above, independent memory operations are effectively performed
322in random order, but this can be a problem for CPU-CPU interaction and for I/O.
323What is required is some way of intervening to instruct the compiler and the
324CPU to restrict the order.
325
326Memory barriers are such interventions. They impose a perceived partial
David Howells2b948952006-06-25 05:48:49 -0700327ordering over the memory operations on either side of the barrier.
328
329Such enforcement is important because the CPUs and other devices in a system
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700330can use a variety of tricks to improve performance, including reordering,
David Howells2b948952006-06-25 05:48:49 -0700331deferral and combination of memory operations; speculative loads; speculative
332branch prediction and various types of caching. Memory barriers are used to
333override or suppress these tricks, allowing the code to sanely control the
334interaction of multiple CPUs and/or devices.
David Howells108b42b2006-03-31 16:00:29 +0100335
336
337VARIETIES OF MEMORY BARRIER
338---------------------------
339
340Memory barriers come in four basic varieties:
341
342 (1) Write (or store) memory barriers.
343
344 A write memory barrier gives a guarantee that all the STORE operations
345 specified before the barrier will appear to happen before all the STORE
346 operations specified after the barrier with respect to the other
347 components of the system.
348
349 A write barrier is a partial ordering on stores only; it is not required
350 to have any effect on loads.
351
David Howells6bc39272006-06-25 05:49:22 -0700352 A CPU can be viewed as committing a sequence of store operations to the
David Howells108b42b2006-03-31 16:00:29 +0100353 memory system as time progresses. All stores before a write barrier will
354 occur in the sequence _before_ all the stores after the write barrier.
355
356 [!] Note that write barriers should normally be paired with read or data
357 dependency barriers; see the "SMP barrier pairing" subsection.
358
359
360 (2) Data dependency barriers.
361
362 A data dependency barrier is a weaker form of read barrier. In the case
363 where two loads are performed such that the second depends on the result
364 of the first (eg: the first load retrieves the address to which the second
365 load will be directed), a data dependency barrier would be required to
366 make sure that the target of the second load is updated before the address
367 obtained by the first load is accessed.
368
369 A data dependency barrier is a partial ordering on interdependent loads
370 only; it is not required to have any effect on stores, independent loads
371 or overlapping loads.
372
373 As mentioned in (1), the other CPUs in the system can be viewed as
374 committing sequences of stores to the memory system that the CPU being
375 considered can then perceive. A data dependency barrier issued by the CPU
376 under consideration guarantees that for any load preceding it, if that
377 load touches one of a sequence of stores from another CPU, then by the
378 time the barrier completes, the effects of all the stores prior to that
379 touched by the load will be perceptible to any loads issued after the data
380 dependency barrier.
381
382 See the "Examples of memory barrier sequences" subsection for diagrams
383 showing the ordering constraints.
384
385 [!] Note that the first load really has to have a _data_ dependency and
386 not a control dependency. If the address for the second load is dependent
387 on the first load, but the dependency is through a conditional rather than
388 actually loading the address itself, then it's a _control_ dependency and
389 a full read barrier or better is required. See the "Control dependencies"
390 subsection for more information.
391
392 [!] Note that data dependency barriers should normally be paired with
393 write barriers; see the "SMP barrier pairing" subsection.
394
395
396 (3) Read (or load) memory barriers.
397
398 A read barrier is a data dependency barrier plus a guarantee that all the
399 LOAD operations specified before the barrier will appear to happen before
400 all the LOAD operations specified after the barrier with respect to the
401 other components of the system.
402
403 A read barrier is a partial ordering on loads only; it is not required to
404 have any effect on stores.
405
406 Read memory barriers imply data dependency barriers, and so can substitute
407 for them.
408
409 [!] Note that read barriers should normally be paired with write barriers;
410 see the "SMP barrier pairing" subsection.
411
412
413 (4) General memory barriers.
414
David Howells670bd952006-06-10 09:54:12 -0700415 A general memory barrier gives a guarantee that all the LOAD and STORE
416 operations specified before the barrier will appear to happen before all
417 the LOAD and STORE operations specified after the barrier with respect to
418 the other components of the system.
419
420 A general memory barrier is a partial ordering over both loads and stores.
David Howells108b42b2006-03-31 16:00:29 +0100421
422 General memory barriers imply both read and write memory barriers, and so
423 can substitute for either.
424
425
426And a couple of implicit varieties:
427
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100428 (5) ACQUIRE operations.
David Howells108b42b2006-03-31 16:00:29 +0100429
430 This acts as a one-way permeable barrier. It guarantees that all memory
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100431 operations after the ACQUIRE operation will appear to happen after the
432 ACQUIRE operation with respect to the other components of the system.
433 ACQUIRE operations include LOCK operations and smp_load_acquire()
434 operations.
David Howells108b42b2006-03-31 16:00:29 +0100435
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100436 Memory operations that occur before an ACQUIRE operation may appear to
437 happen after it completes.
David Howells108b42b2006-03-31 16:00:29 +0100438
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100439 An ACQUIRE operation should almost always be paired with a RELEASE
440 operation.
David Howells108b42b2006-03-31 16:00:29 +0100441
442
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100443 (6) RELEASE operations.
David Howells108b42b2006-03-31 16:00:29 +0100444
445 This also acts as a one-way permeable barrier. It guarantees that all
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100446 memory operations before the RELEASE operation will appear to happen
447 before the RELEASE operation with respect to the other components of the
448 system. RELEASE operations include UNLOCK operations and
449 smp_store_release() operations.
David Howells108b42b2006-03-31 16:00:29 +0100450
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100451 Memory operations that occur after a RELEASE operation may appear to
David Howells108b42b2006-03-31 16:00:29 +0100452 happen before it completes.
453
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100454 The use of ACQUIRE and RELEASE operations generally precludes the need
455 for other sorts of memory barrier (but note the exceptions mentioned in
456 the subsection "MMIO write barrier"). In addition, a RELEASE+ACQUIRE
457 pair is -not- guaranteed to act as a full memory barrier. However, after
458 an ACQUIRE on a given variable, all memory accesses preceding any prior
459 RELEASE on that same variable are guaranteed to be visible. In other
460 words, within a given variable's critical section, all accesses of all
461 previous critical sections for that variable are guaranteed to have
462 completed.
Paul E. McKenney17eb88e2013-12-11 13:59:09 -0800463
Peter Zijlstra2e4f5382013-11-06 14:57:36 +0100464 This means that ACQUIRE acts as a minimal "acquire" operation and
465 RELEASE acts as a minimal "release" operation.
David Howells108b42b2006-03-31 16:00:29 +0100466
467
468Memory barriers are only required where there's a possibility of interaction
469between two CPUs or between a CPU and a device. If it can be guaranteed that
470there won't be any such interaction in any particular piece of code, then
471memory barriers are unnecessary in that piece of code.
472
473
474Note that these are the _minimum_ guarantees. Different architectures may give
475more substantial guarantees, but they may _not_ be relied upon outside of arch
476specific code.
477
478
479WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
480----------------------------------------------
481
482There are certain things that the Linux kernel memory barriers do not guarantee:
483
484 (*) There is no guarantee that any of the memory accesses specified before a
485 memory barrier will be _complete_ by the completion of a memory barrier
486 instruction; the barrier can be considered to draw a line in that CPU's
487 access queue that accesses of the appropriate type may not cross.
488
489 (*) There is no guarantee that issuing a memory barrier on one CPU will have
490 any direct effect on another CPU or any other hardware in the system. The
491 indirect effect will be the order in which the second CPU sees the effects
492 of the first CPU's accesses occur, but see the next point:
493
David Howells6bc39272006-06-25 05:49:22 -0700494 (*) There is no guarantee that a CPU will see the correct order of effects
David Howells108b42b2006-03-31 16:00:29 +0100495 from a second CPU's accesses, even _if_ the second CPU uses a memory
496 barrier, unless the first CPU _also_ uses a matching memory barrier (see
497 the subsection on "SMP Barrier Pairing").
498
499 (*) There is no guarantee that some intervening piece of off-the-CPU
500 hardware[*] will not reorder the memory accesses. CPU cache coherency
501 mechanisms should propagate the indirect effects of a memory barrier
502 between CPUs, but might not do so in order.
503
504 [*] For information on bus mastering DMA and coherency please read:
505
Randy Dunlap4b5ff462008-03-10 17:16:32 -0700506 Documentation/PCI/pci.txt
Paul Bolle395cf962011-08-15 02:02:26 +0200507 Documentation/DMA-API-HOWTO.txt
David Howells108b42b2006-03-31 16:00:29 +0100508 Documentation/DMA-API.txt
509
510
511DATA DEPENDENCY BARRIERS
512------------------------
513
514The usage requirements of data dependency barriers are a little subtle, and
515it's not always obvious that they're needed. To illustrate, consider the
516following sequence of events:
517
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800518 CPU 1 CPU 2
519 =============== ===============
David Howells108b42b2006-03-31 16:00:29 +0100520 { A == 1, B == 2, C = 3, P == &A, Q == &C }
521 B = 4;
522 <write barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700523 WRITE_ONCE(P, &B)
524 Q = READ_ONCE(P);
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800525 D = *Q;
David Howells108b42b2006-03-31 16:00:29 +0100526
527There's a clear data dependency here, and it would seem that by the end of the
528sequence, Q must be either &A or &B, and that:
529
530 (Q == &A) implies (D == 1)
531 (Q == &B) implies (D == 4)
532
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700533But! CPU 2's perception of P may be updated _before_ its perception of B, thus
David Howells108b42b2006-03-31 16:00:29 +0100534leading to the following situation:
535
536 (Q == &B) and (D == 2) ????
537
538Whilst this may seem like a failure of coherency or causality maintenance, it
539isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
540Alpha).
541
David Howells2b948952006-06-25 05:48:49 -0700542To deal with this, a data dependency barrier or better must be inserted
543between the address load and the data load:
David Howells108b42b2006-03-31 16:00:29 +0100544
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800545 CPU 1 CPU 2
546 =============== ===============
David Howells108b42b2006-03-31 16:00:29 +0100547 { A == 1, B == 2, C = 3, P == &A, Q == &C }
548 B = 4;
549 <write barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700550 WRITE_ONCE(P, &B);
551 Q = READ_ONCE(P);
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800552 <data dependency barrier>
553 D = *Q;
David Howells108b42b2006-03-31 16:00:29 +0100554
555This enforces the occurrence of one of the two implications, and prevents the
556third possibility from arising.
557
Paul E. McKenney92a84dd2016-01-14 14:17:04 -0800558A data-dependency barrier must also order against dependent writes:
559
560 CPU 1 CPU 2
561 =============== ===============
562 { A == 1, B == 2, C = 3, P == &A, Q == &C }
563 B = 4;
564 <write barrier>
565 WRITE_ONCE(P, &B);
566 Q = READ_ONCE(P);
567 <data dependency barrier>
568 *Q = 5;
569
570The data-dependency barrier must order the read into Q with the store
571into *Q. This prohibits this outcome:
572
573 (Q == B) && (B == 4)
574
575Please note that this pattern should be rare. After all, the whole point
576of dependency ordering is to -prevent- writes to the data structure, along
577with the expensive cache misses associated with those writes. This pattern
578can be used to record rare error conditions and the like, and the ordering
579prevents such records from being lost.
580
581
David Howells108b42b2006-03-31 16:00:29 +0100582[!] Note that this extremely counterintuitive situation arises most easily on
583machines with split caches, so that, for example, one cache bank processes
584even-numbered cache lines and the other bank processes odd-numbered cache
585lines. The pointer P might be stored in an odd-numbered cache line, and the
586variable B might be stored in an even-numbered cache line. Then, if the
587even-numbered bank of the reading CPU's cache is extremely busy while the
588odd-numbered bank is idle, one can see the new value of the pointer P (&B),
David Howells6bc39272006-06-25 05:49:22 -0700589but the old value of the variable B (2).
David Howells108b42b2006-03-31 16:00:29 +0100590
591
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800592The data dependency barrier is very important to the RCU system,
593for example. See rcu_assign_pointer() and rcu_dereference() in
594include/linux/rcupdate.h. This permits the current target of an RCU'd
595pointer to be replaced with a new modified target, without the replacement
596target appearing to be incompletely initialised.
David Howells108b42b2006-03-31 16:00:29 +0100597
598See also the subsection on "Cache Coherency" for a more thorough example.
599
600
601CONTROL DEPENDENCIES
602--------------------
603
Paul E. McKenneyff382812015-02-17 10:00:06 -0800604A load-load control dependency requires a full read memory barrier, not
605simply a data dependency barrier to make it work correctly. Consider the
606following bit of code:
David Howells108b42b2006-03-31 16:00:29 +0100607
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700608 q = READ_ONCE(a);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800609 if (q) {
610 <data dependency barrier> /* BUG: No data dependency!!! */
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700611 p = READ_ONCE(b);
Paul E. McKenney45c8a362013-07-02 15:24:09 -0700612 }
David Howells108b42b2006-03-31 16:00:29 +0100613
614This will not have the desired effect because there is no actual data
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800615dependency, but rather a control dependency that the CPU may short-circuit
616by attempting to predict the outcome in advance, so that other CPUs see
617the load from b as having happened before the load from a. In such a
618case what's actually required is:
David Howells108b42b2006-03-31 16:00:29 +0100619
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700620 q = READ_ONCE(a);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800621 if (q) {
Paul E. McKenney45c8a362013-07-02 15:24:09 -0700622 <read barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700623 p = READ_ONCE(b);
Paul E. McKenney45c8a362013-07-02 15:24:09 -0700624 }
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800625
626However, stores are not speculated. This means that ordering -is- provided
Paul E. McKenneyff382812015-02-17 10:00:06 -0800627for load-store control dependencies, as in the following example:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800628
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800629 q = READ_ONCE(a);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700630 if (q) {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700631 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800632 }
633
Paul E. McKenney5af46922015-04-25 12:48:29 -0700634Control dependencies pair normally with other types of barriers. That
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800635said, please note that READ_ONCE() is not optional! Without the
636READ_ONCE(), the compiler might combine the load from 'a' with other
637loads from 'a', and the store to 'b' with other stores to 'b', with
638possible highly counterintuitive effects on ordering.
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800639
640Worse yet, if the compiler is able to prove (say) that the value of
641variable 'a' is always non-zero, it would be well within its rights
642to optimize the original example by eliminating the "if" statement
643as follows:
644
645 q = a;
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700646 b = p; /* BUG: Compiler and CPU can both reorder!!! */
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800647
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800648So don't leave out the READ_ONCE().
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700649
650It is tempting to try to enforce ordering on identical stores on both
651branches of the "if" statement as follows:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800652
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800653 q = READ_ONCE(a);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800654 if (q) {
Paul E. McKenney9b2b3bf2014-02-12 20:19:47 -0800655 barrier();
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700656 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800657 do_something();
658 } else {
Paul E. McKenney9b2b3bf2014-02-12 20:19:47 -0800659 barrier();
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700660 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800661 do_something_else();
662 }
663
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700664Unfortunately, current compilers will transform this as follows at high
665optimization levels:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800666
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800667 q = READ_ONCE(a);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700668 barrier();
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700669 WRITE_ONCE(b, p); /* BUG: No ordering vs. load from a!!! */
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800670 if (q) {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700671 /* WRITE_ONCE(b, p); -- moved up, BUG!!! */
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800672 do_something();
673 } else {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700674 /* WRITE_ONCE(b, p); -- moved up, BUG!!! */
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800675 do_something_else();
676 }
677
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700678Now there is no conditional between the load from 'a' and the store to
679'b', which means that the CPU is within its rights to reorder them:
680The conditional is absolutely required, and must be present in the
681assembly code even after all compiler optimizations have been applied.
682Therefore, if you need ordering in this example, you need explicit
683memory barriers, for example, smp_store_release():
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800684
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700685 q = READ_ONCE(a);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700686 if (q) {
687 smp_store_release(&b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800688 do_something();
689 } else {
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700690 smp_store_release(&b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800691 do_something_else();
692 }
693
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700694In contrast, without explicit memory barriers, two-legged-if control
695ordering is guaranteed only when the stores differ, for example:
696
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800697 q = READ_ONCE(a);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700698 if (q) {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700699 WRITE_ONCE(b, p);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700700 do_something();
701 } else {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700702 WRITE_ONCE(b, r);
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700703 do_something_else();
704 }
705
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800706The initial READ_ONCE() is still required to prevent the compiler from
707proving the value of 'a'.
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800708
709In addition, you need to be careful what you do with the local variable 'q',
710otherwise the compiler might be able to guess the value and again remove
711the needed conditional. For example:
712
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800713 q = READ_ONCE(a);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800714 if (q % MAX) {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700715 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800716 do_something();
717 } else {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700718 WRITE_ONCE(b, r);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800719 do_something_else();
720 }
721
722If MAX is defined to be 1, then the compiler knows that (q % MAX) is
723equal to zero, in which case the compiler is within its rights to
724transform the above code into the following:
725
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800726 q = READ_ONCE(a);
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700727 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800728 do_something_else();
729
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700730Given this transformation, the CPU is not required to respect the ordering
731between the load from variable 'a' and the store to variable 'b'. It is
732tempting to add a barrier(), but this does not help. The conditional
733is gone, and the barrier won't bring it back. Therefore, if you are
734relying on this ordering, you should make sure that MAX is greater than
735one, perhaps as follows:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800736
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800737 q = READ_ONCE(a);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800738 BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
739 if (q % MAX) {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700740 WRITE_ONCE(b, p);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800741 do_something();
742 } else {
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700743 WRITE_ONCE(b, r);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800744 do_something_else();
745 }
746
Paul E. McKenney2456d2a2014-08-13 15:40:02 -0700747Please note once again that the stores to 'b' differ. If they were
748identical, as noted earlier, the compiler could pull this store outside
749of the 'if' statement.
750
Paul E. McKenney8b19d1d2014-10-12 07:55:47 -0700751You must also be careful not to rely too much on boolean short-circuit
752evaluation. Consider this example:
753
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800754 q = READ_ONCE(a);
Paul E. McKenney57aecae2015-05-18 18:27:42 -0700755 if (q || 1 > 0)
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700756 WRITE_ONCE(b, 1);
Paul E. McKenney8b19d1d2014-10-12 07:55:47 -0700757
Paul E. McKenney5af46922015-04-25 12:48:29 -0700758Because the first condition cannot fault and the second condition is
759always true, the compiler can transform this example as following,
760defeating control dependency:
Paul E. McKenney8b19d1d2014-10-12 07:55:47 -0700761
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800762 q = READ_ONCE(a);
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700763 WRITE_ONCE(b, 1);
Paul E. McKenney8b19d1d2014-10-12 07:55:47 -0700764
765This example underscores the need to ensure that the compiler cannot
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700766out-guess your code. More generally, although READ_ONCE() does force
Paul E. McKenney8b19d1d2014-10-12 07:55:47 -0700767the compiler to actually emit code for a given load, it does not force
768the compiler to use the results.
769
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800770Finally, control dependencies do -not- provide transitivity. This is
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700771demonstrated by two related examples, with the initial values of
772x and y both being zero:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800773
774 CPU 0 CPU 1
Paul E. McKenney5af46922015-04-25 12:48:29 -0700775 ======================= =======================
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800776 r1 = READ_ONCE(x); r2 = READ_ONCE(y);
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700777 if (r1 > 0) if (r2 > 0)
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700778 WRITE_ONCE(y, 1); WRITE_ONCE(x, 1);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800779
780 assert(!(r1 == 1 && r2 == 1));
781
782The above two-CPU example will never trigger the assert(). However,
783if control dependencies guaranteed transitivity (which they do not),
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700784then adding the following CPU would guarantee a related assertion:
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800785
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700786 CPU 2
787 =====================
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700788 WRITE_ONCE(x, 2);
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800789
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700790 assert(!(r1 == 2 && r2 == 1 && x == 2)); /* FAILS!!! */
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800791
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700792But because control dependencies do -not- provide transitivity, the above
793assertion can fail after the combined three-CPU example completes. If you
794need the three-CPU example to provide ordering, you will need smp_mb()
795between the loads and stores in the CPU 0 and CPU 1 code fragments,
Paul E. McKenney5af46922015-04-25 12:48:29 -0700796that is, just before or just after the "if" statements. Furthermore,
797the original two-CPU example is very fragile and should be avoided.
Paul E. McKenney5646f7a2014-07-25 17:05:24 -0700798
799These two examples are the LB and WWC litmus tests from this paper:
800http://www.cl.cam.ac.uk/users/pes20/ppc-supplemental/test6.pdf and this
801site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800802
803In summary:
804
805 (*) Control dependencies can order prior loads against later stores.
806 However, they do -not- guarantee any other sort of ordering:
807 Not prior loads against later loads, nor prior stores against
808 later anything. If you need these other forms of ordering,
Davidlohr Buesod87510c2014-12-28 01:11:16 -0800809 use smp_rmb(), smp_wmb(), or, in the case of prior stores and
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800810 later loads, smp_mb().
811
Paul E. McKenney7817b792015-12-29 16:23:18 -0800812 (*) If both legs of the "if" statement begin with identical stores to
813 the same variable, then those stores must be ordered, either by
814 preceding both of them with smp_mb() or by using smp_store_release()
815 to carry out the stores. Please note that it is -not- sufficient
816 to use barrier() at beginning of each leg of the "if" statement,
817 as optimizing compilers do not necessarily respect barrier()
818 in this case.
Paul E. McKenney9b2b3bf2014-02-12 20:19:47 -0800819
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800820 (*) Control dependencies require at least one run-time conditional
Paul E. McKenney586dd562014-02-11 12:28:06 -0800821 between the prior load and the subsequent store, and this
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700822 conditional must involve the prior load. If the compiler is able
823 to optimize the conditional away, it will have also optimized
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800824 away the ordering. Careful use of READ_ONCE() and WRITE_ONCE()
825 can help to preserve the needed conditional.
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800826
827 (*) Control dependencies require that the compiler avoid reordering the
Linus Torvalds105ff3c2015-11-03 17:22:17 -0800828 dependency into nonexistence. Careful use of READ_ONCE() or
829 atomic{,64}_read() can help to preserve your control dependency.
Paul E. McKenney895f5542016-01-06 14:23:03 -0800830 Please see the COMPILER BARRIER section for more information.
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800831
Paul E. McKenneyff382812015-02-17 10:00:06 -0800832 (*) Control dependencies pair normally with other types of barriers.
833
Peter Zijlstra18c03c62013-12-11 13:59:06 -0800834 (*) Control dependencies do -not- provide transitivity. If you
835 need transitivity, use smp_mb().
David Howells108b42b2006-03-31 16:00:29 +0100836
837
838SMP BARRIER PAIRING
839-------------------
840
841When dealing with CPU-CPU interactions, certain types of memory barrier should
842always be paired. A lack of appropriate pairing is almost certainly an error.
843
Paul E. McKenneyff382812015-02-17 10:00:06 -0800844General barriers pair with each other, though they also pair with most
845other types of barriers, albeit without transitivity. An acquire barrier
846pairs with a release barrier, but both may also pair with other barriers,
847including of course general barriers. A write barrier pairs with a data
848dependency barrier, a control dependency, an acquire barrier, a release
849barrier, a read barrier, or a general barrier. Similarly a read barrier,
850control dependency, or a data dependency barrier pairs with a write
851barrier, an acquire barrier, a release barrier, or a general barrier:
David Howells108b42b2006-03-31 16:00:29 +0100852
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800853 CPU 1 CPU 2
854 =============== ===============
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700855 WRITE_ONCE(a, 1);
David Howells108b42b2006-03-31 16:00:29 +0100856 <write barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700857 WRITE_ONCE(b, 2); x = READ_ONCE(b);
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800858 <read barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700859 y = READ_ONCE(a);
David Howells108b42b2006-03-31 16:00:29 +0100860
861Or:
862
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800863 CPU 1 CPU 2
864 =============== ===============================
David Howells108b42b2006-03-31 16:00:29 +0100865 a = 1;
866 <write barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700867 WRITE_ONCE(b, &a); x = READ_ONCE(b);
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800868 <data dependency barrier>
869 y = *x;
David Howells108b42b2006-03-31 16:00:29 +0100870
Paul E. McKenneyff382812015-02-17 10:00:06 -0800871Or even:
872
873 CPU 1 CPU 2
874 =============== ===============================
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700875 r1 = READ_ONCE(y);
Paul E. McKenneyff382812015-02-17 10:00:06 -0800876 <general barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700877 WRITE_ONCE(y, 1); if (r2 = READ_ONCE(x)) {
Paul E. McKenneyff382812015-02-17 10:00:06 -0800878 <implicit control dependency>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700879 WRITE_ONCE(y, 1);
Paul E. McKenneyff382812015-02-17 10:00:06 -0800880 }
881
882 assert(r1 == 0 || r2 == 0);
883
David Howells108b42b2006-03-31 16:00:29 +0100884Basically, the read barrier always has to be there, even though it can be of
885the "weaker" type.
886
David Howells670bd952006-06-10 09:54:12 -0700887[!] Note that the stores before the write barrier would normally be expected to
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700888match the loads after the read barrier or the data dependency barrier, and vice
David Howells670bd952006-06-10 09:54:12 -0700889versa:
890
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800891 CPU 1 CPU 2
892 =================== ===================
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700893 WRITE_ONCE(a, 1); }---- --->{ v = READ_ONCE(c);
894 WRITE_ONCE(b, 2); } \ / { w = READ_ONCE(d);
Paul E. McKenney2ecf8102013-12-11 13:59:04 -0800895 <write barrier> \ <read barrier>
Paul E. McKenney9af194c2015-06-18 14:33:24 -0700896 WRITE_ONCE(c, 3); } / \ { x = READ_ONCE(a);
897 WRITE_ONCE(d, 4); }---- --->{ y = READ_ONCE(b);
David Howells670bd952006-06-10 09:54:12 -0700898
David Howells108b42b2006-03-31 16:00:29 +0100899
900EXAMPLES OF MEMORY BARRIER SEQUENCES
901------------------------------------
902
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700903Firstly, write barriers act as partial orderings on store operations.
David Howells108b42b2006-03-31 16:00:29 +0100904Consider the following sequence of events:
905
906 CPU 1
907 =======================
908 STORE A = 1
909 STORE B = 2
910 STORE C = 3
911 <write barrier>
912 STORE D = 4
913 STORE E = 5
914
915This sequence of events is committed to the memory coherence system in an order
916that the rest of the system might perceive as the unordered set of { STORE A,
Adrian Bunk80f72282006-06-30 18:27:16 +0200917STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
David Howells108b42b2006-03-31 16:00:29 +0100918}:
919
920 +-------+ : :
921 | | +------+
922 | |------>| C=3 | } /\
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700923 | | : +------+ }----- \ -----> Events perceptible to
924 | | : | A=1 | } \/ the rest of the system
David Howells108b42b2006-03-31 16:00:29 +0100925 | | : +------+ }
926 | CPU 1 | : | B=2 | }
927 | | +------+ }
928 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier
929 | | +------+ } requires all stores prior to the
930 | | : | E=5 | } barrier to be committed before
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700931 | | : +------+ } further stores may take place
David Howells108b42b2006-03-31 16:00:29 +0100932 | |------>| D=4 | }
933 | | +------+
934 +-------+ : :
935 |
David Howells670bd952006-06-10 09:54:12 -0700936 | Sequence in which stores are committed to the
937 | memory system by CPU 1
David Howells108b42b2006-03-31 16:00:29 +0100938 V
939
940
Jarek Poplawski81fc6322007-05-23 13:58:20 -0700941Secondly, data dependency barriers act as partial orderings on data-dependent
David Howells108b42b2006-03-31 16:00:29 +0100942loads. Consider the following sequence of events:
943
944 CPU 1 CPU 2
945 ======================= =======================
David Howellsc14038c2006-04-10 22:54:24 -0700946 { B = 7; X = 9; Y = 8; C = &Y }
David Howells108b42b2006-03-31 16:00:29 +0100947 STORE A = 1
948 STORE B = 2
949 <write barrier>
950 STORE C = &B LOAD X
951 STORE D = 4 LOAD C (gets &B)
952 LOAD *C (reads B)
953
954Without intervention, CPU 2 may perceive the events on CPU 1 in some
955effectively random order, despite the write barrier issued by CPU 1:
956
957 +-------+ : : : :
958 | | +------+ +-------+ | Sequence of update
959 | |------>| B=2 |----- --->| Y->8 | | of perception on
960 | | : +------+ \ +-------+ | CPU 2
961 | CPU 1 | : | A=1 | \ --->| C->&Y | V
962 | | +------+ | +-------+
963 | | wwwwwwwwwwwwwwww | : :
964 | | +------+ | : :
965 | | : | C=&B |--- | : : +-------+
966 | | : +------+ \ | +-------+ | |
967 | |------>| D=4 | ----------->| C->&B |------>| |
968 | | +------+ | +-------+ | |
969 +-------+ : : | : : | |
970 | : : | |
971 | : : | CPU 2 |
972 | +-------+ | |
973 Apparently incorrect ---> | | B->7 |------>| |
974 perception of B (!) | +-------+ | |
975 | : : | |
976 | +-------+ | |
977 The load of X holds ---> \ | X->9 |------>| |
978 up the maintenance \ +-------+ | |
979 of coherence of B ----->| B->2 | +-------+
980 +-------+
981 : :
982
983
984In the above example, CPU 2 perceives that B is 7, despite the load of *C
Paolo Ornati670e9f32006-10-03 22:57:56 +0200985(which would be B) coming after the LOAD of C.
David Howells108b42b2006-03-31 16:00:29 +0100986
987If, however, a data dependency barrier were to be placed between the load of C
David Howellsc14038c2006-04-10 22:54:24 -0700988and the load of *C (ie: B) on CPU 2:
989
990 CPU 1 CPU 2
991 ======================= =======================
992 { B = 7; X = 9; Y = 8; C = &Y }
993 STORE A = 1
994 STORE B = 2
995 <write barrier>
996 STORE C = &B LOAD X
997 STORE D = 4 LOAD C (gets &B)
998 <data dependency barrier>
999 LOAD *C (reads B)
1000
1001then the following will occur:
David Howells108b42b2006-03-31 16:00:29 +01001002
1003 +-------+ : : : :
1004 | | +------+ +-------+
1005 | |------>| B=2 |----- --->| Y->8 |
1006 | | : +------+ \ +-------+
1007 | CPU 1 | : | A=1 | \ --->| C->&Y |
1008 | | +------+ | +-------+
1009 | | wwwwwwwwwwwwwwww | : :
1010 | | +------+ | : :
1011 | | : | C=&B |--- | : : +-------+
1012 | | : +------+ \ | +-------+ | |
1013 | |------>| D=4 | ----------->| C->&B |------>| |
1014 | | +------+ | +-------+ | |
1015 +-------+ : : | : : | |
1016 | : : | |
1017 | : : | CPU 2 |
1018 | +-------+ | |
David Howells670bd952006-06-10 09:54:12 -07001019 | | X->9 |------>| |
1020 | +-------+ | |
1021 Makes sure all effects ---> \ ddddddddddddddddd | |
1022 prior to the store of C \ +-------+ | |
1023 are perceptible to ----->| B->2 |------>| |
1024 subsequent loads +-------+ | |
David Howells108b42b2006-03-31 16:00:29 +01001025 : : +-------+
1026
1027
1028And thirdly, a read barrier acts as a partial order on loads. Consider the
1029following sequence of events:
1030
1031 CPU 1 CPU 2
1032 ======================= =======================
David Howells670bd952006-06-10 09:54:12 -07001033 { A = 0, B = 9 }
David Howells108b42b2006-03-31 16:00:29 +01001034 STORE A=1
David Howells108b42b2006-03-31 16:00:29 +01001035 <write barrier>
David Howells670bd952006-06-10 09:54:12 -07001036 STORE B=2
David Howells108b42b2006-03-31 16:00:29 +01001037 LOAD B
David Howells670bd952006-06-10 09:54:12 -07001038 LOAD A
David Howells108b42b2006-03-31 16:00:29 +01001039
1040Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
1041some effectively random order, despite the write barrier issued by CPU 1:
1042
David Howells670bd952006-06-10 09:54:12 -07001043 +-------+ : : : :
1044 | | +------+ +-------+
1045 | |------>| A=1 |------ --->| A->0 |
1046 | | +------+ \ +-------+
1047 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1048 | | +------+ | +-------+
1049 | |------>| B=2 |--- | : :
1050 | | +------+ \ | : : +-------+
1051 +-------+ : : \ | +-------+ | |
1052 ---------->| B->2 |------>| |
1053 | +-------+ | CPU 2 |
1054 | | A->0 |------>| |
1055 | +-------+ | |
1056 | : : +-------+
1057 \ : :
1058 \ +-------+
1059 ---->| A->1 |
1060 +-------+
1061 : :
David Howells108b42b2006-03-31 16:00:29 +01001062
1063
David Howells6bc39272006-06-25 05:49:22 -07001064If, however, a read barrier were to be placed between the load of B and the
David Howells670bd952006-06-10 09:54:12 -07001065load of A on CPU 2:
David Howells108b42b2006-03-31 16:00:29 +01001066
David Howells670bd952006-06-10 09:54:12 -07001067 CPU 1 CPU 2
1068 ======================= =======================
1069 { A = 0, B = 9 }
1070 STORE A=1
1071 <write barrier>
1072 STORE B=2
1073 LOAD B
1074 <read barrier>
1075 LOAD A
1076
1077then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
10782:
1079
1080 +-------+ : : : :
1081 | | +------+ +-------+
1082 | |------>| A=1 |------ --->| A->0 |
1083 | | +------+ \ +-------+
1084 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1085 | | +------+ | +-------+
1086 | |------>| B=2 |--- | : :
1087 | | +------+ \ | : : +-------+
1088 +-------+ : : \ | +-------+ | |
1089 ---------->| B->2 |------>| |
1090 | +-------+ | CPU 2 |
1091 | : : | |
1092 | : : | |
1093 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1094 barrier causes all effects \ +-------+ | |
1095 prior to the storage of B ---->| A->1 |------>| |
1096 to be perceptible to CPU 2 +-------+ | |
1097 : : +-------+
1098
1099
1100To illustrate this more completely, consider what could happen if the code
1101contained a load of A either side of the read barrier:
1102
1103 CPU 1 CPU 2
1104 ======================= =======================
1105 { A = 0, B = 9 }
1106 STORE A=1
1107 <write barrier>
1108 STORE B=2
1109 LOAD B
1110 LOAD A [first load of A]
1111 <read barrier>
1112 LOAD A [second load of A]
1113
1114Even though the two loads of A both occur after the load of B, they may both
1115come up with different values:
1116
1117 +-------+ : : : :
1118 | | +------+ +-------+
1119 | |------>| A=1 |------ --->| A->0 |
1120 | | +------+ \ +-------+
1121 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1122 | | +------+ | +-------+
1123 | |------>| B=2 |--- | : :
1124 | | +------+ \ | : : +-------+
1125 +-------+ : : \ | +-------+ | |
1126 ---------->| B->2 |------>| |
1127 | +-------+ | CPU 2 |
1128 | : : | |
1129 | : : | |
1130 | +-------+ | |
1131 | | A->0 |------>| 1st |
1132 | +-------+ | |
1133 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1134 barrier causes all effects \ +-------+ | |
1135 prior to the storage of B ---->| A->1 |------>| 2nd |
1136 to be perceptible to CPU 2 +-------+ | |
1137 : : +-------+
1138
1139
1140But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1141before the read barrier completes anyway:
1142
1143 +-------+ : : : :
1144 | | +------+ +-------+
1145 | |------>| A=1 |------ --->| A->0 |
1146 | | +------+ \ +-------+
1147 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1148 | | +------+ | +-------+
1149 | |------>| B=2 |--- | : :
1150 | | +------+ \ | : : +-------+
1151 +-------+ : : \ | +-------+ | |
1152 ---------->| B->2 |------>| |
1153 | +-------+ | CPU 2 |
1154 | : : | |
1155 \ : : | |
1156 \ +-------+ | |
1157 ---->| A->1 |------>| 1st |
1158 +-------+ | |
1159 rrrrrrrrrrrrrrrrr | |
1160 +-------+ | |
1161 | A->1 |------>| 2nd |
1162 +-------+ | |
1163 : : +-------+
1164
1165
1166The guarantee is that the second load will always come up with A == 1 if the
1167load of B came up with B == 2. No such guarantee exists for the first load of
1168A; that may come up with either A == 0 or A == 1.
1169
1170
1171READ MEMORY BARRIERS VS LOAD SPECULATION
1172----------------------------------------
1173
1174Many CPUs speculate with loads: that is they see that they will need to load an
1175item from memory, and they find a time where they're not using the bus for any
1176other loads, and so do the load in advance - even though they haven't actually
1177got to that point in the instruction execution flow yet. This permits the
1178actual load instruction to potentially complete immediately because the CPU
1179already has the value to hand.
1180
1181It may turn out that the CPU didn't actually need the value - perhaps because a
1182branch circumvented the load - in which case it can discard the value or just
1183cache it for later use.
1184
1185Consider:
1186
Ingo Molnare0edc782013-11-22 11:24:53 +01001187 CPU 1 CPU 2
David Howells670bd952006-06-10 09:54:12 -07001188 ======================= =======================
Ingo Molnare0edc782013-11-22 11:24:53 +01001189 LOAD B
1190 DIVIDE } Divide instructions generally
1191 DIVIDE } take a long time to perform
1192 LOAD A
David Howells670bd952006-06-10 09:54:12 -07001193
1194Which might appear as this:
1195
1196 : : +-------+
1197 +-------+ | |
1198 --->| B->2 |------>| |
1199 +-------+ | CPU 2 |
1200 : :DIVIDE | |
1201 +-------+ | |
1202 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1203 division speculates on the +-------+ ~ | |
1204 LOAD of A : : ~ | |
1205 : :DIVIDE | |
1206 : : ~ | |
1207 Once the divisions are complete --> : : ~-->| |
1208 the CPU can then perform the : : | |
1209 LOAD with immediate effect : : +-------+
1210
1211
1212Placing a read barrier or a data dependency barrier just before the second
1213load:
1214
Ingo Molnare0edc782013-11-22 11:24:53 +01001215 CPU 1 CPU 2
David Howells670bd952006-06-10 09:54:12 -07001216 ======================= =======================
Ingo Molnare0edc782013-11-22 11:24:53 +01001217 LOAD B
1218 DIVIDE
1219 DIVIDE
David Howells670bd952006-06-10 09:54:12 -07001220 <read barrier>
Ingo Molnare0edc782013-11-22 11:24:53 +01001221 LOAD A
David Howells670bd952006-06-10 09:54:12 -07001222
1223will force any value speculatively obtained to be reconsidered to an extent
1224dependent on the type of barrier used. If there was no change made to the
1225speculated memory location, then the speculated value will just be used:
1226
1227 : : +-------+
1228 +-------+ | |
1229 --->| B->2 |------>| |
1230 +-------+ | CPU 2 |
1231 : :DIVIDE | |
1232 +-------+ | |
1233 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1234 division speculates on the +-------+ ~ | |
1235 LOAD of A : : ~ | |
1236 : :DIVIDE | |
1237 : : ~ | |
1238 : : ~ | |
1239 rrrrrrrrrrrrrrrr~ | |
1240 : : ~ | |
1241 : : ~-->| |
1242 : : | |
1243 : : +-------+
1244
1245
1246but if there was an update or an invalidation from another CPU pending, then
1247the speculation will be cancelled and the value reloaded:
1248
1249 : : +-------+
1250 +-------+ | |
1251 --->| B->2 |------>| |
1252 +-------+ | CPU 2 |
1253 : :DIVIDE | |
1254 +-------+ | |
1255 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1256 division speculates on the +-------+ ~ | |
1257 LOAD of A : : ~ | |
1258 : :DIVIDE | |
1259 : : ~ | |
1260 : : ~ | |
1261 rrrrrrrrrrrrrrrrr | |
1262 +-------+ | |
1263 The speculation is discarded ---> --->| A->1 |------>| |
1264 and an updated value is +-------+ | |
1265 retrieved : : +-------+
David Howells108b42b2006-03-31 16:00:29 +01001266
1267
Paul E. McKenney241e6662011-02-10 16:54:50 -08001268TRANSITIVITY
1269------------
1270
1271Transitivity is a deeply intuitive notion about ordering that is not
1272always provided by real computer systems. The following example
Paul E. McKenneyf36fe1e2016-02-15 14:50:36 -08001273demonstrates transitivity:
Paul E. McKenney241e6662011-02-10 16:54:50 -08001274
1275 CPU 1 CPU 2 CPU 3
1276 ======================= ======================= =======================
1277 { X = 0, Y = 0 }
1278 STORE X=1 LOAD X STORE Y=1
1279 <general barrier> <general barrier>
1280 LOAD Y LOAD X
1281
1282Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
1283This indicates that CPU 2's load from X in some sense follows CPU 1's
1284store to X and that CPU 2's load from Y in some sense preceded CPU 3's
1285store to Y. The question is then "Can CPU 3's load from X return 0?"
1286
1287Because CPU 2's load from X in some sense came after CPU 1's store, it
1288is natural to expect that CPU 3's load from X must therefore return 1.
1289This expectation is an example of transitivity: if a load executing on
1290CPU A follows a load from the same variable executing on CPU B, then
1291CPU A's load must either return the same value that CPU B's load did,
1292or must return some later value.
1293
1294In the Linux kernel, use of general memory barriers guarantees
1295transitivity. Therefore, in the above example, if CPU 2's load from X
1296returns 1 and its load from Y returns 0, then CPU 3's load from X must
1297also return 1.
1298
1299However, transitivity is -not- guaranteed for read or write barriers.
1300For example, suppose that CPU 2's general barrier in the above example
1301is changed to a read barrier as shown below:
1302
1303 CPU 1 CPU 2 CPU 3
1304 ======================= ======================= =======================
1305 { X = 0, Y = 0 }
1306 STORE X=1 LOAD X STORE Y=1
1307 <read barrier> <general barrier>
1308 LOAD Y LOAD X
1309
1310This substitution destroys transitivity: in this example, it is perfectly
1311legal for CPU 2's load from X to return 1, its load from Y to return 0,
1312and CPU 3's load from X to return 0.
1313
1314The key point is that although CPU 2's read barrier orders its pair
1315of loads, it does not guarantee to order CPU 1's store. Therefore, if
1316this example runs on a system where CPUs 1 and 2 share a store buffer
1317or a level of cache, CPU 2 might have early access to CPU 1's writes.
1318General barriers are therefore required to ensure that all CPUs agree
1319on the combined order of CPU 1's and CPU 2's accesses.
1320
Paul E. McKenneyc535cc92016-01-15 09:30:42 -08001321General barriers provide "global transitivity", so that all CPUs will
1322agree on the order of operations. In contrast, a chain of release-acquire
1323pairs provides only "local transitivity", so that only those CPUs on
1324the chain are guaranteed to agree on the combined order of the accesses.
1325For example, switching to C code in deference to Herman Hollerith:
1326
1327 int u, v, x, y, z;
1328
1329 void cpu0(void)
1330 {
1331 r0 = smp_load_acquire(&x);
1332 WRITE_ONCE(u, 1);
1333 smp_store_release(&y, 1);
1334 }
1335
1336 void cpu1(void)
1337 {
1338 r1 = smp_load_acquire(&y);
1339 r4 = READ_ONCE(v);
1340 r5 = READ_ONCE(u);
1341 smp_store_release(&z, 1);
1342 }
1343
1344 void cpu2(void)
1345 {
1346 r2 = smp_load_acquire(&z);
1347 smp_store_release(&x, 1);
1348 }
1349
1350 void cpu3(void)
1351 {
1352 WRITE_ONCE(v, 1);
1353 smp_mb();
1354 r3 = READ_ONCE(u);
1355 }
1356
1357Because cpu0(), cpu1(), and cpu2() participate in a local transitive
1358chain of smp_store_release()/smp_load_acquire() pairs, the following
1359outcome is prohibited:
1360
1361 r0 == 1 && r1 == 1 && r2 == 1
1362
1363Furthermore, because of the release-acquire relationship between cpu0()
1364and cpu1(), cpu1() must see cpu0()'s writes, so that the following
1365outcome is prohibited:
1366
1367 r1 == 1 && r5 == 0
1368
1369However, the transitivity of release-acquire is local to the participating
1370CPUs and does not apply to cpu3(). Therefore, the following outcome
1371is possible:
1372
1373 r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
1374
Paul E. McKenney37ef0342016-01-25 22:12:34 -08001375As an aside, the following outcome is also possible:
1376
1377 r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
1378
Paul E. McKenneyc535cc92016-01-15 09:30:42 -08001379Although cpu0(), cpu1(), and cpu2() will see their respective reads and
1380writes in order, CPUs not involved in the release-acquire chain might
1381well disagree on the order. This disagreement stems from the fact that
1382the weak memory-barrier instructions used to implement smp_load_acquire()
1383and smp_store_release() are not required to order prior stores against
1384subsequent loads in all cases. This means that cpu3() can see cpu0()'s
1385store to u as happening -after- cpu1()'s load from v, even though
1386both cpu0() and cpu1() agree that these two operations occurred in the
1387intended order.
1388
1389However, please keep in mind that smp_load_acquire() is not magic.
1390In particular, it simply reads from its argument with ordering. It does
1391-not- ensure that any particular value will be read. Therefore, the
1392following outcome is possible:
1393
1394 r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
1395
1396Note that this outcome can happen even on a mythical sequentially
1397consistent system where nothing is ever reordered.
1398
1399To reiterate, if your code requires global transitivity, use general
1400barriers throughout.
Paul E. McKenney241e6662011-02-10 16:54:50 -08001401
1402
David Howells108b42b2006-03-31 16:00:29 +01001403========================
1404EXPLICIT KERNEL BARRIERS
1405========================
1406
1407The Linux kernel has a variety of different barriers that act at different
1408levels:
1409
1410 (*) Compiler barrier.
1411
1412 (*) CPU memory barriers.
1413
1414 (*) MMIO write barrier.
1415
1416
1417COMPILER BARRIER
1418----------------
1419
1420The Linux kernel has an explicit compiler barrier function that prevents the
1421compiler from moving the memory accesses either side of it to the other side:
1422
1423 barrier();
1424
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001425This is a general barrier -- there are no read-read or write-write
1426variants of barrier(). However, READ_ONCE() and WRITE_ONCE() can be
1427thought of as weak forms of barrier() that affect only the specific
1428accesses flagged by the READ_ONCE() or WRITE_ONCE().
David Howells108b42b2006-03-31 16:00:29 +01001429
Paul E. McKenney692118d2013-12-11 13:59:07 -08001430The barrier() function has the following effects:
1431
1432 (*) Prevents the compiler from reordering accesses following the
1433 barrier() to precede any accesses preceding the barrier().
1434 One example use for this property is to ease communication between
1435 interrupt-handler code and the code that was interrupted.
1436
1437 (*) Within a loop, forces the compiler to load the variables used
1438 in that loop's conditional on each pass through that loop.
1439
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001440The READ_ONCE() and WRITE_ONCE() functions can prevent any number of
1441optimizations that, while perfectly safe in single-threaded code, can
1442be fatal in concurrent code. Here are some examples of these sorts
1443of optimizations:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001444
Paul E. McKenney449f7412014-01-02 15:03:50 -08001445 (*) The compiler is within its rights to reorder loads and stores
1446 to the same variable, and in some cases, the CPU is within its
1447 rights to reorder loads to the same variable. This means that
1448 the following code:
1449
1450 a[0] = x;
1451 a[1] = x;
1452
1453 Might result in an older value of x stored in a[1] than in a[0].
1454 Prevent both the compiler and the CPU from doing this as follows:
1455
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001456 a[0] = READ_ONCE(x);
1457 a[1] = READ_ONCE(x);
Paul E. McKenney449f7412014-01-02 15:03:50 -08001458
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001459 In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
1460 accesses from multiple CPUs to a single variable.
Paul E. McKenney449f7412014-01-02 15:03:50 -08001461
Paul E. McKenney692118d2013-12-11 13:59:07 -08001462 (*) The compiler is within its rights to merge successive loads from
1463 the same variable. Such merging can cause the compiler to "optimize"
1464 the following code:
1465
1466 while (tmp = a)
1467 do_something_with(tmp);
1468
1469 into the following code, which, although in some sense legitimate
1470 for single-threaded code, is almost certainly not what the developer
1471 intended:
1472
1473 if (tmp = a)
1474 for (;;)
1475 do_something_with(tmp);
1476
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001477 Use READ_ONCE() to prevent the compiler from doing this to you:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001478
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001479 while (tmp = READ_ONCE(a))
Paul E. McKenney692118d2013-12-11 13:59:07 -08001480 do_something_with(tmp);
1481
1482 (*) The compiler is within its rights to reload a variable, for example,
1483 in cases where high register pressure prevents the compiler from
1484 keeping all data of interest in registers. The compiler might
1485 therefore optimize the variable 'tmp' out of our previous example:
1486
1487 while (tmp = a)
1488 do_something_with(tmp);
1489
1490 This could result in the following code, which is perfectly safe in
1491 single-threaded code, but can be fatal in concurrent code:
1492
1493 while (a)
1494 do_something_with(a);
1495
1496 For example, the optimized version of this code could result in
1497 passing a zero to do_something_with() in the case where the variable
1498 a was modified by some other CPU between the "while" statement and
1499 the call to do_something_with().
1500
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001501 Again, use READ_ONCE() to prevent the compiler from doing this:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001502
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001503 while (tmp = READ_ONCE(a))
Paul E. McKenney692118d2013-12-11 13:59:07 -08001504 do_something_with(tmp);
1505
1506 Note that if the compiler runs short of registers, it might save
1507 tmp onto the stack. The overhead of this saving and later restoring
1508 is why compilers reload variables. Doing so is perfectly safe for
1509 single-threaded code, so you need to tell the compiler about cases
1510 where it is not safe.
1511
1512 (*) The compiler is within its rights to omit a load entirely if it knows
1513 what the value will be. For example, if the compiler can prove that
1514 the value of variable 'a' is always zero, it can optimize this code:
1515
1516 while (tmp = a)
1517 do_something_with(tmp);
1518
1519 Into this:
1520
1521 do { } while (0);
1522
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001523 This transformation is a win for single-threaded code because it
1524 gets rid of a load and a branch. The problem is that the compiler
1525 will carry out its proof assuming that the current CPU is the only
1526 one updating variable 'a'. If variable 'a' is shared, then the
1527 compiler's proof will be erroneous. Use READ_ONCE() to tell the
1528 compiler that it doesn't know as much as it thinks it does:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001529
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001530 while (tmp = READ_ONCE(a))
Paul E. McKenney692118d2013-12-11 13:59:07 -08001531 do_something_with(tmp);
1532
1533 But please note that the compiler is also closely watching what you
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001534 do with the value after the READ_ONCE(). For example, suppose you
Paul E. McKenney692118d2013-12-11 13:59:07 -08001535 do the following and MAX is a preprocessor macro with the value 1:
1536
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001537 while ((tmp = READ_ONCE(a)) % MAX)
Paul E. McKenney692118d2013-12-11 13:59:07 -08001538 do_something_with(tmp);
1539
1540 Then the compiler knows that the result of the "%" operator applied
1541 to MAX will always be zero, again allowing the compiler to optimize
1542 the code into near-nonexistence. (It will still load from the
1543 variable 'a'.)
1544
1545 (*) Similarly, the compiler is within its rights to omit a store entirely
1546 if it knows that the variable already has the value being stored.
1547 Again, the compiler assumes that the current CPU is the only one
1548 storing into the variable, which can cause the compiler to do the
1549 wrong thing for shared variables. For example, suppose you have
1550 the following:
1551
1552 a = 0;
1553 /* Code that does not store to variable a. */
1554 a = 0;
1555
1556 The compiler sees that the value of variable 'a' is already zero, so
1557 it might well omit the second store. This would come as a fatal
1558 surprise if some other CPU might have stored to variable 'a' in the
1559 meantime.
1560
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001561 Use WRITE_ONCE() to prevent the compiler from making this sort of
Paul E. McKenney692118d2013-12-11 13:59:07 -08001562 wrong guess:
1563
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001564 WRITE_ONCE(a, 0);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001565 /* Code that does not store to variable a. */
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001566 WRITE_ONCE(a, 0);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001567
1568 (*) The compiler is within its rights to reorder memory accesses unless
1569 you tell it not to. For example, consider the following interaction
1570 between process-level code and an interrupt handler:
1571
1572 void process_level(void)
1573 {
1574 msg = get_message();
1575 flag = true;
1576 }
1577
1578 void interrupt_handler(void)
1579 {
1580 if (flag)
1581 process_message(msg);
1582 }
1583
Masanari Iidadf5cbb22014-03-21 10:04:30 +09001584 There is nothing to prevent the compiler from transforming
Paul E. McKenney692118d2013-12-11 13:59:07 -08001585 process_level() to the following, in fact, this might well be a
1586 win for single-threaded code:
1587
1588 void process_level(void)
1589 {
1590 flag = true;
1591 msg = get_message();
1592 }
1593
1594 If the interrupt occurs between these two statement, then
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001595 interrupt_handler() might be passed a garbled msg. Use WRITE_ONCE()
Paul E. McKenney692118d2013-12-11 13:59:07 -08001596 to prevent this as follows:
1597
1598 void process_level(void)
1599 {
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001600 WRITE_ONCE(msg, get_message());
1601 WRITE_ONCE(flag, true);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001602 }
1603
1604 void interrupt_handler(void)
1605 {
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001606 if (READ_ONCE(flag))
1607 process_message(READ_ONCE(msg));
Paul E. McKenney692118d2013-12-11 13:59:07 -08001608 }
1609
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001610 Note that the READ_ONCE() and WRITE_ONCE() wrappers in
1611 interrupt_handler() are needed if this interrupt handler can itself
1612 be interrupted by something that also accesses 'flag' and 'msg',
1613 for example, a nested interrupt or an NMI. Otherwise, READ_ONCE()
1614 and WRITE_ONCE() are not needed in interrupt_handler() other than
1615 for documentation purposes. (Note also that nested interrupts
1616 do not typically occur in modern Linux kernels, in fact, if an
1617 interrupt handler returns with interrupts enabled, you will get a
1618 WARN_ONCE() splat.)
Paul E. McKenney692118d2013-12-11 13:59:07 -08001619
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001620 You should assume that the compiler can move READ_ONCE() and
1621 WRITE_ONCE() past code not containing READ_ONCE(), WRITE_ONCE(),
1622 barrier(), or similar primitives.
Paul E. McKenney692118d2013-12-11 13:59:07 -08001623
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001624 This effect could also be achieved using barrier(), but READ_ONCE()
1625 and WRITE_ONCE() are more selective: With READ_ONCE() and
1626 WRITE_ONCE(), the compiler need only forget the contents of the
1627 indicated memory locations, while with barrier() the compiler must
1628 discard the value of all memory locations that it has currented
1629 cached in any machine registers. Of course, the compiler must also
1630 respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
1631 though the CPU of course need not do so.
Paul E. McKenney692118d2013-12-11 13:59:07 -08001632
1633 (*) The compiler is within its rights to invent stores to a variable,
1634 as in the following example:
1635
1636 if (a)
1637 b = a;
1638 else
1639 b = 42;
1640
1641 The compiler might save a branch by optimizing this as follows:
1642
1643 b = 42;
1644 if (a)
1645 b = a;
1646
1647 In single-threaded code, this is not only safe, but also saves
1648 a branch. Unfortunately, in concurrent code, this optimization
1649 could cause some other CPU to see a spurious value of 42 -- even
1650 if variable 'a' was never zero -- when loading variable 'b'.
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001651 Use WRITE_ONCE() to prevent this as follows:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001652
1653 if (a)
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001654 WRITE_ONCE(b, a);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001655 else
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001656 WRITE_ONCE(b, 42);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001657
1658 The compiler can also invent loads. These are usually less
1659 damaging, but they can result in cache-line bouncing and thus in
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001660 poor performance and scalability. Use READ_ONCE() to prevent
Paul E. McKenney692118d2013-12-11 13:59:07 -08001661 invented loads.
1662
1663 (*) For aligned memory locations whose size allows them to be accessed
1664 with a single memory-reference instruction, prevents "load tearing"
1665 and "store tearing," in which a single large access is replaced by
1666 multiple smaller accesses. For example, given an architecture having
1667 16-bit store instructions with 7-bit immediate fields, the compiler
1668 might be tempted to use two 16-bit store-immediate instructions to
1669 implement the following 32-bit store:
1670
1671 p = 0x00010002;
1672
1673 Please note that GCC really does use this sort of optimization,
1674 which is not surprising given that it would likely take more
1675 than two instructions to build the constant and then store it.
1676 This optimization can therefore be a win in single-threaded code.
1677 In fact, a recent bug (since fixed) caused GCC to incorrectly use
1678 this optimization in a volatile store. In the absence of such bugs,
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001679 use of WRITE_ONCE() prevents store tearing in the following example:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001680
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001681 WRITE_ONCE(p, 0x00010002);
Paul E. McKenney692118d2013-12-11 13:59:07 -08001682
1683 Use of packed structures can also result in load and store tearing,
1684 as in this example:
1685
1686 struct __attribute__((__packed__)) foo {
1687 short a;
1688 int b;
1689 short c;
1690 };
1691 struct foo foo1, foo2;
1692 ...
1693
1694 foo2.a = foo1.a;
1695 foo2.b = foo1.b;
1696 foo2.c = foo1.c;
1697
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001698 Because there are no READ_ONCE() or WRITE_ONCE() wrappers and no
1699 volatile markings, the compiler would be well within its rights to
1700 implement these three assignment statements as a pair of 32-bit
1701 loads followed by a pair of 32-bit stores. This would result in
1702 load tearing on 'foo1.b' and store tearing on 'foo2.b'. READ_ONCE()
1703 and WRITE_ONCE() again prevent tearing in this example:
Paul E. McKenney692118d2013-12-11 13:59:07 -08001704
1705 foo2.a = foo1.a;
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001706 WRITE_ONCE(foo2.b, READ_ONCE(foo1.b));
Paul E. McKenney692118d2013-12-11 13:59:07 -08001707 foo2.c = foo1.c;
1708
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001709All that aside, it is never necessary to use READ_ONCE() and
1710WRITE_ONCE() on a variable that has been marked volatile. For example,
1711because 'jiffies' is marked volatile, it is never necessary to
1712say READ_ONCE(jiffies). The reason for this is that READ_ONCE() and
1713WRITE_ONCE() are implemented as volatile casts, which has no effect when
1714its argument is already marked volatile.
Paul E. McKenney692118d2013-12-11 13:59:07 -08001715
1716Please note that these compiler barriers have no direct effect on the CPU,
1717which may then reorder things however it wishes.
David Howells108b42b2006-03-31 16:00:29 +01001718
1719
1720CPU MEMORY BARRIERS
1721-------------------
1722
1723The Linux kernel has eight basic CPU memory barriers:
1724
1725 TYPE MANDATORY SMP CONDITIONAL
1726 =============== ======================= ===========================
1727 GENERAL mb() smp_mb()
1728 WRITE wmb() smp_wmb()
1729 READ rmb() smp_rmb()
1730 DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends()
1731
1732
Nick Piggin73f10282008-05-14 06:35:11 +02001733All memory barriers except the data dependency barriers imply a compiler
1734barrier. Data dependencies do not impose any additional compiler ordering.
1735
Paul E. McKenney9af194c2015-06-18 14:33:24 -07001736Aside: In the case of data dependencies, the compiler would be expected
1737to issue the loads in the correct order (eg. `a[b]` would have to load
1738the value of b before loading a[b]), however there is no guarantee in
1739the C specification that the compiler may not speculate the value of b
1740(eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1)
1741tmp = a[b]; ). There is also the problem of a compiler reloading b after
1742having loaded a[b], thus having a newer copy of b than a[b]. A consensus
1743has not yet been reached about these problems, however the READ_ONCE()
1744macro is a good place to start looking.
David Howells108b42b2006-03-31 16:00:29 +01001745
1746SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
Jarek Poplawski81fc6322007-05-23 13:58:20 -07001747systems because it is assumed that a CPU will appear to be self-consistent,
David Howells108b42b2006-03-31 16:00:29 +01001748and will order overlapping accesses correctly with respect to itself.
Michael S. Tsirkin6a65d262015-12-27 18:23:01 +02001749However, see the subsection on "Virtual Machine Guests" below.
David Howells108b42b2006-03-31 16:00:29 +01001750
1751[!] Note that SMP memory barriers _must_ be used to control the ordering of
1752references to shared memory on SMP systems, though the use of locking instead
1753is sufficient.
1754
1755Mandatory barriers should not be used to control SMP effects, since mandatory
Michael S. Tsirkin6a65d262015-12-27 18:23:01 +02001756barriers impose unnecessary overhead on both SMP and UP systems. They may,
1757however, be used to control MMIO effects on accesses through relaxed memory I/O
1758windows. These barriers are required even on non-SMP systems as they affect
1759the order in which memory operations appear to a device by prohibiting both the
1760compiler and the CPU from reordering them.
David Howells108b42b2006-03-31 16:00:29 +01001761
1762
1763There are some more advanced barrier functions:
1764
Peter Zijlstrab92b8b32015-05-12 10:51:55 +02001765 (*) smp_store_mb(var, value)
David Howells108b42b2006-03-31 16:00:29 +01001766
Oleg Nesterov75b2bd52006-11-08 17:44:38 -08001767 This assigns the value to the variable and then inserts a full memory
Davidlohr Bueso2d142e52015-10-27 12:53:51 -07001768 barrier after it. It isn't guaranteed to insert anything more than a
1769 compiler barrier in a UP compilation.
David Howells108b42b2006-03-31 16:00:29 +01001770
1771
Peter Zijlstra1b156112014-03-13 19:00:35 +01001772 (*) smp_mb__before_atomic();
1773 (*) smp_mb__after_atomic();
David Howells108b42b2006-03-31 16:00:29 +01001774
Peter Zijlstra1b156112014-03-13 19:00:35 +01001775 These are for use with atomic (such as add, subtract, increment and
1776 decrement) functions that don't return a value, especially when used for
1777 reference counting. These functions do not imply memory barriers.
1778
1779 These are also used for atomic bitop functions that do not return a
1780 value (such as set_bit and clear_bit).
David Howells108b42b2006-03-31 16:00:29 +01001781
1782 As an example, consider a piece of code that marks an object as being dead
1783 and then decrements the object's reference count:
1784
1785 obj->dead = 1;
Peter Zijlstra1b156112014-03-13 19:00:35 +01001786 smp_mb__before_atomic();
David Howells108b42b2006-03-31 16:00:29 +01001787 atomic_dec(&obj->ref_count);
1788
1789 This makes sure that the death mark on the object is perceived to be set
1790 *before* the reference counter is decremented.
1791
1792 See Documentation/atomic_ops.txt for more information. See the "Atomic
1793 operations" subsection for information on where to use these.
1794
1795
Paul E. McKenneyad2ad5d2015-09-17 08:18:32 -07001796 (*) lockless_dereference();
1797 This can be thought of as a pointer-fetch wrapper around the
1798 smp_read_barrier_depends() data-dependency barrier.
1799
1800 This is also similar to rcu_dereference(), but in cases where
1801 object lifetime is handled by some mechanism other than RCU, for
1802 example, when the objects removed only when the system goes down.
1803 In addition, lockless_dereference() is used in some data structures
1804 that can be used both with and without RCU.
1805
1806
Alexander Duyck1077fa32014-12-11 15:02:06 -08001807 (*) dma_wmb();
1808 (*) dma_rmb();
1809
1810 These are for use with consistent memory to guarantee the ordering
1811 of writes or reads of shared memory accessible to both the CPU and a
1812 DMA capable device.
1813
1814 For example, consider a device driver that shares memory with a device
1815 and uses a descriptor status value to indicate if the descriptor belongs
1816 to the device or the CPU, and a doorbell to notify it when new
1817 descriptors are available:
1818
1819 if (desc->status != DEVICE_OWN) {
1820 /* do not read data until we own descriptor */
1821 dma_rmb();
1822
1823 /* read/modify data */
1824 read_data = desc->data;
1825 desc->data = write_data;
1826
1827 /* flush modifications before status update */
1828 dma_wmb();
1829
1830 /* assign ownership */
1831 desc->status = DEVICE_OWN;
1832
1833 /* force memory to sync before notifying device via MMIO */
1834 wmb();
1835
1836 /* notify device of new descriptors */
1837 writel(DESC_NOTIFY, doorbell);
1838 }
1839
1840 The dma_rmb() allows us guarantee the device has released ownership
Sylvain Trias7a458002015-04-08 10:27:57 +02001841 before we read the data from the descriptor, and the dma_wmb() allows
Alexander Duyck1077fa32014-12-11 15:02:06 -08001842 us to guarantee the data is written to the descriptor before the device
1843 can see it now has ownership. The wmb() is needed to guarantee that the
1844 cache coherent memory writes have completed before attempting a write to
1845 the cache incoherent MMIO region.
1846
1847 See Documentation/DMA-API.txt for more information on consistent memory.
1848
David Howells108b42b2006-03-31 16:00:29 +01001849MMIO WRITE BARRIER
1850------------------
1851
1852The Linux kernel also has a special barrier for use with memory-mapped I/O
1853writes:
1854
1855 mmiowb();
1856
1857This is a variation on the mandatory write barrier that causes writes to weakly
1858ordered I/O regions to be partially ordered. Its effects may go beyond the
1859CPU->Hardware interface and actually affect the hardware at some level.
1860
1861See the subsection "Locks vs I/O accesses" for more information.
1862
1863
1864===============================
1865IMPLICIT KERNEL MEMORY BARRIERS
1866===============================
1867
1868Some of the other functions in the linux kernel imply memory barriers, amongst
David Howells670bd952006-06-10 09:54:12 -07001869which are locking and scheduling functions.
David Howells108b42b2006-03-31 16:00:29 +01001870
1871This specification is a _minimum_ guarantee; any particular architecture may
1872provide more substantial guarantees, but these may not be relied upon outside
1873of arch specific code.
1874
1875
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001876ACQUIRING FUNCTIONS
1877-------------------
David Howells108b42b2006-03-31 16:00:29 +01001878
1879The Linux kernel has a number of locking constructs:
1880
1881 (*) spin locks
1882 (*) R/W spin locks
1883 (*) mutexes
1884 (*) semaphores
1885 (*) R/W semaphores
David Howells108b42b2006-03-31 16:00:29 +01001886
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001887In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
David Howells108b42b2006-03-31 16:00:29 +01001888for each construct. These operations all imply certain barriers:
1889
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001890 (1) ACQUIRE operation implication:
David Howells108b42b2006-03-31 16:00:29 +01001891
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001892 Memory operations issued after the ACQUIRE will be completed after the
1893 ACQUIRE operation has completed.
David Howells108b42b2006-03-31 16:00:29 +01001894
Paul E. McKenney8dd853d2014-02-23 08:34:24 -08001895 Memory operations issued before the ACQUIRE may be completed after
1896 the ACQUIRE operation has completed. An smp_mb__before_spinlock(),
Will Deacond9560282015-03-31 09:39:41 +01001897 combined with a following ACQUIRE, orders prior stores against
1898 subsequent loads and stores. Note that this is weaker than smp_mb()!
1899 The smp_mb__before_spinlock() primitive is free on many architectures.
David Howells108b42b2006-03-31 16:00:29 +01001900
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001901 (2) RELEASE operation implication:
David Howells108b42b2006-03-31 16:00:29 +01001902
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001903 Memory operations issued before the RELEASE will be completed before the
1904 RELEASE operation has completed.
David Howells108b42b2006-03-31 16:00:29 +01001905
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001906 Memory operations issued after the RELEASE may be completed before the
1907 RELEASE operation has completed.
David Howells108b42b2006-03-31 16:00:29 +01001908
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001909 (3) ACQUIRE vs ACQUIRE implication:
David Howells108b42b2006-03-31 16:00:29 +01001910
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001911 All ACQUIRE operations issued before another ACQUIRE operation will be
1912 completed before that ACQUIRE operation.
David Howells108b42b2006-03-31 16:00:29 +01001913
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001914 (4) ACQUIRE vs RELEASE implication:
David Howells108b42b2006-03-31 16:00:29 +01001915
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001916 All ACQUIRE operations issued before a RELEASE operation will be
1917 completed before the RELEASE operation.
David Howells108b42b2006-03-31 16:00:29 +01001918
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001919 (5) Failed conditional ACQUIRE implication:
David Howells108b42b2006-03-31 16:00:29 +01001920
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001921 Certain locking variants of the ACQUIRE operation may fail, either due to
1922 being unable to get the lock immediately, or due to receiving an unblocked
David Howells108b42b2006-03-31 16:00:29 +01001923 signal whilst asleep waiting for the lock to become available. Failed
1924 locks do not imply any sort of barrier.
1925
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001926[!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
1927one-way barriers is that the effects of instructions outside of a critical
1928section may seep into the inside of the critical section.
David Howells108b42b2006-03-31 16:00:29 +01001929
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001930An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
1931because it is possible for an access preceding the ACQUIRE to happen after the
1932ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
1933the two accesses can themselves then cross:
David Howells670bd952006-06-10 09:54:12 -07001934
1935 *A = a;
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001936 ACQUIRE M
1937 RELEASE M
David Howells670bd952006-06-10 09:54:12 -07001938 *B = b;
1939
1940may occur as:
1941
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001942 ACQUIRE M, STORE *B, STORE *A, RELEASE M
Paul E. McKenney17eb88e2013-12-11 13:59:09 -08001943
Paul E. McKenney8dd853d2014-02-23 08:34:24 -08001944When the ACQUIRE and RELEASE are a lock acquisition and release,
1945respectively, this same reordering can occur if the lock's ACQUIRE and
1946RELEASE are to the same lock variable, but only from the perspective of
1947another CPU not holding that lock. In short, a ACQUIRE followed by an
1948RELEASE may -not- be assumed to be a full memory barrier.
Paul E. McKenney17eb88e2013-12-11 13:59:09 -08001949
Paul E. McKenney12d560f2015-07-14 18:35:23 -07001950Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
1951not imply a full memory barrier. Therefore, the CPU's execution of the
1952critical sections corresponding to the RELEASE and the ACQUIRE can cross,
1953so that:
Paul E. McKenney17eb88e2013-12-11 13:59:09 -08001954
1955 *A = a;
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001956 RELEASE M
1957 ACQUIRE N
Paul E. McKenney17eb88e2013-12-11 13:59:09 -08001958 *B = b;
1959
1960could occur as:
1961
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01001962 ACQUIRE N, STORE *B, STORE *A, RELEASE M
Paul E. McKenney17eb88e2013-12-11 13:59:09 -08001963
Paul E. McKenney8dd853d2014-02-23 08:34:24 -08001964It might appear that this reordering could introduce a deadlock.
1965However, this cannot happen because if such a deadlock threatened,
1966the RELEASE would simply complete, thereby avoiding the deadlock.
1967
1968 Why does this work?
1969
1970 One key point is that we are only talking about the CPU doing
1971 the reordering, not the compiler. If the compiler (or, for
1972 that matter, the developer) switched the operations, deadlock
1973 -could- occur.
1974
1975 But suppose the CPU reordered the operations. In this case,
1976 the unlock precedes the lock in the assembly code. The CPU
1977 simply elected to try executing the later lock operation first.
1978 If there is a deadlock, this lock operation will simply spin (or
1979 try to sleep, but more on that later). The CPU will eventually
1980 execute the unlock operation (which preceded the lock operation
1981 in the assembly code), which will unravel the potential deadlock,
1982 allowing the lock operation to succeed.
1983
1984 But what if the lock is a sleeplock? In that case, the code will
1985 try to enter the scheduler, where it will eventually encounter
1986 a memory barrier, which will force the earlier unlock operation
1987 to complete, again unraveling the deadlock. There might be
1988 a sleep-unlock race, but the locking primitive needs to resolve
1989 such races properly in any case.
1990
David Howells108b42b2006-03-31 16:00:29 +01001991Locks and semaphores may not provide any guarantee of ordering on UP compiled
1992systems, and so cannot be counted on in such a situation to actually achieve
1993anything at all - especially with respect to I/O accesses - unless combined
1994with interrupt disabling operations.
1995
1996See also the section on "Inter-CPU locking barrier effects".
1997
1998
1999As an example, consider the following:
2000
2001 *A = a;
2002 *B = b;
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002003 ACQUIRE
David Howells108b42b2006-03-31 16:00:29 +01002004 *C = c;
2005 *D = d;
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002006 RELEASE
David Howells108b42b2006-03-31 16:00:29 +01002007 *E = e;
2008 *F = f;
2009
2010The following sequence of events is acceptable:
2011
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002012 ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
David Howells108b42b2006-03-31 16:00:29 +01002013
2014 [+] Note that {*F,*A} indicates a combined access.
2015
2016But none of the following are:
2017
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002018 {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E
2019 *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F
2020 *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F
2021 *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E
David Howells108b42b2006-03-31 16:00:29 +01002022
2023
2024
2025INTERRUPT DISABLING FUNCTIONS
2026-----------------------------
2027
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002028Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
2029(RELEASE equivalent) will act as compiler barriers only. So if memory or I/O
David Howells108b42b2006-03-31 16:00:29 +01002030barriers are required in such a situation, they must be provided from some
2031other means.
2032
2033
David Howells50fa6102009-04-28 15:01:38 +01002034SLEEP AND WAKE-UP FUNCTIONS
2035---------------------------
2036
2037Sleeping and waking on an event flagged in global data can be viewed as an
2038interaction between two pieces of data: the task state of the task waiting for
2039the event and the global data used to indicate the event. To make sure that
2040these appear to happen in the right order, the primitives to begin the process
2041of going to sleep, and the primitives to initiate a wake up imply certain
2042barriers.
2043
2044Firstly, the sleeper normally follows something like this sequence of events:
2045
2046 for (;;) {
2047 set_current_state(TASK_UNINTERRUPTIBLE);
2048 if (event_indicated)
2049 break;
2050 schedule();
2051 }
2052
2053A general memory barrier is interpolated automatically by set_current_state()
2054after it has altered the task state:
2055
2056 CPU 1
2057 ===============================
2058 set_current_state();
Peter Zijlstrab92b8b32015-05-12 10:51:55 +02002059 smp_store_mb();
David Howells50fa6102009-04-28 15:01:38 +01002060 STORE current->state
2061 <general barrier>
2062 LOAD event_indicated
2063
2064set_current_state() may be wrapped by:
2065
2066 prepare_to_wait();
2067 prepare_to_wait_exclusive();
2068
2069which therefore also imply a general memory barrier after setting the state.
2070The whole sequence above is available in various canned forms, all of which
2071interpolate the memory barrier in the right place:
2072
2073 wait_event();
2074 wait_event_interruptible();
2075 wait_event_interruptible_exclusive();
2076 wait_event_interruptible_timeout();
2077 wait_event_killable();
2078 wait_event_timeout();
2079 wait_on_bit();
2080 wait_on_bit_lock();
2081
2082
2083Secondly, code that performs a wake up normally follows something like this:
2084
2085 event_indicated = 1;
2086 wake_up(&event_wait_queue);
2087
2088or:
2089
2090 event_indicated = 1;
2091 wake_up_process(event_daemon);
2092
2093A write memory barrier is implied by wake_up() and co. if and only if they wake
2094something up. The barrier occurs before the task state is cleared, and so sits
2095between the STORE to indicate the event and the STORE to set TASK_RUNNING:
2096
2097 CPU 1 CPU 2
2098 =============================== ===============================
2099 set_current_state(); STORE event_indicated
Peter Zijlstrab92b8b32015-05-12 10:51:55 +02002100 smp_store_mb(); wake_up();
David Howells50fa6102009-04-28 15:01:38 +01002101 STORE current->state <write barrier>
2102 <general barrier> STORE current->state
2103 LOAD event_indicated
2104
Paul E. McKenney5726ce02014-05-13 10:14:51 -07002105To repeat, this write memory barrier is present if and only if something
2106is actually awakened. To see this, consider the following sequence of
2107events, where X and Y are both initially zero:
2108
2109 CPU 1 CPU 2
2110 =============================== ===============================
2111 X = 1; STORE event_indicated
2112 smp_mb(); wake_up();
2113 Y = 1; wait_event(wq, Y == 1);
2114 wake_up(); load from Y sees 1, no memory barrier
2115 load from X might see 0
2116
2117In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
2118to see 1.
2119
David Howells50fa6102009-04-28 15:01:38 +01002120The available waker functions include:
2121
2122 complete();
2123 wake_up();
2124 wake_up_all();
2125 wake_up_bit();
2126 wake_up_interruptible();
2127 wake_up_interruptible_all();
2128 wake_up_interruptible_nr();
2129 wake_up_interruptible_poll();
2130 wake_up_interruptible_sync();
2131 wake_up_interruptible_sync_poll();
2132 wake_up_locked();
2133 wake_up_locked_poll();
2134 wake_up_nr();
2135 wake_up_poll();
2136 wake_up_process();
2137
2138
2139[!] Note that the memory barriers implied by the sleeper and the waker do _not_
2140order multiple stores before the wake-up with respect to loads of those stored
2141values after the sleeper has called set_current_state(). For instance, if the
2142sleeper does:
2143
2144 set_current_state(TASK_INTERRUPTIBLE);
2145 if (event_indicated)
2146 break;
2147 __set_current_state(TASK_RUNNING);
2148 do_something(my_data);
2149
2150and the waker does:
2151
2152 my_data = value;
2153 event_indicated = 1;
2154 wake_up(&event_wait_queue);
2155
2156there's no guarantee that the change to event_indicated will be perceived by
2157the sleeper as coming after the change to my_data. In such a circumstance, the
2158code on both sides must interpolate its own memory barriers between the
2159separate data accesses. Thus the above sleeper ought to do:
2160
2161 set_current_state(TASK_INTERRUPTIBLE);
2162 if (event_indicated) {
2163 smp_rmb();
2164 do_something(my_data);
2165 }
2166
2167and the waker should do:
2168
2169 my_data = value;
2170 smp_wmb();
2171 event_indicated = 1;
2172 wake_up(&event_wait_queue);
2173
2174
David Howells108b42b2006-03-31 16:00:29 +01002175MISCELLANEOUS FUNCTIONS
2176-----------------------
2177
2178Other functions that imply barriers:
2179
2180 (*) schedule() and similar imply full memory barriers.
2181
David Howells108b42b2006-03-31 16:00:29 +01002182
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002183===================================
2184INTER-CPU ACQUIRING BARRIER EFFECTS
2185===================================
David Howells108b42b2006-03-31 16:00:29 +01002186
2187On SMP systems locking primitives give a more substantial form of barrier: one
2188that does affect memory access ordering on other CPUs, within the context of
2189conflict on any particular lock.
2190
2191
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002192ACQUIRES VS MEMORY ACCESSES
2193---------------------------
David Howells108b42b2006-03-31 16:00:29 +01002194
Aneesh Kumar79afecf2006-05-15 09:44:36 -07002195Consider the following: the system has a pair of spinlocks (M) and (Q), and
David Howells108b42b2006-03-31 16:00:29 +01002196three CPUs; then should the following sequence of events occur:
2197
2198 CPU 1 CPU 2
2199 =============================== ===============================
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002200 WRITE_ONCE(*A, a); WRITE_ONCE(*E, e);
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002201 ACQUIRE M ACQUIRE Q
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002202 WRITE_ONCE(*B, b); WRITE_ONCE(*F, f);
2203 WRITE_ONCE(*C, c); WRITE_ONCE(*G, g);
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002204 RELEASE M RELEASE Q
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002205 WRITE_ONCE(*D, d); WRITE_ONCE(*H, h);
David Howells108b42b2006-03-31 16:00:29 +01002206
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002207Then there is no guarantee as to what order CPU 3 will see the accesses to *A
David Howells108b42b2006-03-31 16:00:29 +01002208through *H occur in, other than the constraints imposed by the separate locks
2209on the separate CPUs. It might, for example, see:
2210
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002211 *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
David Howells108b42b2006-03-31 16:00:29 +01002212
2213But it won't see any of:
2214
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002215 *B, *C or *D preceding ACQUIRE M
2216 *A, *B or *C following RELEASE M
2217 *F, *G or *H preceding ACQUIRE Q
2218 *E, *F or *G following RELEASE Q
David Howells108b42b2006-03-31 16:00:29 +01002219
2220
David Howells108b42b2006-03-31 16:00:29 +01002221
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002222ACQUIRES VS I/O ACCESSES
2223------------------------
David Howells108b42b2006-03-31 16:00:29 +01002224
2225Under certain circumstances (especially involving NUMA), I/O accesses within
2226two spinlocked sections on two different CPUs may be seen as interleaved by the
2227PCI bridge, because the PCI bridge does not necessarily participate in the
2228cache-coherence protocol, and is therefore incapable of issuing the required
2229read memory barriers.
2230
2231For example:
2232
2233 CPU 1 CPU 2
2234 =============================== ===============================
2235 spin_lock(Q)
2236 writel(0, ADDR)
2237 writel(1, DATA);
2238 spin_unlock(Q);
2239 spin_lock(Q);
2240 writel(4, ADDR);
2241 writel(5, DATA);
2242 spin_unlock(Q);
2243
2244may be seen by the PCI bridge as follows:
2245
2246 STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5
2247
2248which would probably cause the hardware to malfunction.
2249
2250
2251What is necessary here is to intervene with an mmiowb() before dropping the
2252spinlock, for example:
2253
2254 CPU 1 CPU 2
2255 =============================== ===============================
2256 spin_lock(Q)
2257 writel(0, ADDR)
2258 writel(1, DATA);
2259 mmiowb();
2260 spin_unlock(Q);
2261 spin_lock(Q);
2262 writel(4, ADDR);
2263 writel(5, DATA);
2264 mmiowb();
2265 spin_unlock(Q);
2266
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002267this will ensure that the two stores issued on CPU 1 appear at the PCI bridge
2268before either of the stores issued on CPU 2.
David Howells108b42b2006-03-31 16:00:29 +01002269
2270
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002271Furthermore, following a store by a load from the same device obviates the need
2272for the mmiowb(), because the load forces the store to complete before the load
David Howells108b42b2006-03-31 16:00:29 +01002273is performed:
2274
2275 CPU 1 CPU 2
2276 =============================== ===============================
2277 spin_lock(Q)
2278 writel(0, ADDR)
2279 a = readl(DATA);
2280 spin_unlock(Q);
2281 spin_lock(Q);
2282 writel(4, ADDR);
2283 b = readl(DATA);
2284 spin_unlock(Q);
2285
2286
2287See Documentation/DocBook/deviceiobook.tmpl for more information.
2288
2289
2290=================================
2291WHERE ARE MEMORY BARRIERS NEEDED?
2292=================================
2293
2294Under normal operation, memory operation reordering is generally not going to
2295be a problem as a single-threaded linear piece of code will still appear to
David Howells50fa6102009-04-28 15:01:38 +01002296work correctly, even if it's in an SMP kernel. There are, however, four
David Howells108b42b2006-03-31 16:00:29 +01002297circumstances in which reordering definitely _could_ be a problem:
2298
2299 (*) Interprocessor interaction.
2300
2301 (*) Atomic operations.
2302
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002303 (*) Accessing devices.
David Howells108b42b2006-03-31 16:00:29 +01002304
2305 (*) Interrupts.
2306
2307
2308INTERPROCESSOR INTERACTION
2309--------------------------
2310
2311When there's a system with more than one processor, more than one CPU in the
2312system may be working on the same data set at the same time. This can cause
2313synchronisation problems, and the usual way of dealing with them is to use
2314locks. Locks, however, are quite expensive, and so it may be preferable to
2315operate without the use of a lock if at all possible. In such a case
2316operations that affect both CPUs may have to be carefully ordered to prevent
2317a malfunction.
2318
2319Consider, for example, the R/W semaphore slow path. Here a waiting process is
2320queued on the semaphore, by virtue of it having a piece of its stack linked to
2321the semaphore's list of waiting processes:
2322
2323 struct rw_semaphore {
2324 ...
2325 spinlock_t lock;
2326 struct list_head waiters;
2327 };
2328
2329 struct rwsem_waiter {
2330 struct list_head list;
2331 struct task_struct *task;
2332 };
2333
2334To wake up a particular waiter, the up_read() or up_write() functions have to:
2335
2336 (1) read the next pointer from this waiter's record to know as to where the
2337 next waiter record is;
2338
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002339 (2) read the pointer to the waiter's task structure;
David Howells108b42b2006-03-31 16:00:29 +01002340
2341 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2342
2343 (4) call wake_up_process() on the task; and
2344
2345 (5) release the reference held on the waiter's task struct.
2346
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002347In other words, it has to perform this sequence of events:
David Howells108b42b2006-03-31 16:00:29 +01002348
2349 LOAD waiter->list.next;
2350 LOAD waiter->task;
2351 STORE waiter->task;
2352 CALL wakeup
2353 RELEASE task
2354
2355and if any of these steps occur out of order, then the whole thing may
2356malfunction.
2357
2358Once it has queued itself and dropped the semaphore lock, the waiter does not
2359get the lock again; it instead just waits for its task pointer to be cleared
2360before proceeding. Since the record is on the waiter's stack, this means that
2361if the task pointer is cleared _before_ the next pointer in the list is read,
2362another CPU might start processing the waiter and might clobber the waiter's
2363stack before the up*() function has a chance to read the next pointer.
2364
2365Consider then what might happen to the above sequence of events:
2366
2367 CPU 1 CPU 2
2368 =============================== ===============================
2369 down_xxx()
2370 Queue waiter
2371 Sleep
2372 up_yyy()
2373 LOAD waiter->task;
2374 STORE waiter->task;
2375 Woken up by other event
2376 <preempt>
2377 Resume processing
2378 down_xxx() returns
2379 call foo()
2380 foo() clobbers *waiter
2381 </preempt>
2382 LOAD waiter->list.next;
2383 --- OOPS ---
2384
2385This could be dealt with using the semaphore lock, but then the down_xxx()
2386function has to needlessly get the spinlock again after being woken up.
2387
2388The way to deal with this is to insert a general SMP memory barrier:
2389
2390 LOAD waiter->list.next;
2391 LOAD waiter->task;
2392 smp_mb();
2393 STORE waiter->task;
2394 CALL wakeup
2395 RELEASE task
2396
2397In this case, the barrier makes a guarantee that all memory accesses before the
2398barrier will appear to happen before all the memory accesses after the barrier
2399with respect to the other CPUs on the system. It does _not_ guarantee that all
2400the memory accesses before the barrier will be complete by the time the barrier
2401instruction itself is complete.
2402
2403On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2404compiler barrier, thus making sure the compiler emits the instructions in the
David Howells6bc39272006-06-25 05:49:22 -07002405right order without actually intervening in the CPU. Since there's only one
2406CPU, that CPU's dependency ordering logic will take care of everything else.
David Howells108b42b2006-03-31 16:00:29 +01002407
2408
2409ATOMIC OPERATIONS
2410-----------------
2411
David Howellsdbc87002006-04-10 22:54:23 -07002412Whilst they are technically interprocessor interaction considerations, atomic
2413operations are noted specially as some of them imply full memory barriers and
2414some don't, but they're very heavily relied on as a group throughout the
2415kernel.
2416
2417Any atomic operation that modifies some state in memory and returns information
2418about the state (old or new) implies an SMP-conditional general memory barrier
Nick Piggin26333572007-10-18 03:06:39 -07002419(smp_mb()) on each side of the actual operation (with the exception of
2420explicit lock operations, described later). These include:
David Howells108b42b2006-03-31 16:00:29 +01002421
2422 xchg();
Paul E. McKenneyfb2b5812013-12-11 13:59:05 -08002423 atomic_xchg(); atomic_long_xchg();
Paul E. McKenneyfb2b5812013-12-11 13:59:05 -08002424 atomic_inc_return(); atomic_long_inc_return();
2425 atomic_dec_return(); atomic_long_dec_return();
2426 atomic_add_return(); atomic_long_add_return();
2427 atomic_sub_return(); atomic_long_sub_return();
2428 atomic_inc_and_test(); atomic_long_inc_and_test();
2429 atomic_dec_and_test(); atomic_long_dec_and_test();
2430 atomic_sub_and_test(); atomic_long_sub_and_test();
2431 atomic_add_negative(); atomic_long_add_negative();
David Howellsdbc87002006-04-10 22:54:23 -07002432 test_and_set_bit();
2433 test_and_clear_bit();
2434 test_and_change_bit();
David Howells108b42b2006-03-31 16:00:29 +01002435
Will Deaconed2de9f2015-07-16 16:10:06 +01002436 /* when succeeds */
2437 cmpxchg();
2438 atomic_cmpxchg(); atomic_long_cmpxchg();
Paul E. McKenneyfb2b5812013-12-11 13:59:05 -08002439 atomic_add_unless(); atomic_long_add_unless();
2440
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002441These are used for such things as implementing ACQUIRE-class and RELEASE-class
David Howellsdbc87002006-04-10 22:54:23 -07002442operations and adjusting reference counters towards object destruction, and as
2443such the implicit memory barrier effects are necessary.
David Howells108b42b2006-03-31 16:00:29 +01002444
David Howells108b42b2006-03-31 16:00:29 +01002445
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002446The following operations are potential problems as they do _not_ imply memory
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002447barriers, but might be used for implementing such things as RELEASE-class
David Howellsdbc87002006-04-10 22:54:23 -07002448operations:
2449
2450 atomic_set();
David Howells108b42b2006-03-31 16:00:29 +01002451 set_bit();
2452 clear_bit();
2453 change_bit();
David Howellsdbc87002006-04-10 22:54:23 -07002454
2455With these the appropriate explicit memory barrier should be used if necessary
Peter Zijlstra1b156112014-03-13 19:00:35 +01002456(smp_mb__before_atomic() for instance).
David Howells108b42b2006-03-31 16:00:29 +01002457
2458
David Howellsdbc87002006-04-10 22:54:23 -07002459The following also do _not_ imply memory barriers, and so may require explicit
Peter Zijlstra1b156112014-03-13 19:00:35 +01002460memory barriers under some circumstances (smp_mb__before_atomic() for
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002461instance):
David Howells108b42b2006-03-31 16:00:29 +01002462
2463 atomic_add();
2464 atomic_sub();
2465 atomic_inc();
2466 atomic_dec();
2467
2468If they're used for statistics generation, then they probably don't need memory
2469barriers, unless there's a coupling between statistical data.
2470
2471If they're used for reference counting on an object to control its lifetime,
2472they probably don't need memory barriers because either the reference count
2473will be adjusted inside a locked section, or the caller will already hold
2474sufficient references to make the lock, and thus a memory barrier unnecessary.
2475
2476If they're used for constructing a lock of some description, then they probably
2477do need memory barriers as a lock primitive generally has to do things in a
2478specific order.
2479
David Howells108b42b2006-03-31 16:00:29 +01002480Basically, each usage case has to be carefully considered as to whether memory
David Howellsdbc87002006-04-10 22:54:23 -07002481barriers are needed or not.
2482
Nick Piggin26333572007-10-18 03:06:39 -07002483The following operations are special locking primitives:
2484
2485 test_and_set_bit_lock();
2486 clear_bit_unlock();
2487 __clear_bit_unlock();
2488
Peter Zijlstra2e4f5382013-11-06 14:57:36 +01002489These implement ACQUIRE-class and RELEASE-class operations. These should be used in
Nick Piggin26333572007-10-18 03:06:39 -07002490preference to other operations when implementing locking primitives, because
2491their implementations can be optimised on many architectures.
2492
David Howellsdbc87002006-04-10 22:54:23 -07002493[!] Note that special memory barrier primitives are available for these
2494situations because on some CPUs the atomic instructions used imply full memory
2495barriers, and so barrier instructions are superfluous in conjunction with them,
2496and in such cases the special barrier primitives will be no-ops.
David Howells108b42b2006-03-31 16:00:29 +01002497
2498See Documentation/atomic_ops.txt for more information.
2499
2500
2501ACCESSING DEVICES
2502-----------------
2503
2504Many devices can be memory mapped, and so appear to the CPU as if they're just
2505a set of memory locations. To control such a device, the driver usually has to
2506make the right memory accesses in exactly the right order.
2507
2508However, having a clever CPU or a clever compiler creates a potential problem
2509in that the carefully sequenced accesses in the driver code won't reach the
2510device in the requisite order if the CPU or the compiler thinks it is more
2511efficient to reorder, combine or merge accesses - something that would cause
2512the device to malfunction.
2513
2514Inside of the Linux kernel, I/O should be done through the appropriate accessor
2515routines - such as inb() or writel() - which know how to make such accesses
2516appropriately sequential. Whilst this, for the most part, renders the explicit
2517use of memory barriers unnecessary, there are a couple of situations where they
2518might be needed:
2519
2520 (1) On some systems, I/O stores are not strongly ordered across all CPUs, and
2521 so for _all_ general drivers locks should be used and mmiowb() must be
2522 issued prior to unlocking the critical section.
2523
2524 (2) If the accessor functions are used to refer to an I/O memory window with
2525 relaxed memory access properties, then _mandatory_ memory barriers are
2526 required to enforce ordering.
2527
2528See Documentation/DocBook/deviceiobook.tmpl for more information.
2529
2530
2531INTERRUPTS
2532----------
2533
2534A driver may be interrupted by its own interrupt service routine, and thus the
2535two parts of the driver may interfere with each other's attempts to control or
2536access the device.
2537
2538This may be alleviated - at least in part - by disabling local interrupts (a
2539form of locking), such that the critical operations are all contained within
2540the interrupt-disabled section in the driver. Whilst the driver's interrupt
2541routine is executing, the driver's core may not run on the same CPU, and its
2542interrupt is not permitted to happen again until the current interrupt has been
2543handled, thus the interrupt handler does not need to lock against that.
2544
2545However, consider a driver that was talking to an ethernet card that sports an
2546address register and a data register. If that driver's core talks to the card
2547under interrupt-disablement and then the driver's interrupt handler is invoked:
2548
2549 LOCAL IRQ DISABLE
2550 writew(ADDR, 3);
2551 writew(DATA, y);
2552 LOCAL IRQ ENABLE
2553 <interrupt>
2554 writew(ADDR, 4);
2555 q = readw(DATA);
2556 </interrupt>
2557
2558The store to the data register might happen after the second store to the
2559address register if ordering rules are sufficiently relaxed:
2560
2561 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2562
2563
2564If ordering rules are relaxed, it must be assumed that accesses done inside an
2565interrupt disabled section may leak outside of it and may interleave with
2566accesses performed in an interrupt - and vice versa - unless implicit or
2567explicit barriers are used.
2568
2569Normally this won't be a problem because the I/O accesses done inside such
2570sections will include synchronous load operations on strictly ordered I/O
2571registers that form implicit I/O barriers. If this isn't sufficient then an
2572mmiowb() may need to be used explicitly.
2573
2574
2575A similar situation may occur between an interrupt routine and two routines
2576running on separate CPUs that communicate with each other. If such a case is
2577likely, then interrupt-disabling locks should be used to guarantee ordering.
2578
2579
2580==========================
2581KERNEL I/O BARRIER EFFECTS
2582==========================
2583
2584When accessing I/O memory, drivers should use the appropriate accessor
2585functions:
2586
2587 (*) inX(), outX():
2588
2589 These are intended to talk to I/O space rather than memory space, but
2590 that's primarily a CPU-specific concept. The i386 and x86_64 processors do
2591 indeed have special I/O space access cycles and instructions, but many
2592 CPUs don't have such a concept.
2593
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002594 The PCI bus, amongst others, defines an I/O space concept which - on such
2595 CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O
David Howells6bc39272006-06-25 05:49:22 -07002596 space. However, it may also be mapped as a virtual I/O space in the CPU's
2597 memory map, particularly on those CPUs that don't support alternate I/O
2598 spaces.
David Howells108b42b2006-03-31 16:00:29 +01002599
2600 Accesses to this space may be fully synchronous (as on i386), but
2601 intermediary bridges (such as the PCI host bridge) may not fully honour
2602 that.
2603
2604 They are guaranteed to be fully ordered with respect to each other.
2605
2606 They are not guaranteed to be fully ordered with respect to other types of
2607 memory and I/O operation.
2608
2609 (*) readX(), writeX():
2610
2611 Whether these are guaranteed to be fully ordered and uncombined with
2612 respect to each other on the issuing CPU depends on the characteristics
2613 defined for the memory window through which they're accessing. On later
2614 i386 architecture machines, for example, this is controlled by way of the
2615 MTRR registers.
2616
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002617 Ordinarily, these will be guaranteed to be fully ordered and uncombined,
David Howells108b42b2006-03-31 16:00:29 +01002618 provided they're not accessing a prefetchable device.
2619
2620 However, intermediary hardware (such as a PCI bridge) may indulge in
2621 deferral if it so wishes; to flush a store, a load from the same location
2622 is preferred[*], but a load from the same device or from configuration
2623 space should suffice for PCI.
2624
2625 [*] NOTE! attempting to load from the same location as was written to may
Ingo Molnare0edc782013-11-22 11:24:53 +01002626 cause a malfunction - consider the 16550 Rx/Tx serial registers for
2627 example.
David Howells108b42b2006-03-31 16:00:29 +01002628
2629 Used with prefetchable I/O memory, an mmiowb() barrier may be required to
2630 force stores to be ordered.
2631
2632 Please refer to the PCI specification for more information on interactions
2633 between PCI transactions.
2634
Will Deacona8e0aea2013-09-04 12:30:08 +01002635 (*) readX_relaxed(), writeX_relaxed()
David Howells108b42b2006-03-31 16:00:29 +01002636
Will Deacona8e0aea2013-09-04 12:30:08 +01002637 These are similar to readX() and writeX(), but provide weaker memory
2638 ordering guarantees. Specifically, they do not guarantee ordering with
2639 respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee
2640 ordering with respect to LOCK or UNLOCK operations. If the latter is
2641 required, an mmiowb() barrier can be used. Note that relaxed accesses to
2642 the same peripheral are guaranteed to be ordered with respect to each
2643 other.
David Howells108b42b2006-03-31 16:00:29 +01002644
2645 (*) ioreadX(), iowriteX()
2646
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002647 These will perform appropriately for the type of access they're actually
David Howells108b42b2006-03-31 16:00:29 +01002648 doing, be it inX()/outX() or readX()/writeX().
2649
2650
2651========================================
2652ASSUMED MINIMUM EXECUTION ORDERING MODEL
2653========================================
2654
2655It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2656maintain the appearance of program causality with respect to itself. Some CPUs
2657(such as i386 or x86_64) are more constrained than others (such as powerpc or
2658frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2659of arch-specific code.
2660
2661This means that it must be considered that the CPU will execute its instruction
2662stream in any order it feels like - or even in parallel - provided that if an
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002663instruction in the stream depends on an earlier instruction, then that
David Howells108b42b2006-03-31 16:00:29 +01002664earlier instruction must be sufficiently complete[*] before the later
2665instruction may proceed; in other words: provided that the appearance of
2666causality is maintained.
2667
2668 [*] Some instructions have more than one effect - such as changing the
2669 condition codes, changing registers or changing memory - and different
2670 instructions may depend on different effects.
2671
2672A CPU may also discard any instruction sequence that winds up having no
2673ultimate effect. For example, if two adjacent instructions both load an
2674immediate value into the same register, the first may be discarded.
2675
2676
2677Similarly, it has to be assumed that compiler might reorder the instruction
2678stream in any way it sees fit, again provided the appearance of causality is
2679maintained.
2680
2681
2682============================
2683THE EFFECTS OF THE CPU CACHE
2684============================
2685
2686The way cached memory operations are perceived across the system is affected to
2687a certain extent by the caches that lie between CPUs and memory, and by the
2688memory coherence system that maintains the consistency of state in the system.
2689
2690As far as the way a CPU interacts with another part of the system through the
2691caches goes, the memory system has to include the CPU's caches, and memory
2692barriers for the most part act at the interface between the CPU and its cache
2693(memory barriers logically act on the dotted line in the following diagram):
2694
2695 <--- CPU ---> : <----------- Memory ----------->
2696 :
2697 +--------+ +--------+ : +--------+ +-----------+
2698 | | | | : | | | | +--------+
Ingo Molnare0edc782013-11-22 11:24:53 +01002699 | CPU | | Memory | : | CPU | | | | |
2700 | Core |--->| Access |----->| Cache |<-->| | | |
David Howells108b42b2006-03-31 16:00:29 +01002701 | | | Queue | : | | | |--->| Memory |
Ingo Molnare0edc782013-11-22 11:24:53 +01002702 | | | | : | | | | | |
2703 +--------+ +--------+ : +--------+ | | | |
David Howells108b42b2006-03-31 16:00:29 +01002704 : | Cache | +--------+
2705 : | Coherency |
2706 : | Mechanism | +--------+
2707 +--------+ +--------+ : +--------+ | | | |
2708 | | | | : | | | | | |
2709 | CPU | | Memory | : | CPU | | |--->| Device |
Ingo Molnare0edc782013-11-22 11:24:53 +01002710 | Core |--->| Access |----->| Cache |<-->| | | |
2711 | | | Queue | : | | | | | |
David Howells108b42b2006-03-31 16:00:29 +01002712 | | | | : | | | | +--------+
2713 +--------+ +--------+ : +--------+ +-----------+
2714 :
2715 :
2716
2717Although any particular load or store may not actually appear outside of the
2718CPU that issued it since it may have been satisfied within the CPU's own cache,
2719it will still appear as if the full memory access had taken place as far as the
2720other CPUs are concerned since the cache coherency mechanisms will migrate the
2721cacheline over to the accessing CPU and propagate the effects upon conflict.
2722
2723The CPU core may execute instructions in any order it deems fit, provided the
2724expected program causality appears to be maintained. Some of the instructions
2725generate load and store operations which then go into the queue of memory
2726accesses to be performed. The core may place these in the queue in any order
2727it wishes, and continue execution until it is forced to wait for an instruction
2728to complete.
2729
2730What memory barriers are concerned with is controlling the order in which
2731accesses cross from the CPU side of things to the memory side of things, and
2732the order in which the effects are perceived to happen by the other observers
2733in the system.
2734
2735[!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2736their own loads and stores as if they had happened in program order.
2737
2738[!] MMIO or other device accesses may bypass the cache system. This depends on
2739the properties of the memory window through which devices are accessed and/or
2740the use of any special device communication instructions the CPU may have.
2741
2742
2743CACHE COHERENCY
2744---------------
2745
2746Life isn't quite as simple as it may appear above, however: for while the
2747caches are expected to be coherent, there's no guarantee that that coherency
2748will be ordered. This means that whilst changes made on one CPU will
2749eventually become visible on all CPUs, there's no guarantee that they will
2750become apparent in the same order on those other CPUs.
2751
2752
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002753Consider dealing with a system that has a pair of CPUs (1 & 2), each of which
2754has a pair of parallel data caches (CPU 1 has A/B, and CPU 2 has C/D):
David Howells108b42b2006-03-31 16:00:29 +01002755
2756 :
2757 : +--------+
2758 : +---------+ | |
2759 +--------+ : +--->| Cache A |<------->| |
2760 | | : | +---------+ | |
2761 | CPU 1 |<---+ | |
2762 | | : | +---------+ | |
2763 +--------+ : +--->| Cache B |<------->| |
2764 : +---------+ | |
2765 : | Memory |
2766 : +---------+ | System |
2767 +--------+ : +--->| Cache C |<------->| |
2768 | | : | +---------+ | |
2769 | CPU 2 |<---+ | |
2770 | | : | +---------+ | |
2771 +--------+ : +--->| Cache D |<------->| |
2772 : +---------+ | |
2773 : +--------+
2774 :
2775
2776Imagine the system has the following properties:
2777
2778 (*) an odd-numbered cache line may be in cache A, cache C or it may still be
2779 resident in memory;
2780
2781 (*) an even-numbered cache line may be in cache B, cache D or it may still be
2782 resident in memory;
2783
2784 (*) whilst the CPU core is interrogating one cache, the other cache may be
2785 making use of the bus to access the rest of the system - perhaps to
2786 displace a dirty cacheline or to do a speculative load;
2787
2788 (*) each cache has a queue of operations that need to be applied to that cache
2789 to maintain coherency with the rest of the system;
2790
2791 (*) the coherency queue is not flushed by normal loads to lines already
2792 present in the cache, even though the contents of the queue may
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002793 potentially affect those loads.
David Howells108b42b2006-03-31 16:00:29 +01002794
2795Imagine, then, that two writes are made on the first CPU, with a write barrier
2796between them to guarantee that they will appear to reach that CPU's caches in
2797the requisite order:
2798
2799 CPU 1 CPU 2 COMMENT
2800 =============== =============== =======================================
2801 u == 0, v == 1 and p == &u, q == &u
2802 v = 2;
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002803 smp_wmb(); Make sure change to v is visible before
David Howells108b42b2006-03-31 16:00:29 +01002804 change to p
2805 <A:modify v=2> v is now in cache A exclusively
2806 p = &v;
2807 <B:modify p=&v> p is now in cache B exclusively
2808
2809The write memory barrier forces the other CPUs in the system to perceive that
2810the local CPU's caches have apparently been updated in the correct order. But
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002811now imagine that the second CPU wants to read those values:
David Howells108b42b2006-03-31 16:00:29 +01002812
2813 CPU 1 CPU 2 COMMENT
2814 =============== =============== =======================================
2815 ...
2816 q = p;
2817 x = *q;
2818
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002819The above pair of reads may then fail to happen in the expected order, as the
David Howells108b42b2006-03-31 16:00:29 +01002820cacheline holding p may get updated in one of the second CPU's caches whilst
2821the update to the cacheline holding v is delayed in the other of the second
2822CPU's caches by some other cache event:
2823
2824 CPU 1 CPU 2 COMMENT
2825 =============== =============== =======================================
2826 u == 0, v == 1 and p == &u, q == &u
2827 v = 2;
2828 smp_wmb();
2829 <A:modify v=2> <C:busy>
2830 <C:queue v=2>
Aneesh Kumar79afecf2006-05-15 09:44:36 -07002831 p = &v; q = p;
David Howells108b42b2006-03-31 16:00:29 +01002832 <D:request p>
2833 <B:modify p=&v> <D:commit p=&v>
Ingo Molnare0edc782013-11-22 11:24:53 +01002834 <D:read p>
David Howells108b42b2006-03-31 16:00:29 +01002835 x = *q;
2836 <C:read *q> Reads from v before v updated in cache
2837 <C:unbusy>
2838 <C:commit v=2>
2839
2840Basically, whilst both cachelines will be updated on CPU 2 eventually, there's
2841no guarantee that, without intervention, the order of update will be the same
2842as that committed on CPU 1.
2843
2844
2845To intervene, we need to interpolate a data dependency barrier or a read
2846barrier between the loads. This will force the cache to commit its coherency
2847queue before processing any further requests:
2848
2849 CPU 1 CPU 2 COMMENT
2850 =============== =============== =======================================
2851 u == 0, v == 1 and p == &u, q == &u
2852 v = 2;
2853 smp_wmb();
2854 <A:modify v=2> <C:busy>
2855 <C:queue v=2>
Paolo 'Blaisorblade' Giarrusso3fda9822006-10-19 23:28:19 -07002856 p = &v; q = p;
David Howells108b42b2006-03-31 16:00:29 +01002857 <D:request p>
2858 <B:modify p=&v> <D:commit p=&v>
Ingo Molnare0edc782013-11-22 11:24:53 +01002859 <D:read p>
David Howells108b42b2006-03-31 16:00:29 +01002860 smp_read_barrier_depends()
2861 <C:unbusy>
2862 <C:commit v=2>
2863 x = *q;
2864 <C:read *q> Reads from v after v updated in cache
2865
2866
2867This sort of problem can be encountered on DEC Alpha processors as they have a
2868split cache that improves performance by making better use of the data bus.
2869Whilst most CPUs do imply a data dependency barrier on the read when a memory
2870access depends on a read, not all do, so it may not be relied on.
2871
2872Other CPUs may also have split caches, but must coordinate between the various
Matt LaPlante3f6dee92006-10-03 22:45:33 +02002873cachelets for normal memory accesses. The semantics of the Alpha removes the
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002874need for coordination in the absence of memory barriers.
David Howells108b42b2006-03-31 16:00:29 +01002875
2876
2877CACHE COHERENCY VS DMA
2878----------------------
2879
2880Not all systems maintain cache coherency with respect to devices doing DMA. In
2881such cases, a device attempting DMA may obtain stale data from RAM because
2882dirty cache lines may be resident in the caches of various CPUs, and may not
2883have been written back to RAM yet. To deal with this, the appropriate part of
2884the kernel must flush the overlapping bits of cache on each CPU (and maybe
2885invalidate them as well).
2886
2887In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2888cache lines being written back to RAM from a CPU's cache after the device has
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002889installed its own data, or cache lines present in the CPU's cache may simply
2890obscure the fact that RAM has been updated, until at such time as the cacheline
2891is discarded from the CPU's cache and reloaded. To deal with this, the
2892appropriate part of the kernel must invalidate the overlapping bits of the
David Howells108b42b2006-03-31 16:00:29 +01002893cache on each CPU.
2894
2895See Documentation/cachetlb.txt for more information on cache management.
2896
2897
2898CACHE COHERENCY VS MMIO
2899-----------------------
2900
2901Memory mapped I/O usually takes place through memory locations that are part of
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002902a window in the CPU's memory space that has different properties assigned than
David Howells108b42b2006-03-31 16:00:29 +01002903the usual RAM directed window.
2904
2905Amongst these properties is usually the fact that such accesses bypass the
2906caching entirely and go directly to the device buses. This means MMIO accesses
2907may, in effect, overtake accesses to cached memory that were emitted earlier.
2908A memory barrier isn't sufficient in such a case, but rather the cache must be
2909flushed between the cached memory write and the MMIO access if the two are in
2910any way dependent.
2911
2912
2913=========================
2914THE THINGS CPUS GET UP TO
2915=========================
2916
2917A programmer might take it for granted that the CPU will perform memory
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002918operations in exactly the order specified, so that if the CPU is, for example,
David Howells108b42b2006-03-31 16:00:29 +01002919given the following piece of code to execute:
2920
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002921 a = READ_ONCE(*A);
2922 WRITE_ONCE(*B, b);
2923 c = READ_ONCE(*C);
2924 d = READ_ONCE(*D);
2925 WRITE_ONCE(*E, e);
David Howells108b42b2006-03-31 16:00:29 +01002926
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002927they would then expect that the CPU will complete the memory operation for each
David Howells108b42b2006-03-31 16:00:29 +01002928instruction before moving on to the next one, leading to a definite sequence of
2929operations as seen by external observers in the system:
2930
2931 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2932
2933
2934Reality is, of course, much messier. With many CPUs and compilers, the above
2935assumption doesn't hold because:
2936
2937 (*) loads are more likely to need to be completed immediately to permit
2938 execution progress, whereas stores can often be deferred without a
2939 problem;
2940
2941 (*) loads may be done speculatively, and the result discarded should it prove
2942 to have been unnecessary;
2943
Jarek Poplawski81fc6322007-05-23 13:58:20 -07002944 (*) loads may be done speculatively, leading to the result having been fetched
2945 at the wrong time in the expected sequence of events;
David Howells108b42b2006-03-31 16:00:29 +01002946
2947 (*) the order of the memory accesses may be rearranged to promote better use
2948 of the CPU buses and caches;
2949
2950 (*) loads and stores may be combined to improve performance when talking to
2951 memory or I/O hardware that can do batched accesses of adjacent locations,
2952 thus cutting down on transaction setup costs (memory and PCI devices may
2953 both be able to do this); and
2954
2955 (*) the CPU's data cache may affect the ordering, and whilst cache-coherency
2956 mechanisms may alleviate this - once the store has actually hit the cache
2957 - there's no guarantee that the coherency management will be propagated in
2958 order to other CPUs.
2959
2960So what another CPU, say, might actually observe from the above piece of code
2961is:
2962
2963 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2964
2965 (Where "LOAD {*C,*D}" is a combined load)
2966
2967
2968However, it is guaranteed that a CPU will be self-consistent: it will see its
2969_own_ accesses appear to be correctly ordered, without the need for a memory
2970barrier. For instance with the following code:
2971
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002972 U = READ_ONCE(*A);
2973 WRITE_ONCE(*A, V);
2974 WRITE_ONCE(*A, W);
2975 X = READ_ONCE(*A);
2976 WRITE_ONCE(*A, Y);
2977 Z = READ_ONCE(*A);
David Howells108b42b2006-03-31 16:00:29 +01002978
2979and assuming no intervention by an external influence, it can be assumed that
2980the final result will appear to be:
2981
2982 U == the original value of *A
2983 X == W
2984 Z == Y
2985 *A == Y
2986
2987The code above may cause the CPU to generate the full sequence of memory
2988accesses:
2989
2990 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2991
2992in that order, but, without intervention, the sequence may have almost any
Paul E. McKenney9af194c2015-06-18 14:33:24 -07002993combination of elements combined or discarded, provided the program's view
2994of the world remains consistent. Note that READ_ONCE() and WRITE_ONCE()
2995are -not- optional in the above example, as there are architectures
2996where a given CPU might reorder successive loads to the same location.
2997On such architectures, READ_ONCE() and WRITE_ONCE() do whatever is
2998necessary to prevent this, for example, on Itanium the volatile casts
2999used by READ_ONCE() and WRITE_ONCE() cause GCC to emit the special ld.acq
3000and st.rel instructions (respectively) that prevent such reordering.
David Howells108b42b2006-03-31 16:00:29 +01003001
3002The compiler may also combine, discard or defer elements of the sequence before
3003the CPU even sees them.
3004
3005For instance:
3006
3007 *A = V;
3008 *A = W;
3009
3010may be reduced to:
3011
3012 *A = W;
3013
Paul E. McKenney9af194c2015-06-18 14:33:24 -07003014since, without either a write barrier or an WRITE_ONCE(), it can be
Paul E. McKenney2ecf8102013-12-11 13:59:04 -08003015assumed that the effect of the storage of V to *A is lost. Similarly:
David Howells108b42b2006-03-31 16:00:29 +01003016
3017 *A = Y;
3018 Z = *A;
3019
Paul E. McKenney9af194c2015-06-18 14:33:24 -07003020may, without a memory barrier or an READ_ONCE() and WRITE_ONCE(), be
3021reduced to:
David Howells108b42b2006-03-31 16:00:29 +01003022
3023 *A = Y;
3024 Z = Y;
3025
3026and the LOAD operation never appear outside of the CPU.
3027
3028
3029AND THEN THERE'S THE ALPHA
3030--------------------------
3031
3032The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
3033some versions of the Alpha CPU have a split data cache, permitting them to have
Jarek Poplawski81fc6322007-05-23 13:58:20 -07003034two semantically-related cache lines updated at separate times. This is where
David Howells108b42b2006-03-31 16:00:29 +01003035the data dependency barrier really becomes necessary as this synchronises both
3036caches with the memory coherence system, thus making it seem like pointer
3037changes vs new data occur in the right order.
3038
Jarek Poplawski81fc6322007-05-23 13:58:20 -07003039The Alpha defines the Linux kernel's memory barrier model.
David Howells108b42b2006-03-31 16:00:29 +01003040
3041See the subsection on "Cache Coherency" above.
3042
Michael S. Tsirkin6a65d262015-12-27 18:23:01 +02003043VIRTUAL MACHINE GUESTS
3044-------------------
3045
3046Guests running within virtual machines might be affected by SMP effects even if
3047the guest itself is compiled without SMP support. This is an artifact of
3048interfacing with an SMP host while running an UP kernel. Using mandatory
3049barriers for this use-case would be possible but is often suboptimal.
3050
3051To handle this case optimally, low-level virt_mb() etc macros are available.
3052These have the same effect as smp_mb() etc when SMP is enabled, but generate
3053identical code for SMP and non-SMP systems. For example, virtual machine guests
3054should use virt_mb() rather than smp_mb() when synchronizing against a
3055(possibly SMP) host.
3056
3057These are equivalent to smp_mb() etc counterparts in all other respects,
3058in particular, they do not control MMIO effects: to control
3059MMIO effects, use mandatory barriers.
David Howells108b42b2006-03-31 16:00:29 +01003060
David Howells90fddab2010-03-24 09:43:00 +00003061============
3062EXAMPLE USES
3063============
3064
3065CIRCULAR BUFFERS
3066----------------
3067
3068Memory barriers can be used to implement circular buffering without the need
3069of a lock to serialise the producer with the consumer. See:
3070
3071 Documentation/circular-buffers.txt
3072
3073for details.
3074
3075
David Howells108b42b2006-03-31 16:00:29 +01003076==========
3077REFERENCES
3078==========
3079
3080Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
3081Digital Press)
3082 Chapter 5.2: Physical Address Space Characteristics
3083 Chapter 5.4: Caches and Write Buffers
3084 Chapter 5.5: Data Sharing
3085 Chapter 5.6: Read/Write Ordering
3086
3087AMD64 Architecture Programmer's Manual Volume 2: System Programming
3088 Chapter 7.1: Memory-Access Ordering
3089 Chapter 7.4: Buffering and Combining Memory Writes
3090
3091IA-32 Intel Architecture Software Developer's Manual, Volume 3:
3092System Programming Guide
3093 Chapter 7.1: Locked Atomic Operations
3094 Chapter 7.2: Memory Ordering
3095 Chapter 7.4: Serializing Instructions
3096
3097The SPARC Architecture Manual, Version 9
3098 Chapter 8: Memory Models
3099 Appendix D: Formal Specification of the Memory Models
3100 Appendix J: Programming with the Memory Models
3101
3102UltraSPARC Programmer Reference Manual
3103 Chapter 5: Memory Accesses and Cacheability
3104 Chapter 15: Sparc-V9 Memory Models
3105
3106UltraSPARC III Cu User's Manual
3107 Chapter 9: Memory Models
3108
3109UltraSPARC IIIi Processor User's Manual
3110 Chapter 8: Memory Models
3111
3112UltraSPARC Architecture 2005
3113 Chapter 9: Memory
3114 Appendix D: Formal Specifications of the Memory Models
3115
3116UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
3117 Chapter 8: Memory Models
3118 Appendix F: Caches and Cache Coherency
3119
3120Solaris Internals, Core Kernel Architecture, p63-68:
3121 Chapter 3.3: Hardware Considerations for Locks and
3122 Synchronization
3123
3124Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
3125for Kernel Programmers:
3126 Chapter 13: Other Memory Models
3127
3128Intel Itanium Architecture Software Developer's Manual: Volume 1:
3129 Section 2.6: Speculation
3130 Section 4.4: Memory Access