1			 ============================
2			 LINUX KERNEL MEMORY BARRIERS
3			 ============================
4
5By: David Howells <dhowells@redhat.com>
6    Paul E. McKenney <paulmck@linux.ibm.com>
7    Will Deacon <will.deacon@arm.com>
8    Peter Zijlstra <peterz@infradead.org>
9
10==========
11DISCLAIMER
12==========
13
14This document is not a specification; it is intentionally (for the sake of
15brevity) and unintentionally (due to being human) incomplete. This document is
16meant as a guide to using the various memory barriers provided by Linux, but
17in case of any doubt (and there are many) please ask.  Some doubts may be
18resolved by referring to the formal memory consistency model and related
19documentation at tools/memory-model/.  Nevertheless, even this memory
20model should be viewed as the collective opinion of its maintainers rather
21than as an infallible oracle.
22
23To repeat, this document is not a specification of what Linux expects from
24hardware.
25
26The purpose of this document is twofold:
27
28 (1) to specify the minimum functionality that one can rely on for any
29     particular barrier, and
30
31 (2) to provide a guide as to how to use the barriers that are available.
32
33Note that an architecture can provide more than the minimum requirement
34for any particular barrier, but if the architecture provides less than
35that, that architecture is incorrect.
36
37Note also that it is possible that a barrier may be a no-op for an
38architecture because the way that arch works renders an explicit barrier
39unnecessary in that case.
40
41
42========
43CONTENTS
44========
45
46 (*) Abstract memory access model.
47
48     - Device operations.
49     - Guarantees.
50
51 (*) What are memory barriers?
52
53     - Varieties of memory barrier.
54     - What may not be assumed about memory barriers?
55     - Address-dependency barriers (historical).
56     - Control dependencies.
57     - SMP barrier pairing.
58     - Examples of memory barrier sequences.
59     - Read memory barriers vs load speculation.
60     - Multicopy atomicity.
61
62 (*) Explicit kernel barriers.
63
64     - Compiler barrier.
65     - CPU memory barriers.
66
67 (*) Implicit kernel memory barriers.
68
69     - Lock acquisition functions.
70     - Interrupt disabling functions.
71     - Sleep and wake-up functions.
72     - Miscellaneous functions.
73
74 (*) Inter-CPU acquiring barrier effects.
75
76     - Acquires vs memory accesses.
77
78 (*) Where are memory barriers needed?
79
80     - Interprocessor interaction.
81     - Atomic operations.
82     - Accessing devices.
83     - Interrupts.
84
85 (*) Kernel I/O barrier effects.
86
87 (*) Assumed minimum execution ordering model.
88
89 (*) The effects of the cpu cache.
90
91     - Cache coherency.
92     - Cache coherency vs DMA.
93     - Cache coherency vs MMIO.
94
95 (*) The things CPUs get up to.
96
97     - And then there's the Alpha.
98     - Virtual Machine Guests.
99
100 (*) Example uses.
101
102     - Circular buffers.
103
104 (*) References.
105
106
107============================
108ABSTRACT MEMORY ACCESS MODEL
109============================
110
111Consider the following abstract model of the system:
112
113		            :                :
114		            :                :
115		            :                :
116		+-------+   :   +--------+   :   +-------+
117		|       |   :   |        |   :   |       |
118		|       |   :   |        |   :   |       |
119		| CPU 1 |<----->| Memory |<----->| CPU 2 |
120		|       |   :   |        |   :   |       |
121		|       |   :   |        |   :   |       |
122		+-------+   :   +--------+   :   +-------+
123		    ^       :       ^        :       ^
124		    |       :       |        :       |
125		    |       :       |        :       |
126		    |       :       v        :       |
127		    |       :   +--------+   :       |
128		    |       :   |        |   :       |
129		    |       :   |        |   :       |
130		    +---------->| Device |<----------+
131		            :   |        |   :
132		            :   |        |   :
133		            :   +--------+   :
134		            :                :
135
136Each CPU executes a program that generates memory access operations.  In the
137abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
138perform the memory operations in any order it likes, provided program causality
139appears to be maintained.  Similarly, the compiler may also arrange the
140instructions it emits in any order it likes, provided it doesn't affect the
141apparent operation of the program.
142
143So in the above diagram, the effects of the memory operations performed by a
144CPU are perceived by the rest of the system as the operations cross the
145interface between the CPU and rest of the system (the dotted lines).
146
147
148For example, consider the following sequence of events:
149
150	CPU 1		CPU 2
151	===============	===============
152	{ A == 1; B == 2 }
153	A = 3;		x = B;
154	B = 4;		y = A;
155
156The set of accesses as seen by the memory system in the middle can be arranged
157in 24 different combinations:
158
159	STORE A=3,	STORE B=4,	y=LOAD A->3,	x=LOAD B->4
160	STORE A=3,	STORE B=4,	x=LOAD B->4,	y=LOAD A->3
161	STORE A=3,	y=LOAD A->3,	STORE B=4,	x=LOAD B->4
162	STORE A=3,	y=LOAD A->3,	x=LOAD B->2,	STORE B=4
163	STORE A=3,	x=LOAD B->2,	STORE B=4,	y=LOAD A->3
164	STORE A=3,	x=LOAD B->2,	y=LOAD A->3,	STORE B=4
165	STORE B=4,	STORE A=3,	y=LOAD A->3,	x=LOAD B->4
166	STORE B=4, ...
167	...
168
169and can thus result in four different combinations of values:
170
171	x == 2, y == 1
172	x == 2, y == 3
173	x == 4, y == 1
174	x == 4, y == 3
175
176
177Furthermore, the stores committed by a CPU to the memory system may not be
178perceived by the loads made by another CPU in the same order as the stores were
179committed.
180
181
182As a further example, consider this sequence of events:
183
184	CPU 1		CPU 2
185	===============	===============
186	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
187	B = 4;		Q = P;
188	P = &B;		D = *Q;
189
190There is an obvious address dependency here, as the value loaded into D depends
191on the address retrieved from P by CPU 2.  At the end of the sequence, any of
192the following results are possible:
193
194	(Q == &A) and (D == 1)
195	(Q == &B) and (D == 2)
196	(Q == &B) and (D == 4)
197
198Note that CPU 2 will never try and load C into D because the CPU will load P
199into Q before issuing the load of *Q.
200
201
202DEVICE OPERATIONS
203-----------------
204
205Some devices present their control interfaces as collections of memory
206locations, but the order in which the control registers are accessed is very
207important.  For instance, imagine an ethernet card with a set of internal
208registers that are accessed through an address port register (A) and a data
209port register (D).  To read internal register 5, the following code might then
210be used:
211
212	*A = 5;
213	x = *D;
214
215but this might show up as either of the following two sequences:
216
217	STORE *A = 5, x = LOAD *D
218	x = LOAD *D, STORE *A = 5
219
220the second of which will almost certainly result in a malfunction, since it set
221the address _after_ attempting to read the register.
222
223
224GUARANTEES
225----------
226
227There are some minimal guarantees that may be expected of a CPU:
228
229 (*) On any given CPU, dependent memory accesses will be issued in order, with
230     respect to itself.  This means that for:
231
232	Q = READ_ONCE(P); D = READ_ONCE(*Q);
233
234     the CPU will issue the following memory operations:
235
236	Q = LOAD P, D = LOAD *Q
237
238     and always in that order.  However, on DEC Alpha, READ_ONCE() also
239     emits a memory-barrier instruction, so that a DEC Alpha CPU will
240     instead issue the following memory operations:
241
242	Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER
243
244     Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler
245     mischief.
246
247 (*) Overlapping loads and stores within a particular CPU will appear to be
248     ordered within that CPU.  This means that for:
249
250	a = READ_ONCE(*X); WRITE_ONCE(*X, b);
251
252     the CPU will only issue the following sequence of memory operations:
253
254	a = LOAD *X, STORE *X = b
255
256     And for:
257
258	WRITE_ONCE(*X, c); d = READ_ONCE(*X);
259
260     the CPU will only issue:
261
262	STORE *X = c, d = LOAD *X
263
264     (Loads and stores overlap if they are targeted at overlapping pieces of
265     memory).
266
267And there are a number of things that _must_ or _must_not_ be assumed:
268
269 (*) It _must_not_ be assumed that the compiler will do what you want
270     with memory references that are not protected by READ_ONCE() and
271     WRITE_ONCE().  Without them, the compiler is within its rights to
272     do all sorts of "creative" transformations, which are covered in
273     the COMPILER BARRIER section.
274
275 (*) It _must_not_ be assumed that independent loads and stores will be issued
276     in the order given.  This means that for:
277
278	X = *A; Y = *B; *D = Z;
279
280     we may get any of the following sequences:
281
282	X = LOAD *A,  Y = LOAD *B,  STORE *D = Z
283	X = LOAD *A,  STORE *D = Z, Y = LOAD *B
284	Y = LOAD *B,  X = LOAD *A,  STORE *D = Z
285	Y = LOAD *B,  STORE *D = Z, X = LOAD *A
286	STORE *D = Z, X = LOAD *A,  Y = LOAD *B
287	STORE *D = Z, Y = LOAD *B,  X = LOAD *A
288
289 (*) It _must_ be assumed that overlapping memory accesses may be merged or
290     discarded.  This means that for:
291
292	X = *A; Y = *(A + 4);
293
294     we may get any one of the following sequences:
295
296	X = LOAD *A; Y = LOAD *(A + 4);
297	Y = LOAD *(A + 4); X = LOAD *A;
298	{X, Y} = LOAD {*A, *(A + 4) };
299
300     And for:
301
302	*A = X; *(A + 4) = Y;
303
304     we may get any of:
305
306	STORE *A = X; STORE *(A + 4) = Y;
307	STORE *(A + 4) = Y; STORE *A = X;
308	STORE {*A, *(A + 4) } = {X, Y};
309
310And there are anti-guarantees:
311
312 (*) These guarantees do not apply to bitfields, because compilers often
313     generate code to modify these using non-atomic read-modify-write
314     sequences.  Do not attempt to use bitfields to synchronize parallel
315     algorithms.
316
317 (*) Even in cases where bitfields are protected by locks, all fields
318     in a given bitfield must be protected by one lock.  If two fields
319     in a given bitfield are protected by different locks, the compiler's
320     non-atomic read-modify-write sequences can cause an update to one
321     field to corrupt the value of an adjacent field.
322
323 (*) These guarantees apply only to properly aligned and sized scalar
324     variables.  "Properly sized" currently means variables that are
325     the same size as "char", "short", "int" and "long".  "Properly
326     aligned" means the natural alignment, thus no constraints for
327     "char", two-byte alignment for "short", four-byte alignment for
328     "int", and either four-byte or eight-byte alignment for "long",
329     on 32-bit and 64-bit systems, respectively.  Note that these
330     guarantees were introduced into the C11 standard, so beware when
331     using older pre-C11 compilers (for example, gcc 4.6).  The portion
332     of the standard containing this guarantee is Section 3.14, which
333     defines "memory location" as follows:
334
335     	memory location
336		either an object of scalar type, or a maximal sequence
337		of adjacent bit-fields all having nonzero width
338
339		NOTE 1: Two threads of execution can update and access
340		separate memory locations without interfering with
341		each other.
342
343		NOTE 2: A bit-field and an adjacent non-bit-field member
344		are in separate memory locations. The same applies
345		to two bit-fields, if one is declared inside a nested
346		structure declaration and the other is not, or if the two
347		are separated by a zero-length bit-field declaration,
348		or if they are separated by a non-bit-field member
349		declaration. It is not safe to concurrently update two
350		bit-fields in the same structure if all members declared
351		between them are also bit-fields, no matter what the
352		sizes of those intervening bit-fields happen to be.
353
354
355=========================
356WHAT ARE MEMORY BARRIERS?
357=========================
358
359As can be seen above, independent memory operations are effectively performed
360in random order, but this can be a problem for CPU-CPU interaction and for I/O.
361What is required is some way of intervening to instruct the compiler and the
362CPU to restrict the order.
363
364Memory barriers are such interventions.  They impose a perceived partial
365ordering over the memory operations on either side of the barrier.
366
367Such enforcement is important because the CPUs and other devices in a system
368can use a variety of tricks to improve performance, including reordering,
369deferral and combination of memory operations; speculative loads; speculative
370branch prediction and various types of caching.  Memory barriers are used to
371override or suppress these tricks, allowing the code to sanely control the
372interaction of multiple CPUs and/or devices.
373
374
375VARIETIES OF MEMORY BARRIER
376---------------------------
377
378Memory barriers come in four basic varieties:
379
380 (1) Write (or store) memory barriers.
381
382     A write memory barrier gives a guarantee that all the STORE operations
383     specified before the barrier will appear to happen before all the STORE
384     operations specified after the barrier with respect to the other
385     components of the system.
386
387     A write barrier is a partial ordering on stores only; it is not required
388     to have any effect on loads.
389
390     A CPU can be viewed as committing a sequence of store operations to the
391     memory system as time progresses.  All stores _before_ a write barrier
392     will occur _before_ all the stores after the write barrier.
393
394     [!] Note that write barriers should normally be paired with read or
395     address-dependency barriers; see the "SMP barrier pairing" subsection.
396
397
398 (2) Address-dependency barriers (historical).
399
400     An address-dependency barrier is a weaker form of read barrier.  In the
401     case where two loads are performed such that the second depends on the
402     result of the first (eg: the first load retrieves the address to which
403     the second load will be directed), an address-dependency barrier would
404     be required to make sure that the target of the second load is updated
405     after the address obtained by the first load is accessed.
406
407     An address-dependency barrier is a partial ordering on interdependent
408     loads only; it is not required to have any effect on stores, independent
409     loads or overlapping loads.
410
411     As mentioned in (1), the other CPUs in the system can be viewed as
412     committing sequences of stores to the memory system that the CPU being
413     considered can then perceive.  An address-dependency barrier issued by
414     the CPU under consideration guarantees that for any load preceding it,
415     if that load touches one of a sequence of stores from another CPU, then
416     by the time the barrier completes, the effects of all the stores prior to
417     that touched by the load will be perceptible to any loads issued after
418     the address-dependency barrier.
419
420     See the "Examples of memory barrier sequences" subsection for diagrams
421     showing the ordering constraints.
422
423     [!] Note that the first load really has to have an _address_ dependency and
424     not a control dependency.  If the address for the second load is dependent
425     on the first load, but the dependency is through a conditional rather than
426     actually loading the address itself, then it's a _control_ dependency and
427     a full read barrier or better is required.  See the "Control dependencies"
428     subsection for more information.
429
430     [!] Note that address-dependency barriers should normally be paired with
431     write barriers; see the "SMP barrier pairing" subsection.
432
433     [!] Kernel release v5.9 removed kernel APIs for explicit address-
434     dependency barriers.  Nowadays, APIs for marking loads from shared
435     variables such as READ_ONCE() and rcu_dereference() provide implicit
436     address-dependency barriers.
437
438 (3) Read (or load) memory barriers.
439
440     A read barrier is an address-dependency barrier plus a guarantee that all
441     the LOAD operations specified before the barrier will appear to happen
442     before all the LOAD operations specified after the barrier with respect to
443     the other components of the system.
444
445     A read barrier is a partial ordering on loads only; it is not required to
446     have any effect on stores.
447
448     Read memory barriers imply address-dependency barriers, and so can
449     substitute for them.
450
451     [!] Note that read barriers should normally be paired with write barriers;
452     see the "SMP barrier pairing" subsection.
453
454
455 (4) General memory barriers.
456
457     A general memory barrier gives a guarantee that all the LOAD and STORE
458     operations specified before the barrier will appear to happen before all
459     the LOAD and STORE operations specified after the barrier with respect to
460     the other components of the system.
461
462     A general memory barrier is a partial ordering over both loads and stores.
463
464     General memory barriers imply both read and write memory barriers, and so
465     can substitute for either.
466
467
468And a couple of implicit varieties:
469
470 (5) ACQUIRE operations.
471
472     This acts as a one-way permeable barrier.  It guarantees that all memory
473     operations after the ACQUIRE operation will appear to happen after the
474     ACQUIRE operation with respect to the other components of the system.
475     ACQUIRE operations include LOCK operations and both smp_load_acquire()
476     and smp_cond_load_acquire() operations.
477
478     Memory operations that occur before an ACQUIRE operation may appear to
479     happen after it completes.
480
481     An ACQUIRE operation should almost always be paired with a RELEASE
482     operation.
483
484
485 (6) RELEASE operations.
486
487     This also acts as a one-way permeable barrier.  It guarantees that all
488     memory operations before the RELEASE operation will appear to happen
489     before the RELEASE operation with respect to the other components of the
490     system. RELEASE operations include UNLOCK operations and
491     smp_store_release() operations.
492
493     Memory operations that occur after a RELEASE operation may appear to
494     happen before it completes.
495
496     The use of ACQUIRE and RELEASE operations generally precludes the need
497     for other sorts of memory barrier.  In addition, a RELEASE+ACQUIRE pair is
498     -not- guaranteed to act as a full memory barrier.  However, after an
499     ACQUIRE on a given variable, all memory accesses preceding any prior
500     RELEASE on that same variable are guaranteed to be visible.  In other
501     words, within a given variable's critical section, all accesses of all
502     previous critical sections for that variable are guaranteed to have
503     completed.
504
505     This means that ACQUIRE acts as a minimal "acquire" operation and
506     RELEASE acts as a minimal "release" operation.
507
508A subset of the atomic operations described in atomic_t.txt have ACQUIRE and
509RELEASE variants in addition to fully-ordered and relaxed (no barrier
510semantics) definitions.  For compound atomics performing both a load and a
511store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
512only to the store portion of the operation.
513
514Memory barriers are only required where there's a possibility of interaction
515between two CPUs or between a CPU and a device.  If it can be guaranteed that
516there won't be any such interaction in any particular piece of code, then
517memory barriers are unnecessary in that piece of code.
518
519
520Note that these are the _minimum_ guarantees.  Different architectures may give
521more substantial guarantees, but they may _not_ be relied upon outside of arch
522specific code.
523
524
525WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
526----------------------------------------------
527
528There are certain things that the Linux kernel memory barriers do not guarantee:
529
530 (*) There is no guarantee that any of the memory accesses specified before a
531     memory barrier will be _complete_ by the completion of a memory barrier
532     instruction; the barrier can be considered to draw a line in that CPU's
533     access queue that accesses of the appropriate type may not cross.
534
535 (*) There is no guarantee that issuing a memory barrier on one CPU will have
536     any direct effect on another CPU or any other hardware in the system.  The
537     indirect effect will be the order in which the second CPU sees the effects
538     of the first CPU's accesses occur, but see the next point:
539
540 (*) There is no guarantee that a CPU will see the correct order of effects
541     from a second CPU's accesses, even _if_ the second CPU uses a memory
542     barrier, unless the first CPU _also_ uses a matching memory barrier (see
543     the subsection on "SMP Barrier Pairing").
544
545 (*) There is no guarantee that some intervening piece of off-the-CPU
546     hardware[*] will not reorder the memory accesses.  CPU cache coherency
547     mechanisms should propagate the indirect effects of a memory barrier
548     between CPUs, but might not do so in order.
549
550	[*] For information on bus mastering DMA and coherency please read:
551
552	    Documentation/driver-api/pci/pci.rst
553	    Documentation/core-api/dma-api-howto.rst
554	    Documentation/core-api/dma-api.rst
555
556
557ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
558----------------------------------------
559
560As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
561DEC Alpha, which means that about the only people who need to pay attention
562to this section are those working on DEC Alpha architecture-specific code
563and those working on READ_ONCE() itself.  For those who need it, and for
564those who are interested in the history, here is the story of
565address-dependency barriers.
566
567[!] While address dependencies are observed in both load-to-load and
568load-to-store relations, address-dependency barriers are not necessary
569for load-to-store situations.
570
571The requirement of address-dependency barriers is a little subtle, and
572it's not always obvious that they're needed.  To illustrate, consider the
573following sequence of events:
574
575	CPU 1		      CPU 2
576	===============	      ===============
577	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
578	B = 4;
579	<write barrier>
580	WRITE_ONCE(P, &B);
581			      Q = READ_ONCE_OLD(P);
582			      D = *Q;
583
584[!] READ_ONCE_OLD() corresponds to READ_ONCE() of pre-4.15 kernel, which
585doesn't imply an address-dependency barrier.
586
587There's a clear address dependency here, and it would seem that by the end of
588the sequence, Q must be either &A or &B, and that:
589
590	(Q == &A) implies (D == 1)
591	(Q == &B) implies (D == 4)
592
593But!  CPU 2's perception of P may be updated _before_ its perception of B, thus
594leading to the following situation:
595
596	(Q == &B) and (D == 2) ????
597
598While this may seem like a failure of coherency or causality maintenance, it
599isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
600Alpha).
601
602To deal with this, READ_ONCE() provides an implicit address-dependency barrier
603since kernel release v4.15:
604
605	CPU 1		      CPU 2
606	===============	      ===============
607	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
608	B = 4;
609	<write barrier>
610	WRITE_ONCE(P, &B);
611			      Q = READ_ONCE(P);
612			      <implicit address-dependency barrier>
613			      D = *Q;
614
615This enforces the occurrence of one of the two implications, and prevents the
616third possibility from arising.
617
618
619[!] Note that this extremely counterintuitive situation arises most easily on
620machines with split caches, so that, for example, one cache bank processes
621even-numbered cache lines and the other bank processes odd-numbered cache
622lines.  The pointer P might be stored in an odd-numbered cache line, and the
623variable B might be stored in an even-numbered cache line.  Then, if the
624even-numbered bank of the reading CPU's cache is extremely busy while the
625odd-numbered bank is idle, one can see the new value of the pointer P (&B),
626but the old value of the variable B (2).
627
628
629An address-dependency barrier is not required to order dependent writes
630because the CPUs that the Linux kernel supports don't do writes until they
631are certain (1) that the write will actually happen, (2) of the location of
632the write, and (3) of the value to be written.
633But please carefully read the "CONTROL DEPENDENCIES" section and the
634Documentation/RCU/rcu_dereference.rst file:  The compiler can and does break
635dependencies in a great many highly creative ways.
636
637	CPU 1		      CPU 2
638	===============	      ===============
639	{ A == 1, B == 2, C = 3, P == &A, Q == &C }
640	B = 4;
641	<write barrier>
642	WRITE_ONCE(P, &B);
643			      Q = READ_ONCE_OLD(P);
644			      WRITE_ONCE(*Q, 5);
645
646Therefore, no address-dependency barrier is required to order the read into
647Q with the store into *Q.  In other words, this outcome is prohibited,
648even without an implicit address-dependency barrier of modern READ_ONCE():
649
650	(Q == &B) && (B == 4)
651
652Please note that this pattern should be rare.  After all, the whole point
653of dependency ordering is to -prevent- writes to the data structure, along
654with the expensive cache misses associated with those writes.  This pattern
655can be used to record rare error conditions and the like, and the CPUs'
656naturally occurring ordering prevents such records from being lost.
657
658
659Note well that the ordering provided by an address dependency is local to
660the CPU containing it.  See the section on "Multicopy atomicity" for
661more information.
662
663
664The address-dependency barrier is very important to the RCU system,
665for example.  See rcu_assign_pointer() and rcu_dereference() in
666include/linux/rcupdate.h.  This permits the current target of an RCU'd
667pointer to be replaced with a new modified target, without the replacement
668target appearing to be incompletely initialised.
669
670See also the subsection on "Cache Coherency" for a more thorough example.
671
672
673CONTROL DEPENDENCIES
674--------------------
675
676Control dependencies can be a bit tricky because current compilers do
677not understand them.  The purpose of this section is to help you prevent
678the compiler's ignorance from breaking your code.
679
680A load-load control dependency requires a full read memory barrier, not
681simply an (implicit) address-dependency barrier to make it work correctly.
682Consider the following bit of code:
683
684	q = READ_ONCE(a);
685	<implicit address-dependency barrier>
686	if (q) {
687		/* BUG: No address dependency!!! */
688		p = READ_ONCE(b);
689	}
690
691This will not have the desired effect because there is no actual address
692dependency, but rather a control dependency that the CPU may short-circuit
693by attempting to predict the outcome in advance, so that other CPUs see
694the load from b as having happened before the load from a.  In such a case
695what's actually required is:
696
697	q = READ_ONCE(a);
698	if (q) {
699		<read barrier>
700		p = READ_ONCE(b);
701	}
702
703However, stores are not speculated.  This means that ordering -is- provided
704for load-store control dependencies, as in the following example:
705
706	q = READ_ONCE(a);
707	if (q) {
708		WRITE_ONCE(b, 1);
709	}
710
711Control dependencies pair normally with other types of barriers.
712That said, please note that neither READ_ONCE() nor WRITE_ONCE()
713are optional! Without the READ_ONCE(), the compiler might combine the
714load from 'a' with other loads from 'a'.  Without the WRITE_ONCE(),
715the compiler might combine the store to 'b' with other stores to 'b'.
716Either can result in highly counterintuitive effects on ordering.
717
718Worse yet, if the compiler is able to prove (say) that the value of
719variable 'a' is always non-zero, it would be well within its rights
720to optimize the original example by eliminating the "if" statement
721as follows:
722
723	q = a;
724	b = 1;  /* BUG: Compiler and CPU can both reorder!!! */
725
726So don't leave out the READ_ONCE().
727
728It is tempting to try to enforce ordering on identical stores on both
729branches of the "if" statement as follows:
730
731	q = READ_ONCE(a);
732	if (q) {
733		barrier();
734		WRITE_ONCE(b, 1);
735		do_something();
736	} else {
737		barrier();
738		WRITE_ONCE(b, 1);
739		do_something_else();
740	}
741
742Unfortunately, current compilers will transform this as follows at high
743optimization levels:
744
745	q = READ_ONCE(a);
746	barrier();
747	WRITE_ONCE(b, 1);  /* BUG: No ordering vs. load from a!!! */
748	if (q) {
749		/* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
750		do_something();
751	} else {
752		/* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
753		do_something_else();
754	}
755
756Now there is no conditional between the load from 'a' and the store to
757'b', which means that the CPU is within its rights to reorder them:
758The conditional is absolutely required, and must be present in the
759assembly code even after all compiler optimizations have been applied.
760Therefore, if you need ordering in this example, you need explicit
761memory barriers, for example, smp_store_release():
762
763	q = READ_ONCE(a);
764	if (q) {
765		smp_store_release(&b, 1);
766		do_something();
767	} else {
768		smp_store_release(&b, 1);
769		do_something_else();
770	}
771
772In contrast, without explicit memory barriers, two-legged-if control
773ordering is guaranteed only when the stores differ, for example:
774
775	q = READ_ONCE(a);
776	if (q) {
777		WRITE_ONCE(b, 1);
778		do_something();
779	} else {
780		WRITE_ONCE(b, 2);
781		do_something_else();
782	}
783
784The initial READ_ONCE() is still required to prevent the compiler from
785proving the value of 'a'.
786
787In addition, you need to be careful what you do with the local variable 'q',
788otherwise the compiler might be able to guess the value and again remove
789the needed conditional.  For example:
790
791	q = READ_ONCE(a);
792	if (q % MAX) {
793		WRITE_ONCE(b, 1);
794		do_something();
795	} else {
796		WRITE_ONCE(b, 2);
797		do_something_else();
798	}
799
800If MAX is defined to be 1, then the compiler knows that (q % MAX) is
801equal to zero, in which case the compiler is within its rights to
802transform the above code into the following:
803
804	q = READ_ONCE(a);
805	WRITE_ONCE(b, 2);
806	do_something_else();
807
808Given this transformation, the CPU is not required to respect the ordering
809between the load from variable 'a' and the store to variable 'b'.  It is
810tempting to add a barrier(), but this does not help.  The conditional
811is gone, and the barrier won't bring it back.  Therefore, if you are
812relying on this ordering, you should make sure that MAX is greater than
813one, perhaps as follows:
814
815	q = READ_ONCE(a);
816	BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
817	if (q % MAX) {
818		WRITE_ONCE(b, 1);
819		do_something();
820	} else {
821		WRITE_ONCE(b, 2);
822		do_something_else();
823	}
824
825Please note once again that the stores to 'b' differ.  If they were
826identical, as noted earlier, the compiler could pull this store outside
827of the 'if' statement.
828
829You must also be careful not to rely too much on boolean short-circuit
830evaluation.  Consider this example:
831
832	q = READ_ONCE(a);
833	if (q || 1 > 0)
834		WRITE_ONCE(b, 1);
835
836Because the first condition cannot fault and the second condition is
837always true, the compiler can transform this example as following,
838defeating control dependency:
839
840	q = READ_ONCE(a);
841	WRITE_ONCE(b, 1);
842
843This example underscores the need to ensure that the compiler cannot
844out-guess your code.  More generally, although READ_ONCE() does force
845the compiler to actually emit code for a given load, it does not force
846the compiler to use the results.
847
848In addition, control dependencies apply only to the then-clause and
849else-clause of the if-statement in question.  In particular, it does
850not necessarily apply to code following the if-statement:
851
852	q = READ_ONCE(a);
853	if (q) {
854		WRITE_ONCE(b, 1);
855	} else {
856		WRITE_ONCE(b, 2);
857	}
858	WRITE_ONCE(c, 1);  /* BUG: No ordering against the read from 'a'. */
859
860It is tempting to argue that there in fact is ordering because the
861compiler cannot reorder volatile accesses and also cannot reorder
862the writes to 'b' with the condition.  Unfortunately for this line
863of reasoning, the compiler might compile the two writes to 'b' as
864conditional-move instructions, as in this fanciful pseudo-assembly
865language:
866
867	ld r1,a
868	cmp r1,$0
869	cmov,ne r4,$1
870	cmov,eq r4,$2
871	st r4,b
872	st $1,c
873
874A weakly ordered CPU would have no dependency of any sort between the load
875from 'a' and the store to 'c'.  The control dependencies would extend
876only to the pair of cmov instructions and the store depending on them.
877In short, control dependencies apply only to the stores in the then-clause
878and else-clause of the if-statement in question (including functions
879invoked by those two clauses), not to code following that if-statement.
880
881
882Note well that the ordering provided by a control dependency is local
883to the CPU containing it.  See the section on "Multicopy atomicity"
884for more information.
885
886
887In summary:
888
889  (*) Control dependencies can order prior loads against later stores.
890      However, they do -not- guarantee any other sort of ordering:
891      Not prior loads against later loads, nor prior stores against
892      later anything.  If you need these other forms of ordering,
893      use smp_rmb(), smp_wmb(), or, in the case of prior stores and
894      later loads, smp_mb().
895
896  (*) If both legs of the "if" statement begin with identical stores to
897      the same variable, then those stores must be ordered, either by
898      preceding both of them with smp_mb() or by using smp_store_release()
899      to carry out the stores.  Please note that it is -not- sufficient
900      to use barrier() at beginning of each leg of the "if" statement
901      because, as shown by the example above, optimizing compilers can
902      destroy the control dependency while respecting the letter of the
903      barrier() law.
904
905  (*) Control dependencies require at least one run-time conditional
906      between the prior load and the subsequent store, and this
907      conditional must involve the prior load.  If the compiler is able
908      to optimize the conditional away, it will have also optimized
909      away the ordering.  Careful use of READ_ONCE() and WRITE_ONCE()
910      can help to preserve the needed conditional.
911
912  (*) Control dependencies require that the compiler avoid reordering the
913      dependency into nonexistence.  Careful use of READ_ONCE() or
914      atomic{,64}_read() can help to preserve your control dependency.
915      Please see the COMPILER BARRIER section for more information.
916
917  (*) Control dependencies apply only to the then-clause and else-clause
918      of the if-statement containing the control dependency, including
919      any functions that these two clauses call.  Control dependencies
920      do -not- apply to code following the if-statement containing the
921      control dependency.
922
923  (*) Control dependencies pair normally with other types of barriers.
924
925  (*) Control dependencies do -not- provide multicopy atomicity.  If you
926      need all the CPUs to see a given store at the same time, use smp_mb().
927
928  (*) Compilers do not understand control dependencies.  It is therefore
929      your job to ensure that they do not break your code.
930
931
932SMP BARRIER PAIRING
933-------------------
934
935When dealing with CPU-CPU interactions, certain types of memory barrier should
936always be paired.  A lack of appropriate pairing is almost certainly an error.
937
938General barriers pair with each other, though they also pair with most
939other types of barriers, albeit without multicopy atomicity.  An acquire
940barrier pairs with a release barrier, but both may also pair with other
941barriers, including of course general barriers.  A write barrier pairs
942with an address-dependency barrier, a control dependency, an acquire barrier,
943a release barrier, a read barrier, or a general barrier.  Similarly a
944read barrier, control dependency, or an address-dependency barrier pairs
945with a write barrier, an acquire barrier, a release barrier, or a
946general barrier:
947
948	CPU 1		      CPU 2
949	===============	      ===============
950	WRITE_ONCE(a, 1);
951	<write barrier>
952	WRITE_ONCE(b, 2);     x = READ_ONCE(b);
953			      <read barrier>
954			      y = READ_ONCE(a);
955
956Or:
957
958	CPU 1		      CPU 2
959	===============	      ===============================
960	a = 1;
961	<write barrier>
962	WRITE_ONCE(b, &a);    x = READ_ONCE(b);
963			      <implicit address-dependency barrier>
964			      y = *x;
965
966Or even:
967
968	CPU 1		      CPU 2
969	===============	      ===============================
970	r1 = READ_ONCE(y);
971	<general barrier>
972	WRITE_ONCE(x, 1);     if (r2 = READ_ONCE(x)) {
973			         <implicit control dependency>
974			         WRITE_ONCE(y, 1);
975			      }
976
977	assert(r1 == 0 || r2 == 0);
978
979Basically, the read barrier always has to be there, even though it can be of
980the "weaker" type.
981
982[!] Note that the stores before the write barrier would normally be expected to
983match the loads after the read barrier or the address-dependency barrier, and
984vice versa:
985
986	CPU 1                               CPU 2
987	===================                 ===================
988	WRITE_ONCE(a, 1);    }----   --->{  v = READ_ONCE(c);
989	WRITE_ONCE(b, 2);    }    \ /    {  w = READ_ONCE(d);
990	<write barrier>            \        <read barrier>
991	WRITE_ONCE(c, 3);    }    / \    {  x = READ_ONCE(a);
992	WRITE_ONCE(d, 4);    }----   --->{  y = READ_ONCE(b);
993
994
995EXAMPLES OF MEMORY BARRIER SEQUENCES
996------------------------------------
997
998Firstly, write barriers act as partial orderings on store operations.
999Consider the following sequence of events:
1000
1001	CPU 1
1002	=======================
1003	STORE A = 1
1004	STORE B = 2
1005	STORE C = 3
1006	<write barrier>
1007	STORE D = 4
1008	STORE E = 5
1009
1010This sequence of events is committed to the memory coherence system in an order
1011that the rest of the system might perceive as the unordered set of { STORE A,
1012STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
1013}:
1014
1015	+-------+       :      :
1016	|       |       +------+
1017	|       |------>| C=3  |     }     /\
1018	|       |  :    +------+     }-----  \  -----> Events perceptible to
1019	|       |  :    | A=1  |     }        \/       the rest of the system
1020	|       |  :    +------+     }
1021	| CPU 1 |  :    | B=2  |     }
1022	|       |       +------+     }
1023	|       |   wwwwwwwwwwwwwwww }   <--- At this point the write barrier
1024	|       |       +------+     }        requires all stores prior to the
1025	|       |  :    | E=5  |     }        barrier to be committed before
1026	|       |  :    +------+     }        further stores may take place
1027	|       |------>| D=4  |     }
1028	|       |       +------+
1029	+-------+       :      :
1030	                   |
1031	                   | Sequence in which stores are committed to the
1032	                   | memory system by CPU 1
1033	                   V
1034
1035
1036Secondly, address-dependency barriers act as partial orderings on address-
1037dependent loads.  Consider the following sequence of events:
1038
1039	CPU 1			CPU 2
1040	=======================	=======================
1041		{ B = 7; X = 9; Y = 8; C = &Y }
1042	STORE A = 1
1043	STORE B = 2
1044	<write barrier>
1045	STORE C = &B		LOAD X
1046	STORE D = 4		LOAD C (gets &B)
1047				LOAD *C (reads B)
1048
1049Without intervention, CPU 2 may perceive the events on CPU 1 in some
1050effectively random order, despite the write barrier issued by CPU 1:
1051
1052	+-------+       :      :                :       :
1053	|       |       +------+                +-------+  | Sequence of update
1054	|       |------>| B=2  |-----       --->| Y->8  |  | of perception on
1055	|       |  :    +------+     \          +-------+  | CPU 2
1056	| CPU 1 |  :    | A=1  |      \     --->| C->&Y |  V
1057	|       |       +------+       |        +-------+
1058	|       |   wwwwwwwwwwwwwwww   |        :       :
1059	|       |       +------+       |        :       :
1060	|       |  :    | C=&B |---    |        :       :       +-------+
1061	|       |  :    +------+   \   |        +-------+       |       |
1062	|       |------>| D=4  |    ----------->| C->&B |------>|       |
1063	|       |       +------+       |        +-------+       |       |
1064	+-------+       :      :       |        :       :       |       |
1065	                               |        :       :       |       |
1066	                               |        :       :       | CPU 2 |
1067	                               |        +-------+       |       |
1068	    Apparently incorrect --->  |        | B->7  |------>|       |
1069	    perception of B (!)        |        +-------+       |       |
1070	                               |        :       :       |       |
1071	                               |        +-------+       |       |
1072	    The load of X holds --->    \       | X->9  |------>|       |
1073	    up the maintenance           \      +-------+       |       |
1074	    of coherence of B             ----->| B->2  |       +-------+
1075	                                        +-------+
1076	                                        :       :
1077
1078
1079In the above example, CPU 2 perceives that B is 7, despite the load of *C
1080(which would be B) coming after the LOAD of C.
1081
1082If, however, an address-dependency barrier were to be placed between the load
1083of C and the load of *C (ie: B) on CPU 2:
1084
1085	CPU 1			CPU 2
1086	=======================	=======================
1087		{ B = 7; X = 9; Y = 8; C = &Y }
1088	STORE A = 1
1089	STORE B = 2
1090	<write barrier>
1091	STORE C = &B		LOAD X
1092	STORE D = 4		LOAD C (gets &B)
1093				<address-dependency barrier>
1094				LOAD *C (reads B)
1095
1096then the following will occur:
1097
1098	+-------+       :      :                :       :
1099	|       |       +------+                +-------+
1100	|       |------>| B=2  |-----       --->| Y->8  |
1101	|       |  :    +------+     \          +-------+
1102	| CPU 1 |  :    | A=1  |      \     --->| C->&Y |
1103	|       |       +------+       |        +-------+
1104	|       |   wwwwwwwwwwwwwwww   |        :       :
1105	|       |       +------+       |        :       :
1106	|       |  :    | C=&B |---    |        :       :       +-------+
1107	|       |  :    +------+   \   |        +-------+       |       |
1108	|       |------>| D=4  |    ----------->| C->&B |------>|       |
1109	|       |       +------+       |        +-------+       |       |
1110	+-------+       :      :       |        :       :       |       |
1111	                               |        :       :       |       |
1112	                               |        :       :       | CPU 2 |
1113	                               |        +-------+       |       |
1114	                               |        | X->9  |------>|       |
1115	                               |        +-------+       |       |
1116	  Makes sure all effects --->   \   aaaaaaaaaaaaaaaaa   |       |
1117	  prior to the store of C        \      +-------+       |       |
1118	  are perceptible to              ----->| B->2  |------>|       |
1119	  subsequent loads                      +-------+       |       |
1120	                                        :       :       +-------+
1121
1122
1123And thirdly, a read barrier acts as a partial order on loads.  Consider the
1124following sequence of events:
1125
1126	CPU 1			CPU 2
1127	=======================	=======================
1128		{ A = 0, B = 9 }
1129	STORE A=1
1130	<write barrier>
1131	STORE B=2
1132				LOAD B
1133				LOAD A
1134
1135Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
1136some effectively random order, despite the write barrier issued by CPU 1:
1137
1138	+-------+       :      :                :       :
1139	|       |       +------+                +-------+
1140	|       |------>| A=1  |------      --->| A->0  |
1141	|       |       +------+      \         +-------+
1142	| CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1143	|       |       +------+        |       +-------+
1144	|       |------>| B=2  |---     |       :       :
1145	|       |       +------+   \    |       :       :       +-------+
1146	+-------+       :      :    \   |       +-------+       |       |
1147	                             ---------->| B->2  |------>|       |
1148	                                |       +-------+       | CPU 2 |
1149	                                |       | A->0  |------>|       |
1150	                                |       +-------+       |       |
1151	                                |       :       :       +-------+
1152	                                 \      :       :
1153	                                  \     +-------+
1154	                                   ---->| A->1  |
1155	                                        +-------+
1156	                                        :       :
1157
1158
1159If, however, a read barrier were to be placed between the load of B and the
1160load of A on CPU 2:
1161
1162	CPU 1			CPU 2
1163	=======================	=======================
1164		{ A = 0, B = 9 }
1165	STORE A=1
1166	<write barrier>
1167	STORE B=2
1168				LOAD B
1169				<read barrier>
1170				LOAD A
1171
1172then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
11732:
1174
1175	+-------+       :      :                :       :
1176	|       |       +------+                +-------+
1177	|       |------>| A=1  |------      --->| A->0  |
1178	|       |       +------+      \         +-------+
1179	| CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1180	|       |       +------+        |       +-------+
1181	|       |------>| B=2  |---     |       :       :
1182	|       |       +------+   \    |       :       :       +-------+
1183	+-------+       :      :    \   |       +-------+       |       |
1184	                             ---------->| B->2  |------>|       |
1185	                                |       +-------+       | CPU 2 |
1186	                                |       :       :       |       |
1187	                                |       :       :       |       |
1188	  At this point the read ---->   \  rrrrrrrrrrrrrrrrr   |       |
1189	  barrier causes all effects      \     +-------+       |       |
1190	  prior to the storage of B        ---->| A->1  |------>|       |
1191	  to be perceptible to CPU 2            +-------+       |       |
1192	                                        :       :       +-------+
1193
1194
1195To illustrate this more completely, consider what could happen if the code
1196contained a load of A either side of the read barrier:
1197
1198	CPU 1			CPU 2
1199	=======================	=======================
1200		{ A = 0, B = 9 }
1201	STORE A=1
1202	<write barrier>
1203	STORE B=2
1204				LOAD B
1205				LOAD A [first load of A]
1206				<read barrier>
1207				LOAD A [second load of A]
1208
1209Even though the two loads of A both occur after the load of B, they may both
1210come up with different values:
1211
1212	+-------+       :      :                :       :
1213	|       |       +------+                +-------+
1214	|       |------>| A=1  |------      --->| A->0  |
1215	|       |       +------+      \         +-------+
1216	| CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1217	|       |       +------+        |       +-------+
1218	|       |------>| B=2  |---     |       :       :
1219	|       |       +------+   \    |       :       :       +-------+
1220	+-------+       :      :    \   |       +-------+       |       |
1221	                             ---------->| B->2  |------>|       |
1222	                                |       +-------+       | CPU 2 |
1223	                                |       :       :       |       |
1224	                                |       :       :       |       |
1225	                                |       +-------+       |       |
1226	                                |       | A->0  |------>| 1st   |
1227	                                |       +-------+       |       |
1228	  At this point the read ---->   \  rrrrrrrrrrrrrrrrr   |       |
1229	  barrier causes all effects      \     +-------+       |       |
1230	  prior to the storage of B        ---->| A->1  |------>| 2nd   |
1231	  to be perceptible to CPU 2            +-------+       |       |
1232	                                        :       :       +-------+
1233
1234
1235But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1236before the read barrier completes anyway:
1237
1238	+-------+       :      :                :       :
1239	|       |       +------+                +-------+
1240	|       |------>| A=1  |------      --->| A->0  |
1241	|       |       +------+      \         +-------+
1242	| CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1243	|       |       +------+        |       +-------+
1244	|       |------>| B=2  |---     |       :       :
1245	|       |       +------+   \    |       :       :       +-------+
1246	+-------+       :      :    \   |       +-------+       |       |
1247	                             ---------->| B->2  |------>|       |
1248	                                |       +-------+       | CPU 2 |
1249	                                |       :       :       |       |
1250	                                 \      :       :       |       |
1251	                                  \     +-------+       |       |
1252	                                   ---->| A->1  |------>| 1st   |
1253	                                        +-------+       |       |
1254	                                    rrrrrrrrrrrrrrrrr   |       |
1255	                                        +-------+       |       |
1256	                                        | A->1  |------>| 2nd   |
1257	                                        +-------+       |       |
1258	                                        :       :       +-------+
1259
1260
1261The guarantee is that the second load will always come up with A == 1 if the
1262load of B came up with B == 2.  No such guarantee exists for the first load of
1263A; that may come up with either A == 0 or A == 1.
1264
1265
1266READ MEMORY BARRIERS VS LOAD SPECULATION
1267----------------------------------------
1268
1269Many CPUs speculate with loads: that is they see that they will need to load an
1270item from memory, and they find a time where they're not using the bus for any
1271other loads, and so do the load in advance - even though they haven't actually
1272got to that point in the instruction execution flow yet.  This permits the
1273actual load instruction to potentially complete immediately because the CPU
1274already has the value to hand.
1275
1276It may turn out that the CPU didn't actually need the value - perhaps because a
1277branch circumvented the load - in which case it can discard the value or just
1278cache it for later use.
1279
1280Consider:
1281
1282	CPU 1			CPU 2
1283	=======================	=======================
1284				LOAD B
1285				DIVIDE		} Divide instructions generally
1286				DIVIDE		} take a long time to perform
1287				LOAD A
1288
1289Which might appear as this:
1290
1291	                                        :       :       +-------+
1292	                                        +-------+       |       |
1293	                                    --->| B->2  |------>|       |
1294	                                        +-------+       | CPU 2 |
1295	                                        :       :DIVIDE |       |
1296	                                        +-------+       |       |
1297	The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1298	division speculates on the              +-------+   ~   |       |
1299	LOAD of A                               :       :   ~   |       |
1300	                                        :       :DIVIDE |       |
1301	                                        :       :   ~   |       |
1302	Once the divisions are complete -->     :       :   ~-->|       |
1303	the CPU can then perform the            :       :       |       |
1304	LOAD with immediate effect              :       :       +-------+
1305
1306
1307Placing a read barrier or an address-dependency barrier just before the second
1308load:
1309
1310	CPU 1			CPU 2
1311	=======================	=======================
1312				LOAD B
1313				DIVIDE
1314				DIVIDE
1315				<read barrier>
1316				LOAD A
1317
1318will force any value speculatively obtained to be reconsidered to an extent
1319dependent on the type of barrier used.  If there was no change made to the
1320speculated memory location, then the speculated value will just be used:
1321
1322	                                        :       :       +-------+
1323	                                        +-------+       |       |
1324	                                    --->| B->2  |------>|       |
1325	                                        +-------+       | CPU 2 |
1326	                                        :       :DIVIDE |       |
1327	                                        +-------+       |       |
1328	The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1329	division speculates on the              +-------+   ~   |       |
1330	LOAD of A                               :       :   ~   |       |
1331	                                        :       :DIVIDE |       |
1332	                                        :       :   ~   |       |
1333	                                        :       :   ~   |       |
1334	                                    rrrrrrrrrrrrrrrr~   |       |
1335	                                        :       :   ~   |       |
1336	                                        :       :   ~-->|       |
1337	                                        :       :       |       |
1338	                                        :       :       +-------+
1339
1340
1341but if there was an update or an invalidation from another CPU pending, then
1342the speculation will be cancelled and the value reloaded:
1343
1344	                                        :       :       +-------+
1345	                                        +-------+       |       |
1346	                                    --->| B->2  |------>|       |
1347	                                        +-------+       | CPU 2 |
1348	                                        :       :DIVIDE |       |
1349	                                        +-------+       |       |
1350	The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1351	division speculates on the              +-------+   ~   |       |
1352	LOAD of A                               :       :   ~   |       |
1353	                                        :       :DIVIDE |       |
1354	                                        :       :   ~   |       |
1355	                                        :       :   ~   |       |
1356	                                    rrrrrrrrrrrrrrrrr   |       |
1357	                                        +-------+       |       |
1358	The speculation is discarded --->   --->| A->1  |------>|       |
1359	and an updated value is                 +-------+       |       |
1360	retrieved                               :       :       +-------+
1361
1362
1363MULTICOPY ATOMICITY
1364--------------------
1365
1366Multicopy atomicity is a deeply intuitive notion about ordering that is
1367not always provided by real computer systems, namely that a given store
1368becomes visible at the same time to all CPUs, or, alternatively, that all
1369CPUs agree on the order in which all stores become visible.  However,
1370support of full multicopy atomicity would rule out valuable hardware
1371optimizations, so a weaker form called ``other multicopy atomicity''
1372instead guarantees only that a given store becomes visible at the same
1373time to all -other- CPUs.  The remainder of this document discusses this
1374weaker form, but for brevity will call it simply ``multicopy atomicity''.
1375
1376The following example demonstrates multicopy atomicity:
1377
1378	CPU 1			CPU 2			CPU 3
1379	=======================	=======================	=======================
1380		{ X = 0, Y = 0 }
1381	STORE X=1		r1=LOAD X (reads 1)	LOAD Y (reads 1)
1382				<general barrier>	<read barrier>
1383				STORE Y=r1		LOAD X
1384
1385Suppose that CPU 2's load from X returns 1, which it then stores to Y,
1386and CPU 3's load from Y returns 1.  This indicates that CPU 1's store
1387to X precedes CPU 2's load from X and that CPU 2's store to Y precedes
1388CPU 3's load from Y.  In addition, the memory barriers guarantee that
1389CPU 2 executes its load before its store, and CPU 3 loads from Y before
1390it loads from X.  The question is then "Can CPU 3's load from X return 0?"
1391
1392Because CPU 3's load from X in some sense comes after CPU 2's load, it
1393is natural to expect that CPU 3's load from X must therefore return 1.
1394This expectation follows from multicopy atomicity: if a load executing
1395on CPU B follows a load from the same variable executing on CPU A (and
1396CPU A did not originally store the value which it read), then on
1397multicopy-atomic systems, CPU B's load must return either the same value
1398that CPU A's load did or some later value.  However, the Linux kernel
1399does not require systems to be multicopy atomic.
1400
1401The use of a general memory barrier in the example above compensates
1402for any lack of multicopy atomicity.  In the example, if CPU 2's load
1403from X returns 1 and CPU 3's load from Y returns 1, then CPU 3's load
1404from X must indeed also return 1.
1405
1406However, dependencies, read barriers, and write barriers are not always
1407able to compensate for non-multicopy atomicity.  For example, suppose
1408that CPU 2's general barrier is removed from the above example, leaving
1409only the data dependency shown below:
1410
1411	CPU 1			CPU 2			CPU 3
1412	=======================	=======================	=======================
1413		{ X = 0, Y = 0 }
1414	STORE X=1		r1=LOAD X (reads 1)	LOAD Y (reads 1)
1415				<data dependency>	<read barrier>
1416				STORE Y=r1		LOAD X (reads 0)
1417
1418This substitution allows non-multicopy atomicity to run rampant: in
1419this example, it is perfectly legal for CPU 2's load from X to return 1,
1420CPU 3's load from Y to return 1, and its load from X to return 0.
1421
1422The key point is that although CPU 2's data dependency orders its load
1423and store, it does not guarantee to order CPU 1's store.  Thus, if this
1424example runs on a non-multicopy-atomic system where CPUs 1 and 2 share a
1425store buffer or a level of cache, CPU 2 might have early access to CPU 1's
1426writes.  General barriers are therefore required to ensure that all CPUs
1427agree on the combined order of multiple accesses.
1428
1429General barriers can compensate not only for non-multicopy atomicity,
1430but can also generate additional ordering that can ensure that -all-
1431CPUs will perceive the same order of -all- operations.  In contrast, a
1432chain of release-acquire pairs do not provide this additional ordering,
1433which means that only those CPUs on the chain are guaranteed to agree
1434on the combined order of the accesses.  For example, switching to C code
1435in deference to the ghost of Herman Hollerith:
1436
1437	int u, v, x, y, z;
1438
1439	void cpu0(void)
1440	{
1441		r0 = smp_load_acquire(&x);
1442		WRITE_ONCE(u, 1);
1443		smp_store_release(&y, 1);
1444	}
1445
1446	void cpu1(void)
1447	{
1448		r1 = smp_load_acquire(&y);
1449		r4 = READ_ONCE(v);
1450		r5 = READ_ONCE(u);
1451		smp_store_release(&z, 1);
1452	}
1453
1454	void cpu2(void)
1455	{
1456		r2 = smp_load_acquire(&z);
1457		smp_store_release(&x, 1);
1458	}
1459
1460	void cpu3(void)
1461	{
1462		WRITE_ONCE(v, 1);
1463		smp_mb();
1464		r3 = READ_ONCE(u);
1465	}
1466
1467Because cpu0(), cpu1(), and cpu2() participate in a chain of
1468smp_store_release()/smp_load_acquire() pairs, the following outcome
1469is prohibited:
1470
1471	r0 == 1 && r1 == 1 && r2 == 1
1472
1473Furthermore, because of the release-acquire relationship between cpu0()
1474and cpu1(), cpu1() must see cpu0()'s writes, so that the following
1475outcome is prohibited:
1476
1477	r1 == 1 && r5 == 0
1478
1479However, the ordering provided by a release-acquire chain is local
1480to the CPUs participating in that chain and does not apply to cpu3(),
1481at least aside from stores.  Therefore, the following outcome is possible:
1482
1483	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
1484
1485As an aside, the following outcome is also possible:
1486
1487	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
1488
1489Although cpu0(), cpu1(), and cpu2() will see their respective reads and
1490writes in order, CPUs not involved in the release-acquire chain might
1491well disagree on the order.  This disagreement stems from the fact that
1492the weak memory-barrier instructions used to implement smp_load_acquire()
1493and smp_store_release() are not required to order prior stores against
1494subsequent loads in all cases.  This means that cpu3() can see cpu0()'s
1495store to u as happening -after- cpu1()'s load from v, even though
1496both cpu0() and cpu1() agree that these two operations occurred in the
1497intended order.
1498
1499However, please keep in mind that smp_load_acquire() is not magic.
1500In particular, it simply reads from its argument with ordering.  It does
1501-not- ensure that any particular value will be read.  Therefore, the
1502following outcome is possible:
1503
1504	r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
1505
1506Note that this outcome can happen even on a mythical sequentially
1507consistent system where nothing is ever reordered.
1508
1509To reiterate, if your code requires full ordering of all operations,
1510use general barriers throughout.
1511
1512
1513========================
1514EXPLICIT KERNEL BARRIERS
1515========================
1516
1517The Linux kernel has a variety of different barriers that act at different
1518levels:
1519
1520  (*) Compiler barrier.
1521
1522  (*) CPU memory barriers.
1523
1524
1525COMPILER BARRIER
1526----------------
1527
1528The Linux kernel has an explicit compiler barrier function that prevents the
1529compiler from moving the memory accesses either side of it to the other side:
1530
1531	barrier();
1532
1533This is a general barrier -- there are no read-read or write-write
1534variants of barrier().  However, READ_ONCE() and WRITE_ONCE() can be
1535thought of as weak forms of barrier() that affect only the specific
1536accesses flagged by the READ_ONCE() or WRITE_ONCE().
1537
1538The barrier() function has the following effects:
1539
1540 (*) Prevents the compiler from reordering accesses following the
1541     barrier() to precede any accesses preceding the barrier().
1542     One example use for this property is to ease communication between
1543     interrupt-handler code and the code that was interrupted.
1544
1545 (*) Within a loop, forces the compiler to load the variables used
1546     in that loop's conditional on each pass through that loop.
1547
1548The READ_ONCE() and WRITE_ONCE() functions can prevent any number of
1549optimizations that, while perfectly safe in single-threaded code, can
1550be fatal in concurrent code.  Here are some examples of these sorts
1551of optimizations:
1552
1553 (*) The compiler is within its rights to reorder loads and stores
1554     to the same variable, and in some cases, the CPU is within its
1555     rights to reorder loads to the same variable.  This means that
1556     the following code:
1557
1558	a[0] = x;
1559	a[1] = x;
1560
1561     Might result in an older value of x stored in a[1] than in a[0].
1562     Prevent both the compiler and the CPU from doing this as follows:
1563
1564	a[0] = READ_ONCE(x);
1565	a[1] = READ_ONCE(x);
1566
1567     In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
1568     accesses from multiple CPUs to a single variable.
1569
1570 (*) The compiler is within its rights to merge successive loads from
1571     the same variable.  Such merging can cause the compiler to "optimize"
1572     the following code:
1573
1574	while (tmp = a)
1575		do_something_with(tmp);
1576
1577     into the following code, which, although in some sense legitimate
1578     for single-threaded code, is almost certainly not what the developer
1579     intended:
1580
1581	if (tmp = a)
1582		for (;;)
1583			do_something_with(tmp);
1584
1585     Use READ_ONCE() to prevent the compiler from doing this to you:
1586
1587	while (tmp = READ_ONCE(a))
1588		do_something_with(tmp);
1589
1590 (*) The compiler is within its rights to reload a variable, for example,
1591     in cases where high register pressure prevents the compiler from
1592     keeping all data of interest in registers.  The compiler might
1593     therefore optimize the variable 'tmp' out of our previous example:
1594
1595	while (tmp = a)
1596		do_something_with(tmp);
1597
1598     This could result in the following code, which is perfectly safe in
1599     single-threaded code, but can be fatal in concurrent code:
1600
1601	while (a)
1602		do_something_with(a);
1603
1604     For example, the optimized version of this code could result in
1605     passing a zero to do_something_with() in the case where the variable
1606     a was modified by some other CPU between the "while" statement and
1607     the call to do_something_with().
1608
1609     Again, use READ_ONCE() to prevent the compiler from doing this:
1610
1611	while (tmp = READ_ONCE(a))
1612		do_something_with(tmp);
1613
1614     Note that if the compiler runs short of registers, it might save
1615     tmp onto the stack.  The overhead of this saving and later restoring
1616     is why compilers reload variables.  Doing so is perfectly safe for
1617     single-threaded code, so you need to tell the compiler about cases
1618     where it is not safe.
1619
1620 (*) The compiler is within its rights to omit a load entirely if it knows
1621     what the value will be.  For example, if the compiler can prove that
1622     the value of variable 'a' is always zero, it can optimize this code:
1623
1624	while (tmp = a)
1625		do_something_with(tmp);
1626
1627     Into this:
1628
1629	do { } while (0);
1630
1631     This transformation is a win for single-threaded code because it
1632     gets rid of a load and a branch.  The problem is that the compiler
1633     will carry out its proof assuming that the current CPU is the only
1634     one updating variable 'a'.  If variable 'a' is shared, then the
1635     compiler's proof will be erroneous.  Use READ_ONCE() to tell the
1636     compiler that it doesn't know as much as it thinks it does:
1637
1638	while (tmp = READ_ONCE(a))
1639		do_something_with(tmp);
1640
1641     But please note that the compiler is also closely watching what you
1642     do with the value after the READ_ONCE().  For example, suppose you
1643     do the following and MAX is a preprocessor macro with the value 1:
1644
1645	while ((tmp = READ_ONCE(a)) % MAX)
1646		do_something_with(tmp);
1647
1648     Then the compiler knows that the result of the "%" operator applied
1649     to MAX will always be zero, again allowing the compiler to optimize
1650     the code into near-nonexistence.  (It will still load from the
1651     variable 'a'.)
1652
1653 (*) Similarly, the compiler is within its rights to omit a store entirely
1654     if it knows that the variable already has the value being stored.
1655     Again, the compiler assumes that the current CPU is the only one
1656     storing into the variable, which can cause the compiler to do the
1657     wrong thing for shared variables.  For example, suppose you have
1658     the following:
1659
1660	a = 0;
1661	... Code that does not store to variable a ...
1662	a = 0;
1663
1664     The compiler sees that the value of variable 'a' is already zero, so
1665     it might well omit the second store.  This would come as a fatal
1666     surprise if some other CPU might have stored to variable 'a' in the
1667     meantime.
1668
1669     Use WRITE_ONCE() to prevent the compiler from making this sort of
1670     wrong guess:
1671
1672	WRITE_ONCE(a, 0);
1673	... Code that does not store to variable a ...
1674	WRITE_ONCE(a, 0);
1675
1676 (*) The compiler is within its rights to reorder memory accesses unless
1677     you tell it not to.  For example, consider the following interaction
1678     between process-level code and an interrupt handler:
1679
1680	void process_level(void)
1681	{
1682		msg = get_message();
1683		flag = true;
1684	}
1685
1686	void interrupt_handler(void)
1687	{
1688		if (flag)
1689			process_message(msg);
1690	}
1691
1692     There is nothing to prevent the compiler from transforming
1693     process_level() to the following, in fact, this might well be a
1694     win for single-threaded code:
1695
1696	void process_level(void)
1697	{
1698		flag = true;
1699		msg = get_message();
1700	}
1701
1702     If the interrupt occurs between these two statement, then
1703     interrupt_handler() might be passed a garbled msg.  Use WRITE_ONCE()
1704     to prevent this as follows:
1705
1706	void process_level(void)
1707	{
1708		WRITE_ONCE(msg, get_message());
1709		WRITE_ONCE(flag, true);
1710	}
1711
1712	void interrupt_handler(void)
1713	{
1714		if (READ_ONCE(flag))
1715			process_message(READ_ONCE(msg));
1716	}
1717
1718     Note that the READ_ONCE() and WRITE_ONCE() wrappers in
1719     interrupt_handler() are needed if this interrupt handler can itself
1720     be interrupted by something that also accesses 'flag' and 'msg',
1721     for example, a nested interrupt or an NMI.  Otherwise, READ_ONCE()
1722     and WRITE_ONCE() are not needed in interrupt_handler() other than
1723     for documentation purposes.  (Note also that nested interrupts
1724     do not typically occur in modern Linux kernels, in fact, if an
1725     interrupt handler returns with interrupts enabled, you will get a
1726     WARN_ONCE() splat.)
1727
1728     You should assume that the compiler can move READ_ONCE() and
1729     WRITE_ONCE() past code not containing READ_ONCE(), WRITE_ONCE(),
1730     barrier(), or similar primitives.
1731
1732     This effect could also be achieved using barrier(), but READ_ONCE()
1733     and WRITE_ONCE() are more selective:  With READ_ONCE() and
1734     WRITE_ONCE(), the compiler need only forget the contents of the
1735     indicated memory locations, while with barrier() the compiler must
1736     discard the value of all memory locations that it has currently
1737     cached in any machine registers.  Of course, the compiler must also
1738     respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
1739     though the CPU of course need not do so.
1740
1741 (*) The compiler is within its rights to invent stores to a variable,
1742     as in the following example:
1743
1744	if (a)
1745		b = a;
1746	else
1747		b = 42;
1748
1749     The compiler might save a branch by optimizing this as follows:
1750
1751	b = 42;
1752	if (a)
1753		b = a;
1754
1755     In single-threaded code, this is not only safe, but also saves
1756     a branch.  Unfortunately, in concurrent code, this optimization
1757     could cause some other CPU to see a spurious value of 42 -- even
1758     if variable 'a' was never zero -- when loading variable 'b'.
1759     Use WRITE_ONCE() to prevent this as follows:
1760
1761	if (a)
1762		WRITE_ONCE(b, a);
1763	else
1764		WRITE_ONCE(b, 42);
1765
1766     The compiler can also invent loads.  These are usually less
1767     damaging, but they can result in cache-line bouncing and thus in
1768     poor performance and scalability.  Use READ_ONCE() to prevent
1769     invented loads.
1770
1771 (*) For aligned memory locations whose size allows them to be accessed
1772     with a single memory-reference instruction, prevents "load tearing"
1773     and "store tearing," in which a single large access is replaced by
1774     multiple smaller accesses.  For example, given an architecture having
1775     16-bit store instructions with 7-bit immediate fields, the compiler
1776     might be tempted to use two 16-bit store-immediate instructions to
1777     implement the following 32-bit store:
1778
1779	p = 0x00010002;
1780
1781     Please note that GCC really does use this sort of optimization,
1782     which is not surprising given that it would likely take more
1783     than two instructions to build the constant and then store it.
1784     This optimization can therefore be a win in single-threaded code.
1785     In fact, a recent bug (since fixed) caused GCC to incorrectly use
1786     this optimization in a volatile store.  In the absence of such bugs,
1787     use of WRITE_ONCE() prevents store tearing in the following example:
1788
1789	WRITE_ONCE(p, 0x00010002);
1790
1791     Use of packed structures can also result in load and store tearing,
1792     as in this example:
1793
1794	struct __attribute__((__packed__)) foo {
1795		short a;
1796		int b;
1797		short c;
1798	};
1799	struct foo foo1, foo2;
1800	...
1801
1802	foo2.a = foo1.a;
1803	foo2.b = foo1.b;
1804	foo2.c = foo1.c;
1805
1806     Because there are no READ_ONCE() or WRITE_ONCE() wrappers and no
1807     volatile markings, the compiler would be well within its rights to
1808     implement these three assignment statements as a pair of 32-bit
1809     loads followed by a pair of 32-bit stores.  This would result in
1810     load tearing on 'foo1.b' and store tearing on 'foo2.b'.  READ_ONCE()
1811     and WRITE_ONCE() again prevent tearing in this example:
1812
1813	foo2.a = foo1.a;
1814	WRITE_ONCE(foo2.b, READ_ONCE(foo1.b));
1815	foo2.c = foo1.c;
1816
1817All that aside, it is never necessary to use READ_ONCE() and
1818WRITE_ONCE() on a variable that has been marked volatile.  For example,
1819because 'jiffies' is marked volatile, it is never necessary to
1820say READ_ONCE(jiffies).  The reason for this is that READ_ONCE() and
1821WRITE_ONCE() are implemented as volatile casts, which has no effect when
1822its argument is already marked volatile.
1823
1824Please note that these compiler barriers have no direct effect on the CPU,
1825which may then reorder things however it wishes.
1826
1827
1828CPU MEMORY BARRIERS
1829-------------------
1830
1831The Linux kernel has seven basic CPU memory barriers:
1832
1833	TYPE			MANDATORY	SMP CONDITIONAL
1834	=======================	===============	===============
1835	GENERAL			mb()		smp_mb()
1836	WRITE			wmb()		smp_wmb()
1837	READ			rmb()		smp_rmb()
1838	ADDRESS DEPENDENCY			READ_ONCE()
1839
1840
1841All memory barriers except the address-dependency barriers imply a compiler
1842barrier.  Address dependencies do not impose any additional compiler ordering.
1843
1844Aside: In the case of address dependencies, the compiler would be expected
1845to issue the loads in the correct order (eg. `a[b]` would have to load
1846the value of b before loading a[b]), however there is no guarantee in
1847the C specification that the compiler may not speculate the value of b
1848(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1)
1849tmp = a[b]; ).  There is also the problem of a compiler reloading b after
1850having loaded a[b], thus having a newer copy of b than a[b].  A consensus
1851has not yet been reached about these problems, however the READ_ONCE()
1852macro is a good place to start looking.
1853
1854SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1855systems because it is assumed that a CPU will appear to be self-consistent,
1856and will order overlapping accesses correctly with respect to itself.
1857However, see the subsection on "Virtual Machine Guests" below.
1858
1859[!] Note that SMP memory barriers _must_ be used to control the ordering of
1860references to shared memory on SMP systems, though the use of locking instead
1861is sufficient.
1862
1863Mandatory barriers should not be used to control SMP effects, since mandatory
1864barriers impose unnecessary overhead on both SMP and UP systems. They may,
1865however, be used to control MMIO effects on accesses through relaxed memory I/O
1866windows.  These barriers are required even on non-SMP systems as they affect
1867the order in which memory operations appear to a device by prohibiting both the
1868compiler and the CPU from reordering them.
1869
1870
1871There are some more advanced barrier functions:
1872
1873 (*) smp_store_mb(var, value)
1874
1875     This assigns the value to the variable and then inserts a full memory
1876     barrier after it.  It isn't guaranteed to insert anything more than a
1877     compiler barrier in a UP compilation.
1878
1879
1880 (*) smp_mb__before_atomic();
1881 (*) smp_mb__after_atomic();
1882
1883     These are for use with atomic RMW functions that do not imply memory
1884     barriers, but where the code needs a memory barrier. Examples for atomic
1885     RMW functions that do not imply a memory barrier are e.g. add,
1886     subtract, (failed) conditional operations, _relaxed functions,
1887     but not atomic_read or atomic_set. A common example where a memory
1888     barrier may be required is when atomic ops are used for reference
1889     counting.
1890
1891     These are also used for atomic RMW bitop functions that do not imply a
1892     memory barrier (such as set_bit and clear_bit).
1893
1894     As an example, consider a piece of code that marks an object as being dead
1895     and then decrements the object's reference count:
1896
1897	obj->dead = 1;
1898	smp_mb__before_atomic();
1899	atomic_dec(&obj->ref_count);
1900
1901     This makes sure that the death mark on the object is perceived to be set
1902     *before* the reference counter is decremented.
1903
1904     See Documentation/atomic_{t,bitops}.txt for more information.
1905
1906
1907 (*) dma_wmb();
1908 (*) dma_rmb();
1909 (*) dma_mb();
1910
1911     These are for use with consistent memory to guarantee the ordering
1912     of writes or reads of shared memory accessible to both the CPU and a
1913     DMA capable device. See Documentation/core-api/dma-api.rst file for more
1914     information about consistent memory.
1915
1916     For example, consider a device driver that shares memory with a device
1917     and uses a descriptor status value to indicate if the descriptor belongs
1918     to the device or the CPU, and a doorbell to notify it when new
1919     descriptors are available:
1920
1921	if (desc->status != DEVICE_OWN) {
1922		/* do not read data until we own descriptor */
1923		dma_rmb();
1924
1925		/* read/modify data */
1926		read_data = desc->data;
1927		desc->data = write_data;
1928
1929		/* flush modifications before status update */
1930		dma_wmb();
1931
1932		/* assign ownership */
1933		desc->status = DEVICE_OWN;
1934
1935		/* Make descriptor status visible to the device followed by
1936		 * notify device of new descriptor
1937		 */
1938		writel(DESC_NOTIFY, doorbell);
1939	}
1940
1941     The dma_rmb() allows us to guarantee that the device has released ownership
1942     before we read the data from the descriptor, and the dma_wmb() allows
1943     us to guarantee the data is written to the descriptor before the device
1944     can see it now has ownership.  The dma_mb() implies both a dma_rmb() and
1945     a dma_wmb().
1946
1947     Note that the dma_*() barriers do not provide any ordering guarantees for
1948     accesses to MMIO regions.  See the later "KERNEL I/O BARRIER EFFECTS"
1949     subsection for more information about I/O accessors and MMIO ordering.
1950
1951 (*) pmem_wmb();
1952
1953     This is for use with persistent memory to ensure that stores for which
1954     modifications are written to persistent storage reached a platform
1955     durability domain.
1956
1957     For example, after a non-temporal write to pmem region, we use pmem_wmb()
1958     to ensure that stores have reached a platform durability domain. This ensures
1959     that stores have updated persistent storage before any data access or
1960     data transfer caused by subsequent instructions is initiated. This is
1961     in addition to the ordering done by wmb().
1962
1963     For load from persistent memory, existing read memory barriers are sufficient
1964     to ensure read ordering.
1965
1966 (*) io_stop_wc();
1967
1968     For memory accesses with write-combining attributes (e.g. those returned
1969     by ioremap_wc()), the CPU may wait for prior accesses to be merged with
1970     subsequent ones. io_stop_wc() can be used to prevent the merging of
1971     write-combining memory accesses before this macro with those after it when
1972     such wait has performance implications.
1973
1974===============================
1975IMPLICIT KERNEL MEMORY BARRIERS
1976===============================
1977
1978Some of the other functions in the linux kernel imply memory barriers, amongst
1979which are locking and scheduling functions.
1980
1981This specification is a _minimum_ guarantee; any particular architecture may
1982provide more substantial guarantees, but these may not be relied upon outside
1983of arch specific code.
1984
1985
1986LOCK ACQUISITION FUNCTIONS
1987--------------------------
1988
1989The Linux kernel has a number of locking constructs:
1990
1991 (*) spin locks
1992 (*) R/W spin locks
1993 (*) mutexes
1994 (*) semaphores
1995 (*) R/W semaphores
1996
1997In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
1998for each construct.  These operations all imply certain barriers:
1999
2000 (1) ACQUIRE operation implication:
2001
2002     Memory operations issued after the ACQUIRE will be completed after the
2003     ACQUIRE operation has completed.
2004
2005     Memory operations issued before the ACQUIRE may be completed after
2006     the ACQUIRE operation has completed.
2007
2008 (2) RELEASE operation implication:
2009
2010     Memory operations issued before the RELEASE will be completed before the
2011     RELEASE operation has completed.
2012
2013     Memory operations issued after the RELEASE may be completed before the
2014     RELEASE operation has completed.
2015
2016 (3) ACQUIRE vs ACQUIRE implication:
2017
2018     All ACQUIRE operations issued before another ACQUIRE operation will be
2019     completed before that ACQUIRE operation.
2020
2021 (4) ACQUIRE vs RELEASE implication:
2022
2023     All ACQUIRE operations issued before a RELEASE operation will be
2024     completed before the RELEASE operation.
2025
2026 (5) Failed conditional ACQUIRE implication:
2027
2028     Certain locking variants of the ACQUIRE operation may fail, either due to
2029     being unable to get the lock immediately, or due to receiving an unblocked
2030     signal while asleep waiting for the lock to become available.  Failed
2031     locks do not imply any sort of barrier.
2032
2033[!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
2034one-way barriers is that the effects of instructions outside of a critical
2035section may seep into the inside of the critical section.
2036
2037An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
2038because it is possible for an access preceding the ACQUIRE to happen after the
2039ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
2040the two accesses can themselves then cross:
2041
2042	*A = a;
2043	ACQUIRE M
2044	RELEASE M
2045	*B = b;
2046
2047may occur as:
2048
2049	ACQUIRE M, STORE *B, STORE *A, RELEASE M
2050
2051When the ACQUIRE and RELEASE are a lock acquisition and release,
2052respectively, this same reordering can occur if the lock's ACQUIRE and
2053RELEASE are to the same lock variable, but only from the perspective of
2054another CPU not holding that lock.  In short, a ACQUIRE followed by an
2055RELEASE may -not- be assumed to be a full memory barrier.
2056
2057Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
2058not imply a full memory barrier.  Therefore, the CPU's execution of the
2059critical sections corresponding to the RELEASE and the ACQUIRE can cross,
2060so that:
2061
2062	*A = a;
2063	RELEASE M
2064	ACQUIRE N
2065	*B = b;
2066
2067could occur as:
2068
2069	ACQUIRE N, STORE *B, STORE *A, RELEASE M
2070
2071It might appear that this reordering could introduce a deadlock.
2072However, this cannot happen because if such a deadlock threatened,
2073the RELEASE would simply complete, thereby avoiding the deadlock.
2074
2075	Why does this work?
2076
2077	One key point is that we are only talking about the CPU doing
2078	the reordering, not the compiler.  If the compiler (or, for
2079	that matter, the developer) switched the operations, deadlock
2080	-could- occur.
2081
2082	But suppose the CPU reordered the operations.  In this case,
2083	the unlock precedes the lock in the assembly code.  The CPU
2084	simply elected to try executing the later lock operation first.
2085	If there is a deadlock, this lock operation will simply spin (or
2086	try to sleep, but more on that later).	The CPU will eventually
2087	execute the unlock operation (which preceded the lock operation
2088	in the assembly code), which will unravel the potential deadlock,
2089	allowing the lock operation to succeed.
2090
2091	But what if the lock is a sleeplock?  In that case, the code will
2092	try to enter the scheduler, where it will eventually encounter
2093	a memory barrier, which will force the earlier unlock operation
2094	to complete, again unraveling the deadlock.  There might be
2095	a sleep-unlock race, but the locking primitive needs to resolve
2096	such races properly in any case.
2097
2098Locks and semaphores may not provide any guarantee of ordering on UP compiled
2099systems, and so cannot be counted on in such a situation to actually achieve
2100anything at all - especially with respect to I/O accesses - unless combined
2101with interrupt disabling operations.
2102
2103See also the section on "Inter-CPU acquiring barrier effects".
2104
2105
2106As an example, consider the following:
2107
2108	*A = a;
2109	*B = b;
2110	ACQUIRE
2111	*C = c;
2112	*D = d;
2113	RELEASE
2114	*E = e;
2115	*F = f;
2116
2117The following sequence of events is acceptable:
2118
2119	ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
2120
2121	[+] Note that {*F,*A} indicates a combined access.
2122
2123But none of the following are:
2124
2125	{*F,*A}, *B,	ACQUIRE, *C, *D,	RELEASE, *E
2126	*A, *B, *C,	ACQUIRE, *D,		RELEASE, *E, *F
2127	*A, *B,		ACQUIRE, *C,		RELEASE, *D, *E, *F
2128	*B,		ACQUIRE, *C, *D,	RELEASE, {*F,*A}, *E
2129
2130
2131
2132INTERRUPT DISABLING FUNCTIONS
2133-----------------------------
2134
2135Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
2136(RELEASE equivalent) will act as compiler barriers only.  So if memory or I/O
2137barriers are required in such a situation, they must be provided from some
2138other means.
2139
2140
2141SLEEP AND WAKE-UP FUNCTIONS
2142---------------------------
2143
2144Sleeping and waking on an event flagged in global data can be viewed as an
2145interaction between two pieces of data: the task state of the task waiting for
2146the event and the global data used to indicate the event.  To make sure that
2147these appear to happen in the right order, the primitives to begin the process
2148of going to sleep, and the primitives to initiate a wake up imply certain
2149barriers.
2150
2151Firstly, the sleeper normally follows something like this sequence of events:
2152
2153	for (;;) {
2154		set_current_state(TASK_UNINTERRUPTIBLE);
2155		if (event_indicated)
2156			break;
2157		schedule();
2158	}
2159
2160A general memory barrier is interpolated automatically by set_current_state()
2161after it has altered the task state:
2162
2163	CPU 1
2164	===============================
2165	set_current_state();
2166	  smp_store_mb();
2167	    STORE current->state
2168	    <general barrier>
2169	LOAD event_indicated
2170
2171set_current_state() may be wrapped by:
2172
2173	prepare_to_wait();
2174	prepare_to_wait_exclusive();
2175
2176which therefore also imply a general memory barrier after setting the state.
2177The whole sequence above is available in various canned forms, all of which
2178interpolate the memory barrier in the right place:
2179
2180	wait_event();
2181	wait_event_interruptible();
2182	wait_event_interruptible_exclusive();
2183	wait_event_interruptible_timeout();
2184	wait_event_killable();
2185	wait_event_timeout();
2186	wait_on_bit();
2187	wait_on_bit_lock();
2188
2189
2190Secondly, code that performs a wake up normally follows something like this:
2191
2192	event_indicated = 1;
2193	wake_up(&event_wait_queue);
2194
2195or:
2196
2197	event_indicated = 1;
2198	wake_up_process(event_daemon);
2199
2200A general memory barrier is executed by wake_up() if it wakes something up.
2201If it doesn't wake anything up then a memory barrier may or may not be
2202executed; you must not rely on it.  The barrier occurs before the task state
2203is accessed, in particular, it sits between the STORE to indicate the event
2204and the STORE to set TASK_RUNNING:
2205
2206	CPU 1 (Sleeper)			CPU 2 (Waker)
2207	===============================	===============================
2208	set_current_state();		STORE event_indicated
2209	  smp_store_mb();		wake_up();
2210	    STORE current->state	  ...
2211	    <general barrier>		  <general barrier>
2212	LOAD event_indicated		  if ((LOAD task->state) & TASK_NORMAL)
2213					    STORE task->state
2214
2215where "task" is the thread being woken up and it equals CPU 1's "current".
2216
2217To repeat, a general memory barrier is guaranteed to be executed by wake_up()
2218if something is actually awakened, but otherwise there is no such guarantee.
2219To see this, consider the following sequence of events, where X and Y are both
2220initially zero:
2221
2222	CPU 1				CPU 2
2223	===============================	===============================
2224	X = 1;				Y = 1;
2225	smp_mb();			wake_up();
2226	LOAD Y				LOAD X
2227
2228If a wakeup does occur, one (at least) of the two loads must see 1.  If, on
2229the other hand, a wakeup does not occur, both loads might see 0.
2230
2231wake_up_process() always executes a general memory barrier.  The barrier again
2232occurs before the task state is accessed.  In particular, if the wake_up() in
2233the previous snippet were replaced by a call to wake_up_process() then one of
2234the two loads would be guaranteed to see 1.
2235
2236The available waker functions include:
2237
2238	complete();
2239	wake_up();
2240	wake_up_all();
2241	wake_up_bit();
2242	wake_up_interruptible();
2243	wake_up_interruptible_all();
2244	wake_up_interruptible_nr();
2245	wake_up_interruptible_poll();
2246	wake_up_interruptible_sync();
2247	wake_up_interruptible_sync_poll();
2248	wake_up_locked();
2249	wake_up_locked_poll();
2250	wake_up_nr();
2251	wake_up_poll();
2252	wake_up_process();
2253
2254In terms of memory ordering, these functions all provide the same guarantees of
2255a wake_up() (or stronger).
2256
2257[!] Note that the memory barriers implied by the sleeper and the waker do _not_
2258order multiple stores before the wake-up with respect to loads of those stored
2259values after the sleeper has called set_current_state().  For instance, if the
2260sleeper does:
2261
2262	set_current_state(TASK_INTERRUPTIBLE);
2263	if (event_indicated)
2264		break;
2265	__set_current_state(TASK_RUNNING);
2266	do_something(my_data);
2267
2268and the waker does:
2269
2270	my_data = value;
2271	event_indicated = 1;
2272	wake_up(&event_wait_queue);
2273
2274there's no guarantee that the change to event_indicated will be perceived by
2275the sleeper as coming after the change to my_data.  In such a circumstance, the
2276code on both sides must interpolate its own memory barriers between the
2277separate data accesses.  Thus the above sleeper ought to do:
2278
2279	set_current_state(TASK_INTERRUPTIBLE);
2280	if (event_indicated) {
2281		smp_rmb();
2282		do_something(my_data);
2283	}
2284
2285and the waker should do:
2286
2287	my_data = value;
2288	smp_wmb();
2289	event_indicated = 1;
2290	wake_up(&event_wait_queue);
2291
2292
2293MISCELLANEOUS FUNCTIONS
2294-----------------------
2295
2296Other functions that imply barriers:
2297
2298 (*) schedule() and similar imply full memory barriers.
2299
2300
2301===================================
2302INTER-CPU ACQUIRING BARRIER EFFECTS
2303===================================
2304
2305On SMP systems locking primitives give a more substantial form of barrier: one
2306that does affect memory access ordering on other CPUs, within the context of
2307conflict on any particular lock.
2308
2309
2310ACQUIRES VS MEMORY ACCESSES
2311---------------------------
2312
2313Consider the following: the system has a pair of spinlocks (M) and (Q), and
2314three CPUs; then should the following sequence of events occur:
2315
2316	CPU 1				CPU 2
2317	===============================	===============================
2318	WRITE_ONCE(*A, a);		WRITE_ONCE(*E, e);
2319	ACQUIRE M			ACQUIRE Q
2320	WRITE_ONCE(*B, b);		WRITE_ONCE(*F, f);
2321	WRITE_ONCE(*C, c);		WRITE_ONCE(*G, g);
2322	RELEASE M			RELEASE Q
2323	WRITE_ONCE(*D, d);		WRITE_ONCE(*H, h);
2324
2325Then there is no guarantee as to what order CPU 3 will see the accesses to *A
2326through *H occur in, other than the constraints imposed by the separate locks
2327on the separate CPUs.  It might, for example, see:
2328
2329	*E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
2330
2331But it won't see any of:
2332
2333	*B, *C or *D preceding ACQUIRE M
2334	*A, *B or *C following RELEASE M
2335	*F, *G or *H preceding ACQUIRE Q
2336	*E, *F or *G following RELEASE Q
2337
2338
2339=================================
2340WHERE ARE MEMORY BARRIERS NEEDED?
2341=================================
2342
2343Under normal operation, memory operation reordering is generally not going to
2344be a problem as a single-threaded linear piece of code will still appear to
2345work correctly, even if it's in an SMP kernel.  There are, however, four
2346circumstances in which reordering definitely _could_ be a problem:
2347
2348 (*) Interprocessor interaction.
2349
2350 (*) Atomic operations.
2351
2352 (*) Accessing devices.
2353
2354 (*) Interrupts.
2355
2356
2357INTERPROCESSOR INTERACTION
2358--------------------------
2359
2360When there's a system with more than one processor, more than one CPU in the
2361system may be working on the same data set at the same time.  This can cause
2362synchronisation problems, and the usual way of dealing with them is to use
2363locks.  Locks, however, are quite expensive, and so it may be preferable to
2364operate without the use of a lock if at all possible.  In such a case
2365operations that affect both CPUs may have to be carefully ordered to prevent
2366a malfunction.
2367
2368Consider, for example, the R/W semaphore slow path.  Here a waiting process is
2369queued on the semaphore, by virtue of it having a piece of its stack linked to
2370the semaphore's list of waiting processes:
2371
2372	struct rw_semaphore {
2373		...
2374		spinlock_t lock;
2375		struct list_head waiters;
2376	};
2377
2378	struct rwsem_waiter {
2379		struct list_head list;
2380		struct task_struct *task;
2381	};
2382
2383To wake up a particular waiter, the up_read() or up_write() functions have to:
2384
2385 (1) read the next pointer from this waiter's record to know as to where the
2386     next waiter record is;
2387
2388 (2) read the pointer to the waiter's task structure;
2389
2390 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2391
2392 (4) call wake_up_process() on the task; and
2393
2394 (5) release the reference held on the waiter's task struct.
2395
2396In other words, it has to perform this sequence of events:
2397
2398	LOAD waiter->list.next;
2399	LOAD waiter->task;
2400	STORE waiter->task;
2401	CALL wakeup
2402	RELEASE task
2403
2404and if any of these steps occur out of order, then the whole thing may
2405malfunction.
2406
2407Once it has queued itself and dropped the semaphore lock, the waiter does not
2408get the lock again; it instead just waits for its task pointer to be cleared
2409before proceeding.  Since the record is on the waiter's stack, this means that
2410if the task pointer is cleared _before_ the next pointer in the list is read,
2411another CPU might start processing the waiter and might clobber the waiter's
2412stack before the up*() function has a chance to read the next pointer.
2413
2414Consider then what might happen to the above sequence of events:
2415
2416	CPU 1				CPU 2
2417	===============================	===============================
2418					down_xxx()
2419					Queue waiter
2420					Sleep
2421	up_yyy()
2422	LOAD waiter->task;
2423	STORE waiter->task;
2424					Woken up by other event
2425	<preempt>
2426					Resume processing
2427					down_xxx() returns
2428					call foo()
2429					foo() clobbers *waiter
2430	</preempt>
2431	LOAD waiter->list.next;
2432	--- OOPS ---
2433
2434This could be dealt with using the semaphore lock, but then the down_xxx()
2435function has to needlessly get the spinlock again after being woken up.
2436
2437The way to deal with this is to insert a general SMP memory barrier:
2438
2439	LOAD waiter->list.next;
2440	LOAD waiter->task;
2441	smp_mb();
2442	STORE waiter->task;
2443	CALL wakeup
2444	RELEASE task
2445
2446In this case, the barrier makes a guarantee that all memory accesses before the
2447barrier will appear to happen before all the memory accesses after the barrier
2448with respect to the other CPUs on the system.  It does _not_ guarantee that all
2449the memory accesses before the barrier will be complete by the time the barrier
2450instruction itself is complete.
2451
2452On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2453compiler barrier, thus making sure the compiler emits the instructions in the
2454right order without actually intervening in the CPU.  Since there's only one
2455CPU, that CPU's dependency ordering logic will take care of everything else.
2456
2457
2458ATOMIC OPERATIONS
2459-----------------
2460
2461While they are technically interprocessor interaction considerations, atomic
2462operations are noted specially as some of them imply full memory barriers and
2463some don't, but they're very heavily relied on as a group throughout the
2464kernel.
2465
2466See Documentation/atomic_t.txt for more information.
2467
2468
2469ACCESSING DEVICES
2470-----------------
2471
2472Many devices can be memory mapped, and so appear to the CPU as if they're just
2473a set of memory locations.  To control such a device, the driver usually has to
2474make the right memory accesses in exactly the right order.
2475
2476However, having a clever CPU or a clever compiler creates a potential problem
2477in that the carefully sequenced accesses in the driver code won't reach the
2478device in the requisite order if the CPU or the compiler thinks it is more
2479efficient to reorder, combine or merge accesses - something that would cause
2480the device to malfunction.
2481
2482Inside of the Linux kernel, I/O should be done through the appropriate accessor
2483routines - such as inb() or writel() - which know how to make such accesses
2484appropriately sequential.  While this, for the most part, renders the explicit
2485use of memory barriers unnecessary, if the accessor functions are used to refer
2486to an I/O memory window with relaxed memory access properties, then _mandatory_
2487memory barriers are required to enforce ordering.
2488
2489See Documentation/driver-api/device-io.rst for more information.
2490
2491
2492INTERRUPTS
2493----------
2494
2495A driver may be interrupted by its own interrupt service routine, and thus the
2496two parts of the driver may interfere with each other's attempts to control or
2497access the device.
2498
2499This may be alleviated - at least in part - by disabling local interrupts (a
2500form of locking), such that the critical operations are all contained within
2501the interrupt-disabled section in the driver.  While the driver's interrupt
2502routine is executing, the driver's core may not run on the same CPU, and its
2503interrupt is not permitted to happen again until the current interrupt has been
2504handled, thus the interrupt handler does not need to lock against that.
2505
2506However, consider a driver that was talking to an ethernet card that sports an
2507address register and a data register.  If that driver's core talks to the card
2508under interrupt-disablement and then the driver's interrupt handler is invoked:
2509
2510	LOCAL IRQ DISABLE
2511	writew(ADDR, 3);
2512	writew(DATA, y);
2513	LOCAL IRQ ENABLE
2514	<interrupt>
2515	writew(ADDR, 4);
2516	q = readw(DATA);
2517	</interrupt>
2518
2519The store to the data register might happen after the second store to the
2520address register if ordering rules are sufficiently relaxed:
2521
2522	STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2523
2524
2525If ordering rules are relaxed, it must be assumed that accesses done inside an
2526interrupt disabled section may leak outside of it and may interleave with
2527accesses performed in an interrupt - and vice versa - unless implicit or
2528explicit barriers are used.
2529
2530Normally this won't be a problem because the I/O accesses done inside such
2531sections will include synchronous load operations on strictly ordered I/O
2532registers that form implicit I/O barriers.
2533
2534
2535A similar situation may occur between an interrupt routine and two routines
2536running on separate CPUs that communicate with each other.  If such a case is
2537likely, then interrupt-disabling locks should be used to guarantee ordering.
2538
2539
2540==========================
2541KERNEL I/O BARRIER EFFECTS
2542==========================
2543
2544Interfacing with peripherals via I/O accesses is deeply architecture and device
2545specific. Therefore, drivers which are inherently non-portable may rely on
2546specific behaviours of their target systems in order to achieve synchronization
2547in the most lightweight manner possible. For drivers intending to be portable
2548between multiple architectures and bus implementations, the kernel offers a
2549series of accessor functions that provide various degrees of ordering
2550guarantees:
2551
2552 (*) readX(), writeX():
2553
2554	The readX() and writeX() MMIO accessors take a pointer to the
2555	peripheral being accessed as an __iomem * parameter. For pointers
2556	mapped with the default I/O attributes (e.g. those returned by
2557	ioremap()), the ordering guarantees are as follows:
2558
2559	1. All readX() and writeX() accesses to the same peripheral are ordered
2560	   with respect to each other. This ensures that MMIO register accesses
2561	   by the same CPU thread to a particular device will arrive in program
2562	   order.
2563
2564	2. A writeX() issued by a CPU thread holding a spinlock is ordered
2565	   before a writeX() to the same peripheral from another CPU thread
2566	   issued after a later acquisition of the same spinlock. This ensures
2567	   that MMIO register writes to a particular device issued while holding
2568	   a spinlock will arrive in an order consistent with acquisitions of
2569	   the lock.
2570
2571	3. A writeX() by a CPU thread to the peripheral will first wait for the
2572	   completion of all prior writes to memory either issued by, or
2573	   propagated to, the same thread. This ensures that writes by the CPU
2574	   to an outbound DMA buffer allocated by dma_alloc_coherent() will be
2575	   visible to a DMA engine when the CPU writes to its MMIO control
2576	   register to trigger the transfer.
2577
2578	4. A readX() by a CPU thread from the peripheral will complete before
2579	   any subsequent reads from memory by the same thread can begin. This
2580	   ensures that reads by the CPU from an incoming DMA buffer allocated
2581	   by dma_alloc_coherent() will not see stale data after reading from
2582	   the DMA engine's MMIO status register to establish that the DMA
2583	   transfer has completed.
2584
2585	5. A readX() by a CPU thread from the peripheral will complete before
2586	   any subsequent delay() loop can begin execution on the same thread.
2587	   This ensures that two MMIO register writes by the CPU to a peripheral
2588	   will arrive at least 1us apart if the first write is immediately read
2589	   back with readX() and udelay(1) is called prior to the second
2590	   writeX():
2591
2592		writel(42, DEVICE_REGISTER_0); // Arrives at the device...
2593		readl(DEVICE_REGISTER_0);
2594		udelay(1);
2595		writel(42, DEVICE_REGISTER_1); // ...at least 1us before this.
2596
2597	The ordering properties of __iomem pointers obtained with non-default
2598	attributes (e.g. those returned by ioremap_wc()) are specific to the
2599	underlying architecture and therefore the guarantees listed above cannot
2600	generally be relied upon for accesses to these types of mappings.
2601
2602 (*) readX_relaxed(), writeX_relaxed():
2603
2604	These are similar to readX() and writeX(), but provide weaker memory
2605	ordering guarantees. Specifically, they do not guarantee ordering with
2606	respect to locking, normal memory accesses or delay() loops (i.e.
2607	bullets 2-5 above) but they are still guaranteed to be ordered with
2608	respect to other accesses from the same CPU thread to the same
2609	peripheral when operating on __iomem pointers mapped with the default
2610	I/O attributes.
2611
2612 (*) readsX(), writesX():
2613
2614	The readsX() and writesX() MMIO accessors are designed for accessing
2615	register-based, memory-mapped FIFOs residing on peripherals that are not
2616	capable of performing DMA. Consequently, they provide only the ordering
2617	guarantees of readX_relaxed() and writeX_relaxed(), as documented above.
2618
2619 (*) inX(), outX():
2620
2621	The inX() and outX() accessors are intended to access legacy port-mapped
2622	I/O peripherals, which may require special instructions on some
2623	architectures (notably x86). The port number of the peripheral being
2624	accessed is passed as an argument.
2625
2626	Since many CPU architectures ultimately access these peripherals via an
2627	internal virtual memory mapping, the portable ordering guarantees
2628	provided by inX() and outX() are the same as those provided by readX()
2629	and writeX() respectively when accessing a mapping with the default I/O
2630	attributes.
2631
2632	Device drivers may expect outX() to emit a non-posted write transaction
2633	that waits for a completion response from the I/O peripheral before
2634	returning. This is not guaranteed by all architectures and is therefore
2635	not part of the portable ordering semantics.
2636
2637 (*) insX(), outsX():
2638
2639	As above, the insX() and outsX() accessors provide the same ordering
2640	guarantees as readsX() and writesX() respectively when accessing a
2641	mapping with the default I/O attributes.
2642
2643 (*) ioreadX(), iowriteX():
2644
2645	These will perform appropriately for the type of access they're actually
2646	doing, be it inX()/outX() or readX()/writeX().
2647
2648With the exception of the string accessors (insX(), outsX(), readsX() and
2649writesX()), all of the above assume that the underlying peripheral is
2650little-endian and will therefore perform byte-swapping operations on big-endian
2651architectures.
2652
2653
2654========================================
2655ASSUMED MINIMUM EXECUTION ORDERING MODEL
2656========================================
2657
2658It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2659maintain the appearance of program causality with respect to itself.  Some CPUs
2660(such as i386 or x86_64) are more constrained than others (such as powerpc or
2661frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2662of arch-specific code.
2663
2664This means that it must be considered that the CPU will execute its instruction
2665stream in any order it feels like - or even in parallel - provided that if an
2666instruction in the stream depends on an earlier instruction, then that
2667earlier instruction must be sufficiently complete[*] before the later
2668instruction may proceed; in other words: provided that the appearance of
2669causality is maintained.
2670
2671 [*] Some instructions have more than one effect - such as changing the
2672     condition codes, changing registers or changing memory - and different
2673     instructions may depend on different effects.
2674
2675A CPU may also discard any instruction sequence that winds up having no
2676ultimate effect.  For example, if two adjacent instructions both load an
2677immediate value into the same register, the first may be discarded.
2678
2679
2680Similarly, it has to be assumed that compiler might reorder the instruction
2681stream in any way it sees fit, again provided the appearance of causality is
2682maintained.
2683
2684
2685============================
2686THE EFFECTS OF THE CPU CACHE
2687============================
2688
2689The way cached memory operations are perceived across the system is affected to
2690a certain extent by the caches that lie between CPUs and memory, and by the
2691memory coherence system that maintains the consistency of state in the system.
2692
2693As far as the way a CPU interacts with another part of the system through the
2694caches goes, the memory system has to include the CPU's caches, and memory
2695barriers for the most part act at the interface between the CPU and its cache
2696(memory barriers logically act on the dotted line in the following diagram):
2697
2698	    <--- CPU --->         :       <----------- Memory ----------->
2699	                          :
2700	+--------+    +--------+  :   +--------+    +-----------+
2701	|        |    |        |  :   |        |    |           |    +--------+
2702	|  CPU   |    | Memory |  :   | CPU    |    |           |    |        |
2703	|  Core  |--->| Access |----->| Cache  |<-->|           |    |        |
2704	|        |    | Queue  |  :   |        |    |           |--->| Memory |
2705	|        |    |        |  :   |        |    |           |    |        |
2706	+--------+    +--------+  :   +--------+    |           |    |        |
2707	                          :                 | Cache     |    +--------+
2708	                          :                 | Coherency |
2709	                          :                 | Mechanism |    +--------+
2710	+--------+    +--------+  :   +--------+    |           |    |	      |
2711	|        |    |        |  :   |        |    |           |    |        |
2712	|  CPU   |    | Memory |  :   | CPU    |    |           |--->| Device |
2713	|  Core  |--->| Access |----->| Cache  |<-->|           |    |        |
2714	|        |    | Queue  |  :   |        |    |           |    |        |
2715	|        |    |        |  :   |        |    |           |    +--------+
2716	+--------+    +--------+  :   +--------+    +-----------+
2717	                          :
2718	                          :
2719
2720Although any particular load or store may not actually appear outside of the
2721CPU that issued it since it may have been satisfied within the CPU's own cache,
2722it will still appear as if the full memory access had taken place as far as the
2723other CPUs are concerned since the cache coherency mechanisms will migrate the
2724cacheline over to the accessing CPU and propagate the effects upon conflict.
2725
2726The CPU core may execute instructions in any order it deems fit, provided the
2727expected program causality appears to be maintained.  Some of the instructions
2728generate load and store operations which then go into the queue of memory
2729accesses to be performed.  The core may place these in the queue in any order
2730it wishes, and continue execution until it is forced to wait for an instruction
2731to complete.
2732
2733What memory barriers are concerned with is controlling the order in which
2734accesses cross from the CPU side of things to the memory side of things, and
2735the order in which the effects are perceived to happen by the other observers
2736in the system.
2737
2738[!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2739their own loads and stores as if they had happened in program order.
2740
2741[!] MMIO or other device accesses may bypass the cache system.  This depends on
2742the properties of the memory window through which devices are accessed and/or
2743the use of any special device communication instructions the CPU may have.
2744
2745
2746CACHE COHERENCY VS DMA
2747----------------------
2748
2749Not all systems maintain cache coherency with respect to devices doing DMA.  In
2750such cases, a device attempting DMA may obtain stale data from RAM because
2751dirty cache lines may be resident in the caches of various CPUs, and may not
2752have been written back to RAM yet.  To deal with this, the appropriate part of
2753the kernel must flush the overlapping bits of cache on each CPU (and maybe
2754invalidate them as well).
2755
2756In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2757cache lines being written back to RAM from a CPU's cache after the device has
2758installed its own data, or cache lines present in the CPU's cache may simply
2759obscure the fact that RAM has been updated, until at such time as the cacheline
2760is discarded from the CPU's cache and reloaded.  To deal with this, the
2761appropriate part of the kernel must invalidate the overlapping bits of the
2762cache on each CPU.
2763
2764See Documentation/core-api/cachetlb.rst for more information on cache
2765management.
2766
2767
2768CACHE COHERENCY VS MMIO
2769-----------------------
2770
2771Memory mapped I/O usually takes place through memory locations that are part of
2772a window in the CPU's memory space that has different properties assigned than
2773the usual RAM directed window.
2774
2775Amongst these properties is usually the fact that such accesses bypass the
2776caching entirely and go directly to the device buses.  This means MMIO accesses
2777may, in effect, overtake accesses to cached memory that were emitted earlier.
2778A memory barrier isn't sufficient in such a case, but rather the cache must be
2779flushed between the cached memory write and the MMIO access if the two are in
2780any way dependent.
2781
2782
2783=========================
2784THE THINGS CPUS GET UP TO
2785=========================
2786
2787A programmer might take it for granted that the CPU will perform memory
2788operations in exactly the order specified, so that if the CPU is, for example,
2789given the following piece of code to execute:
2790
2791	a = READ_ONCE(*A);
2792	WRITE_ONCE(*B, b);
2793	c = READ_ONCE(*C);
2794	d = READ_ONCE(*D);
2795	WRITE_ONCE(*E, e);
2796
2797they would then expect that the CPU will complete the memory operation for each
2798instruction before moving on to the next one, leading to a definite sequence of
2799operations as seen by external observers in the system:
2800
2801	LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2802
2803
2804Reality is, of course, much messier.  With many CPUs and compilers, the above
2805assumption doesn't hold because:
2806
2807 (*) loads are more likely to need to be completed immediately to permit
2808     execution progress, whereas stores can often be deferred without a
2809     problem;
2810
2811 (*) loads may be done speculatively, and the result discarded should it prove
2812     to have been unnecessary;
2813
2814 (*) loads may be done speculatively, leading to the result having been fetched
2815     at the wrong time in the expected sequence of events;
2816
2817 (*) the order of the memory accesses may be rearranged to promote better use
2818     of the CPU buses and caches;
2819
2820 (*) loads and stores may be combined to improve performance when talking to
2821     memory or I/O hardware that can do batched accesses of adjacent locations,
2822     thus cutting down on transaction setup costs (memory and PCI devices may
2823     both be able to do this); and
2824
2825 (*) the CPU's data cache may affect the ordering, and while cache-coherency
2826     mechanisms may alleviate this - once the store has actually hit the cache
2827     - there's no guarantee that the coherency management will be propagated in
2828     order to other CPUs.
2829
2830So what another CPU, say, might actually observe from the above piece of code
2831is:
2832
2833	LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2834
2835	(Where "LOAD {*C,*D}" is a combined load)
2836
2837
2838However, it is guaranteed that a CPU will be self-consistent: it will see its
2839_own_ accesses appear to be correctly ordered, without the need for a memory
2840barrier.  For instance with the following code:
2841
2842	U = READ_ONCE(*A);
2843	WRITE_ONCE(*A, V);
2844	WRITE_ONCE(*A, W);
2845	X = READ_ONCE(*A);
2846	WRITE_ONCE(*A, Y);
2847	Z = READ_ONCE(*A);
2848
2849and assuming no intervention by an external influence, it can be assumed that
2850the final result will appear to be:
2851
2852	U == the original value of *A
2853	X == W
2854	Z == Y
2855	*A == Y
2856
2857The code above may cause the CPU to generate the full sequence of memory
2858accesses:
2859
2860	U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2861
2862in that order, but, without intervention, the sequence may have almost any
2863combination of elements combined or discarded, provided the program's view
2864of the world remains consistent.  Note that READ_ONCE() and WRITE_ONCE()
2865are -not- optional in the above example, as there are architectures
2866where a given CPU might reorder successive loads to the same location.
2867On such architectures, READ_ONCE() and WRITE_ONCE() do whatever is
2868necessary to prevent this, for example, on Itanium the volatile casts
2869used by READ_ONCE() and WRITE_ONCE() cause GCC to emit the special ld.acq
2870and st.rel instructions (respectively) that prevent such reordering.
2871
2872The compiler may also combine, discard or defer elements of the sequence before
2873the CPU even sees them.
2874
2875For instance:
2876
2877	*A = V;
2878	*A = W;
2879
2880may be reduced to:
2881
2882	*A = W;
2883
2884since, without either a write barrier or an WRITE_ONCE(), it can be
2885assumed that the effect of the storage of V to *A is lost.  Similarly:
2886
2887	*A = Y;
2888	Z = *A;
2889
2890may, without a memory barrier or an READ_ONCE() and WRITE_ONCE(), be
2891reduced to:
2892
2893	*A = Y;
2894	Z = Y;
2895
2896and the LOAD operation never appear outside of the CPU.
2897
2898
2899AND THEN THERE'S THE ALPHA
2900--------------------------
2901
2902The DEC Alpha CPU is one of the most relaxed CPUs there is.  Not only that,
2903some versions of the Alpha CPU have a split data cache, permitting them to have
2904two semantically-related cache lines updated at separate times.  This is where
2905the address-dependency barrier really becomes necessary as this synchronises
2906both caches with the memory coherence system, thus making it seem like pointer
2907changes vs new data occur in the right order.
2908
2909The Alpha defines the Linux kernel's memory model, although as of v4.15
2910the Linux kernel's addition of smp_mb() to READ_ONCE() on Alpha greatly
2911reduced its impact on the memory model.
2912
2913
2914VIRTUAL MACHINE GUESTS
2915----------------------
2916
2917Guests running within virtual machines might be affected by SMP effects even if
2918the guest itself is compiled without SMP support.  This is an artifact of
2919interfacing with an SMP host while running an UP kernel.  Using mandatory
2920barriers for this use-case would be possible but is often suboptimal.
2921
2922To handle this case optimally, low-level virt_mb() etc macros are available.
2923These have the same effect as smp_mb() etc when SMP is enabled, but generate
2924identical code for SMP and non-SMP systems.  For example, virtual machine guests
2925should use virt_mb() rather than smp_mb() when synchronizing against a
2926(possibly SMP) host.
2927
2928These are equivalent to smp_mb() etc counterparts in all other respects,
2929in particular, they do not control MMIO effects: to control
2930MMIO effects, use mandatory barriers.
2931
2932
2933============
2934EXAMPLE USES
2935============
2936
2937CIRCULAR BUFFERS
2938----------------
2939
2940Memory barriers can be used to implement circular buffering without the need
2941of a lock to serialise the producer with the consumer.  See:
2942
2943	Documentation/core-api/circular-buffers.rst
2944
2945for details.
2946
2947
2948==========
2949REFERENCES
2950==========
2951
2952Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2953Digital Press)
2954	Chapter 5.2: Physical Address Space Characteristics
2955	Chapter 5.4: Caches and Write Buffers
2956	Chapter 5.5: Data Sharing
2957	Chapter 5.6: Read/Write Ordering
2958
2959AMD64 Architecture Programmer's Manual Volume 2: System Programming
2960	Chapter 7.1: Memory-Access Ordering
2961	Chapter 7.4: Buffering and Combining Memory Writes
2962
2963ARM Architecture Reference Manual (ARMv8, for ARMv8-A architecture profile)
2964	Chapter B2: The AArch64 Application Level Memory Model
2965
2966IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2967System Programming Guide
2968	Chapter 7.1: Locked Atomic Operations
2969	Chapter 7.2: Memory Ordering
2970	Chapter 7.4: Serializing Instructions
2971
2972The SPARC Architecture Manual, Version 9
2973	Chapter 8: Memory Models
2974	Appendix D: Formal Specification of the Memory Models
2975	Appendix J: Programming with the Memory Models
2976
2977Storage in the PowerPC (Stone and Fitzgerald)
2978
2979UltraSPARC Programmer Reference Manual
2980	Chapter 5: Memory Accesses and Cacheability
2981	Chapter 15: Sparc-V9 Memory Models
2982
2983UltraSPARC III Cu User's Manual
2984	Chapter 9: Memory Models
2985
2986UltraSPARC IIIi Processor User's Manual
2987	Chapter 8: Memory Models
2988
2989UltraSPARC Architecture 2005
2990	Chapter 9: Memory
2991	Appendix D: Formal Specifications of the Memory Models
2992
2993UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
2994	Chapter 8: Memory Models
2995	Appendix F: Caches and Cache Coherency
2996
2997Solaris Internals, Core Kernel Architecture, p63-68:
2998	Chapter 3.3: Hardware Considerations for Locks and
2999			Synchronization
3000
3001Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
3002for Kernel Programmers:
3003	Chapter 13: Other Memory Models
3004
3005Intel Itanium Architecture Software Developer's Manual: Volume 1:
3006	Section 2.6: Speculation
3007	Section 4.4: Memory Access
3008