1 ============================
2 LINUX KERNEL MEMORY BARRIERS
3 ============================
5 By: David Howells <dhowells@redhat.com>
6 Paul E. McKenney <paulmck@linux.vnet.ibm.com>
10 (*) Abstract memory access model.
15 (*) What are memory barriers?
17 - Varieties of memory barrier.
18 - What may not be assumed about memory barriers?
19 - Data dependency barriers.
20 - Control dependencies.
21 - SMP barrier pairing.
22 - Examples of memory barrier sequences.
23 - Read memory barriers vs load speculation.
26 (*) Explicit kernel barriers.
29 - CPU memory barriers.
32 (*) Implicit kernel memory barriers.
35 - Interrupt disabling functions.
36 - Sleep and wake-up functions.
37 - Miscellaneous functions.
39 (*) Inter-CPU locking barrier effects.
41 - Locks vs memory accesses.
42 - Locks vs I/O accesses.
44 (*) Where are memory barriers needed?
46 - Interprocessor interaction.
51 (*) Kernel I/O barrier effects.
53 (*) Assumed minimum execution ordering model.
55 (*) The effects of the cpu cache.
58 - Cache coherency vs DMA.
59 - Cache coherency vs MMIO.
61 (*) The things CPUs get up to.
63 - And then there's the Alpha.
72 ============================
73 ABSTRACT MEMORY ACCESS MODEL
74 ============================
76 Consider the following abstract model of the system:
81 +-------+ : +--------+ : +-------+
84 | CPU 1 |<----->| Memory |<----->| CPU 2 |
87 +-------+ : +--------+ : +-------+
95 +---------->| Device |<----------+
101 Each CPU executes a program that generates memory access operations. In the
102 abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
103 perform the memory operations in any order it likes, provided program causality
104 appears to be maintained. Similarly, the compiler may also arrange the
105 instructions it emits in any order it likes, provided it doesn't affect the
106 apparent operation of the program.
108 So in the above diagram, the effects of the memory operations performed by a
109 CPU are perceived by the rest of the system as the operations cross the
110 interface between the CPU and rest of the system (the dotted lines).
113 For example, consider the following sequence of events:
116 =============== ===============
121 The set of accesses as seen by the memory system in the middle can be arranged
122 in 24 different combinations:
124 STORE A=3, STORE B=4, y=LOAD A->3, x=LOAD B->4
125 STORE A=3, STORE B=4, x=LOAD B->4, y=LOAD A->3
126 STORE A=3, y=LOAD A->3, STORE B=4, x=LOAD B->4
127 STORE A=3, y=LOAD A->3, x=LOAD B->2, STORE B=4
128 STORE A=3, x=LOAD B->2, STORE B=4, y=LOAD A->3
129 STORE A=3, x=LOAD B->2, y=LOAD A->3, STORE B=4
130 STORE B=4, STORE A=3, y=LOAD A->3, x=LOAD B->4
134 and can thus result in four different combinations of values:
142 Furthermore, the stores committed by a CPU to the memory system may not be
143 perceived by the loads made by another CPU in the same order as the stores were
147 As a further example, consider this sequence of events:
150 =============== ===============
151 { A == 1, B == 2, C = 3, P == &A, Q == &C }
155 There is an obvious data dependency here, as the value loaded into D depends on
156 the address retrieved from P by CPU 2. At the end of the sequence, any of the
157 following results are possible:
159 (Q == &A) and (D == 1)
160 (Q == &B) and (D == 2)
161 (Q == &B) and (D == 4)
163 Note that CPU 2 will never try and load C into D because the CPU will load P
164 into Q before issuing the load of *Q.
170 Some devices present their control interfaces as collections of memory
171 locations, but the order in which the control registers are accessed is very
172 important. For instance, imagine an ethernet card with a set of internal
173 registers that are accessed through an address port register (A) and a data
174 port register (D). To read internal register 5, the following code might then
180 but this might show up as either of the following two sequences:
182 STORE *A = 5, x = LOAD *D
183 x = LOAD *D, STORE *A = 5
185 the second of which will almost certainly result in a malfunction, since it set
186 the address _after_ attempting to read the register.
192 There are some minimal guarantees that may be expected of a CPU:
194 (*) On any given CPU, dependent memory accesses will be issued in order, with
195 respect to itself. This means that for:
197 ACCESS_ONCE(Q) = P; smp_read_barrier_depends(); D = ACCESS_ONCE(*Q);
199 the CPU will issue the following memory operations:
201 Q = LOAD P, D = LOAD *Q
203 and always in that order. On most systems, smp_read_barrier_depends()
204 does nothing, but it is required for DEC Alpha. The ACCESS_ONCE()
205 is required to prevent compiler mischief. Please note that you
206 should normally use something like rcu_dereference() instead of
207 open-coding smp_read_barrier_depends().
209 (*) Overlapping loads and stores within a particular CPU will appear to be
210 ordered within that CPU. This means that for:
212 a = ACCESS_ONCE(*X); ACCESS_ONCE(*X) = b;
214 the CPU will only issue the following sequence of memory operations:
216 a = LOAD *X, STORE *X = b
220 ACCESS_ONCE(*X) = c; d = ACCESS_ONCE(*X);
222 the CPU will only issue:
224 STORE *X = c, d = LOAD *X
226 (Loads and stores overlap if they are targeted at overlapping pieces of
229 And there are a number of things that _must_ or _must_not_ be assumed:
231 (*) It _must_not_ be assumed that the compiler will do what you want with
232 memory references that are not protected by ACCESS_ONCE(). Without
233 ACCESS_ONCE(), the compiler is within its rights to do all sorts
234 of "creative" transformations, which are covered in the Compiler
237 (*) It _must_not_ be assumed that independent loads and stores will be issued
238 in the order given. This means that for:
240 X = *A; Y = *B; *D = Z;
242 we may get any of the following sequences:
244 X = LOAD *A, Y = LOAD *B, STORE *D = Z
245 X = LOAD *A, STORE *D = Z, Y = LOAD *B
246 Y = LOAD *B, X = LOAD *A, STORE *D = Z
247 Y = LOAD *B, STORE *D = Z, X = LOAD *A
248 STORE *D = Z, X = LOAD *A, Y = LOAD *B
249 STORE *D = Z, Y = LOAD *B, X = LOAD *A
251 (*) It _must_ be assumed that overlapping memory accesses may be merged or
252 discarded. This means that for:
254 X = *A; Y = *(A + 4);
256 we may get any one of the following sequences:
258 X = LOAD *A; Y = LOAD *(A + 4);
259 Y = LOAD *(A + 4); X = LOAD *A;
260 {X, Y} = LOAD {*A, *(A + 4) };
264 *A = X; *(A + 4) = Y;
268 STORE *A = X; STORE *(A + 4) = Y;
269 STORE *(A + 4) = Y; STORE *A = X;
270 STORE {*A, *(A + 4) } = {X, Y};
273 =========================
274 WHAT ARE MEMORY BARRIERS?
275 =========================
277 As can be seen above, independent memory operations are effectively performed
278 in random order, but this can be a problem for CPU-CPU interaction and for I/O.
279 What is required is some way of intervening to instruct the compiler and the
280 CPU to restrict the order.
282 Memory barriers are such interventions. They impose a perceived partial
283 ordering over the memory operations on either side of the barrier.
285 Such enforcement is important because the CPUs and other devices in a system
286 can use a variety of tricks to improve performance, including reordering,
287 deferral and combination of memory operations; speculative loads; speculative
288 branch prediction and various types of caching. Memory barriers are used to
289 override or suppress these tricks, allowing the code to sanely control the
290 interaction of multiple CPUs and/or devices.
293 VARIETIES OF MEMORY BARRIER
294 ---------------------------
296 Memory barriers come in four basic varieties:
298 (1) Write (or store) memory barriers.
300 A write memory barrier gives a guarantee that all the STORE operations
301 specified before the barrier will appear to happen before all the STORE
302 operations specified after the barrier with respect to the other
303 components of the system.
305 A write barrier is a partial ordering on stores only; it is not required
306 to have any effect on loads.
308 A CPU can be viewed as committing a sequence of store operations to the
309 memory system as time progresses. All stores before a write barrier will
310 occur in the sequence _before_ all the stores after the write barrier.
312 [!] Note that write barriers should normally be paired with read or data
313 dependency barriers; see the "SMP barrier pairing" subsection.
316 (2) Data dependency barriers.
318 A data dependency barrier is a weaker form of read barrier. In the case
319 where two loads are performed such that the second depends on the result
320 of the first (eg: the first load retrieves the address to which the second
321 load will be directed), a data dependency barrier would be required to
322 make sure that the target of the second load is updated before the address
323 obtained by the first load is accessed.
325 A data dependency barrier is a partial ordering on interdependent loads
326 only; it is not required to have any effect on stores, independent loads
327 or overlapping loads.
329 As mentioned in (1), the other CPUs in the system can be viewed as
330 committing sequences of stores to the memory system that the CPU being
331 considered can then perceive. A data dependency barrier issued by the CPU
332 under consideration guarantees that for any load preceding it, if that
333 load touches one of a sequence of stores from another CPU, then by the
334 time the barrier completes, the effects of all the stores prior to that
335 touched by the load will be perceptible to any loads issued after the data
338 See the "Examples of memory barrier sequences" subsection for diagrams
339 showing the ordering constraints.
341 [!] Note that the first load really has to have a _data_ dependency and
342 not a control dependency. If the address for the second load is dependent
343 on the first load, but the dependency is through a conditional rather than
344 actually loading the address itself, then it's a _control_ dependency and
345 a full read barrier or better is required. See the "Control dependencies"
346 subsection for more information.
348 [!] Note that data dependency barriers should normally be paired with
349 write barriers; see the "SMP barrier pairing" subsection.
352 (3) Read (or load) memory barriers.
354 A read barrier is a data dependency barrier plus a guarantee that all the
355 LOAD operations specified before the barrier will appear to happen before
356 all the LOAD operations specified after the barrier with respect to the
357 other components of the system.
359 A read barrier is a partial ordering on loads only; it is not required to
360 have any effect on stores.
362 Read memory barriers imply data dependency barriers, and so can substitute
365 [!] Note that read barriers should normally be paired with write barriers;
366 see the "SMP barrier pairing" subsection.
369 (4) General memory barriers.
371 A general memory barrier gives a guarantee that all the LOAD and STORE
372 operations specified before the barrier will appear to happen before all
373 the LOAD and STORE operations specified after the barrier with respect to
374 the other components of the system.
376 A general memory barrier is a partial ordering over both loads and stores.
378 General memory barriers imply both read and write memory barriers, and so
379 can substitute for either.
382 And a couple of implicit varieties:
384 (5) ACQUIRE operations.
386 This acts as a one-way permeable barrier. It guarantees that all memory
387 operations after the ACQUIRE operation will appear to happen after the
388 ACQUIRE operation with respect to the other components of the system.
389 ACQUIRE operations include LOCK operations and smp_load_acquire()
392 Memory operations that occur before an ACQUIRE operation may appear to
393 happen after it completes.
395 An ACQUIRE operation should almost always be paired with a RELEASE
399 (6) RELEASE operations.
401 This also acts as a one-way permeable barrier. It guarantees that all
402 memory operations before the RELEASE operation will appear to happen
403 before the RELEASE operation with respect to the other components of the
404 system. RELEASE operations include UNLOCK operations and
405 smp_store_release() operations.
407 Memory operations that occur after a RELEASE operation may appear to
408 happen before it completes.
410 The use of ACQUIRE and RELEASE operations generally precludes the need
411 for other sorts of memory barrier (but note the exceptions mentioned in
412 the subsection "MMIO write barrier"). In addition, a RELEASE+ACQUIRE
413 pair is -not- guaranteed to act as a full memory barrier. However, after
414 an ACQUIRE on a given variable, all memory accesses preceding any prior
415 RELEASE on that same variable are guaranteed to be visible. In other
416 words, within a given variable's critical section, all accesses of all
417 previous critical sections for that variable are guaranteed to have
420 This means that ACQUIRE acts as a minimal "acquire" operation and
421 RELEASE acts as a minimal "release" operation.
424 Memory barriers are only required where there's a possibility of interaction
425 between two CPUs or between a CPU and a device. If it can be guaranteed that
426 there won't be any such interaction in any particular piece of code, then
427 memory barriers are unnecessary in that piece of code.
430 Note that these are the _minimum_ guarantees. Different architectures may give
431 more substantial guarantees, but they may _not_ be relied upon outside of arch
435 WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
436 ----------------------------------------------
438 There are certain things that the Linux kernel memory barriers do not guarantee:
440 (*) There is no guarantee that any of the memory accesses specified before a
441 memory barrier will be _complete_ by the completion of a memory barrier
442 instruction; the barrier can be considered to draw a line in that CPU's
443 access queue that accesses of the appropriate type may not cross.
445 (*) There is no guarantee that issuing a memory barrier on one CPU will have
446 any direct effect on another CPU or any other hardware in the system. The
447 indirect effect will be the order in which the second CPU sees the effects
448 of the first CPU's accesses occur, but see the next point:
450 (*) There is no guarantee that a CPU will see the correct order of effects
451 from a second CPU's accesses, even _if_ the second CPU uses a memory
452 barrier, unless the first CPU _also_ uses a matching memory barrier (see
453 the subsection on "SMP Barrier Pairing").
455 (*) There is no guarantee that some intervening piece of off-the-CPU
456 hardware[*] will not reorder the memory accesses. CPU cache coherency
457 mechanisms should propagate the indirect effects of a memory barrier
458 between CPUs, but might not do so in order.
460 [*] For information on bus mastering DMA and coherency please read:
462 Documentation/PCI/pci.txt
463 Documentation/DMA-API-HOWTO.txt
464 Documentation/DMA-API.txt
467 DATA DEPENDENCY BARRIERS
468 ------------------------
470 The usage requirements of data dependency barriers are a little subtle, and
471 it's not always obvious that they're needed. To illustrate, consider the
472 following sequence of events:
475 =============== ===============
476 { A == 1, B == 2, C = 3, P == &A, Q == &C }
483 There's a clear data dependency here, and it would seem that by the end of the
484 sequence, Q must be either &A or &B, and that:
486 (Q == &A) implies (D == 1)
487 (Q == &B) implies (D == 4)
489 But! CPU 2's perception of P may be updated _before_ its perception of B, thus
490 leading to the following situation:
492 (Q == &B) and (D == 2) ????
494 Whilst this may seem like a failure of coherency or causality maintenance, it
495 isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
498 To deal with this, a data dependency barrier or better must be inserted
499 between the address load and the data load:
502 =============== ===============
503 { A == 1, B == 2, C = 3, P == &A, Q == &C }
508 <data dependency barrier>
511 This enforces the occurrence of one of the two implications, and prevents the
512 third possibility from arising.
514 [!] Note that this extremely counterintuitive situation arises most easily on
515 machines with split caches, so that, for example, one cache bank processes
516 even-numbered cache lines and the other bank processes odd-numbered cache
517 lines. The pointer P might be stored in an odd-numbered cache line, and the
518 variable B might be stored in an even-numbered cache line. Then, if the
519 even-numbered bank of the reading CPU's cache is extremely busy while the
520 odd-numbered bank is idle, one can see the new value of the pointer P (&B),
521 but the old value of the variable B (2).
524 Another example of where data dependency barriers might be required is where a
525 number is read from memory and then used to calculate the index for an array
529 =============== ===============
530 { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
535 <data dependency barrier>
539 The data dependency barrier is very important to the RCU system,
540 for example. See rcu_assign_pointer() and rcu_dereference() in
541 include/linux/rcupdate.h. This permits the current target of an RCU'd
542 pointer to be replaced with a new modified target, without the replacement
543 target appearing to be incompletely initialised.
545 See also the subsection on "Cache Coherency" for a more thorough example.
551 A control dependency requires a full read memory barrier, not simply a data
552 dependency barrier to make it work correctly. Consider the following bit of
557 <data dependency barrier> /* BUG: No data dependency!!! */
561 This will not have the desired effect because there is no actual data
562 dependency, but rather a control dependency that the CPU may short-circuit
563 by attempting to predict the outcome in advance, so that other CPUs see
564 the load from b as having happened before the load from a. In such a
565 case what's actually required is:
573 However, stores are not speculated. This means that ordering -is- provided
574 in the following example:
581 Please note that ACCESS_ONCE() is not optional! Without the
582 ACCESS_ONCE(), might combine the load from 'a' with other loads from
583 'a', and the store to 'b' with other stores to 'b', with possible highly
584 counterintuitive effects on ordering.
586 Worse yet, if the compiler is able to prove (say) that the value of
587 variable 'a' is always non-zero, it would be well within its rights
588 to optimize the original example by eliminating the "if" statement
592 b = p; /* BUG: Compiler and CPU can both reorder!!! */
594 So don't leave out the ACCESS_ONCE().
596 It is tempting to try to enforce ordering on identical stores on both
597 branches of the "if" statement as follows:
610 Unfortunately, current compilers will transform this as follows at high
615 ACCESS_ONCE(b) = p; /* BUG: No ordering vs. load from a!!! */
617 /* ACCESS_ONCE(b) = p; -- moved up, BUG!!! */
620 /* ACCESS_ONCE(b) = p; -- moved up, BUG!!! */
624 Now there is no conditional between the load from 'a' and the store to
625 'b', which means that the CPU is within its rights to reorder them:
626 The conditional is absolutely required, and must be present in the
627 assembly code even after all compiler optimizations have been applied.
628 Therefore, if you need ordering in this example, you need explicit
629 memory barriers, for example, smp_store_release():
633 smp_store_release(&b, p);
636 smp_store_release(&b, p);
640 In contrast, without explicit memory barriers, two-legged-if control
641 ordering is guaranteed only when the stores differ, for example:
652 The initial ACCESS_ONCE() is still required to prevent the compiler from
653 proving the value of 'a'.
655 In addition, you need to be careful what you do with the local variable 'q',
656 otherwise the compiler might be able to guess the value and again remove
657 the needed conditional. For example:
668 If MAX is defined to be 1, then the compiler knows that (q % MAX) is
669 equal to zero, in which case the compiler is within its rights to
670 transform the above code into the following:
676 Given this transformation, the CPU is not required to respect the ordering
677 between the load from variable 'a' and the store to variable 'b'. It is
678 tempting to add a barrier(), but this does not help. The conditional
679 is gone, and the barrier won't bring it back. Therefore, if you are
680 relying on this ordering, you should make sure that MAX is greater than
681 one, perhaps as follows:
684 BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
693 Please note once again that the stores to 'b' differ. If they were
694 identical, as noted earlier, the compiler could pull this store outside
695 of the 'if' statement.
697 You must also be careful not to rely too much on boolean short-circuit
698 evaluation. Consider this example:
704 Because the second condition is always true, the compiler can transform
705 this example as following, defeating control dependency:
710 This example underscores the need to ensure that the compiler cannot
711 out-guess your code. More generally, although ACCESS_ONCE() does force
712 the compiler to actually emit code for a given load, it does not force
713 the compiler to use the results.
715 Finally, control dependencies do -not- provide transitivity. This is
716 demonstrated by two related examples, with the initial values of
717 x and y both being zero:
720 ===================== =====================
721 r1 = ACCESS_ONCE(x); r2 = ACCESS_ONCE(y);
722 if (r1 > 0) if (r2 > 0)
723 ACCESS_ONCE(y) = 1; ACCESS_ONCE(x) = 1;
725 assert(!(r1 == 1 && r2 == 1));
727 The above two-CPU example will never trigger the assert(). However,
728 if control dependencies guaranteed transitivity (which they do not),
729 then adding the following CPU would guarantee a related assertion:
732 =====================
735 assert(!(r1 == 2 && r2 == 1 && x == 2)); /* FAILS!!! */
737 But because control dependencies do -not- provide transitivity, the above
738 assertion can fail after the combined three-CPU example completes. If you
739 need the three-CPU example to provide ordering, you will need smp_mb()
740 between the loads and stores in the CPU 0 and CPU 1 code fragments,
741 that is, just before or just after the "if" statements.
743 These two examples are the LB and WWC litmus tests from this paper:
744 http://www.cl.cam.ac.uk/users/pes20/ppc-supplemental/test6.pdf and this
745 site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.
749 (*) Control dependencies can order prior loads against later stores.
750 However, they do -not- guarantee any other sort of ordering:
751 Not prior loads against later loads, nor prior stores against
752 later anything. If you need these other forms of ordering,
753 use smb_rmb(), smp_wmb(), or, in the case of prior stores and
754 later loads, smp_mb().
756 (*) If both legs of the "if" statement begin with identical stores
757 to the same variable, a barrier() statement is required at the
758 beginning of each leg of the "if" statement.
760 (*) Control dependencies require at least one run-time conditional
761 between the prior load and the subsequent store, and this
762 conditional must involve the prior load. If the compiler
763 is able to optimize the conditional away, it will have also
764 optimized away the ordering. Careful use of ACCESS_ONCE() can
765 help to preserve the needed conditional.
767 (*) Control dependencies require that the compiler avoid reordering the
768 dependency into nonexistence. Careful use of ACCESS_ONCE() or
769 barrier() can help to preserve your control dependency. Please
770 see the Compiler Barrier section for more information.
772 (*) Control dependencies do -not- provide transitivity. If you
773 need transitivity, use smp_mb().
779 When dealing with CPU-CPU interactions, certain types of memory barrier should
780 always be paired. A lack of appropriate pairing is almost certainly an error.
782 General barriers pair with each other, though they also pair with
783 most other types of barriers, albeit without transitivity. An acquire
784 barrier pairs with a release barrier, but both may also pair with other
785 barriers, including of course general barriers. A write barrier pairs
786 with a data dependency barrier, an acquire barrier, a release barrier,
787 a read barrier, or a general barrier. Similarly a read barrier or a
788 data dependency barrier pairs with a write barrier, an acquire barrier,
789 a release barrier, or a general barrier:
792 =============== ===============
795 ACCESS_ONCE(b) = 2; x = ACCESS_ONCE(b);
802 =============== ===============================
805 ACCESS_ONCE(b) = &a; x = ACCESS_ONCE(b);
806 <data dependency barrier>
809 Basically, the read barrier always has to be there, even though it can be of
812 [!] Note that the stores before the write barrier would normally be expected to
813 match the loads after the read barrier or the data dependency barrier, and vice
817 =================== ===================
818 ACCESS_ONCE(a) = 1; }---- --->{ v = ACCESS_ONCE(c);
819 ACCESS_ONCE(b) = 2; } \ / { w = ACCESS_ONCE(d);
820 <write barrier> \ <read barrier>
821 ACCESS_ONCE(c) = 3; } / \ { x = ACCESS_ONCE(a);
822 ACCESS_ONCE(d) = 4; }---- --->{ y = ACCESS_ONCE(b);
825 EXAMPLES OF MEMORY BARRIER SEQUENCES
826 ------------------------------------
828 Firstly, write barriers act as partial orderings on store operations.
829 Consider the following sequence of events:
832 =======================
840 This sequence of events is committed to the memory coherence system in an order
841 that the rest of the system might perceive as the unordered set of { STORE A,
842 STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
847 | |------>| C=3 | } /\
848 | | : +------+ }----- \ -----> Events perceptible to
849 | | : | A=1 | } \/ the rest of the system
851 | CPU 1 | : | B=2 | }
853 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier
854 | | +------+ } requires all stores prior to the
855 | | : | E=5 | } barrier to be committed before
856 | | : +------+ } further stores may take place
861 | Sequence in which stores are committed to the
862 | memory system by CPU 1
866 Secondly, data dependency barriers act as partial orderings on data-dependent
867 loads. Consider the following sequence of events:
870 ======================= =======================
871 { B = 7; X = 9; Y = 8; C = &Y }
876 STORE D = 4 LOAD C (gets &B)
879 Without intervention, CPU 2 may perceive the events on CPU 1 in some
880 effectively random order, despite the write barrier issued by CPU 1:
883 | | +------+ +-------+ | Sequence of update
884 | |------>| B=2 |----- --->| Y->8 | | of perception on
885 | | : +------+ \ +-------+ | CPU 2
886 | CPU 1 | : | A=1 | \ --->| C->&Y | V
887 | | +------+ | +-------+
888 | | wwwwwwwwwwwwwwww | : :
890 | | : | C=&B |--- | : : +-------+
891 | | : +------+ \ | +-------+ | |
892 | |------>| D=4 | ----------->| C->&B |------>| |
893 | | +------+ | +-------+ | |
894 +-------+ : : | : : | |
898 Apparently incorrect ---> | | B->7 |------>| |
899 perception of B (!) | +-------+ | |
902 The load of X holds ---> \ | X->9 |------>| |
903 up the maintenance \ +-------+ | |
904 of coherence of B ----->| B->2 | +-------+
909 In the above example, CPU 2 perceives that B is 7, despite the load of *C
910 (which would be B) coming after the LOAD of C.
912 If, however, a data dependency barrier were to be placed between the load of C
913 and the load of *C (ie: B) on CPU 2:
916 ======================= =======================
917 { B = 7; X = 9; Y = 8; C = &Y }
922 STORE D = 4 LOAD C (gets &B)
923 <data dependency barrier>
926 then the following will occur:
929 | | +------+ +-------+
930 | |------>| B=2 |----- --->| Y->8 |
931 | | : +------+ \ +-------+
932 | CPU 1 | : | A=1 | \ --->| C->&Y |
933 | | +------+ | +-------+
934 | | wwwwwwwwwwwwwwww | : :
936 | | : | C=&B |--- | : : +-------+
937 | | : +------+ \ | +-------+ | |
938 | |------>| D=4 | ----------->| C->&B |------>| |
939 | | +------+ | +-------+ | |
940 +-------+ : : | : : | |
946 Makes sure all effects ---> \ ddddddddddddddddd | |
947 prior to the store of C \ +-------+ | |
948 are perceptible to ----->| B->2 |------>| |
949 subsequent loads +-------+ | |
953 And thirdly, a read barrier acts as a partial order on loads. Consider the
954 following sequence of events:
957 ======================= =======================
965 Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
966 some effectively random order, despite the write barrier issued by CPU 1:
969 | | +------+ +-------+
970 | |------>| A=1 |------ --->| A->0 |
971 | | +------+ \ +-------+
972 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
973 | | +------+ | +-------+
974 | |------>| B=2 |--- | : :
975 | | +------+ \ | : : +-------+
976 +-------+ : : \ | +-------+ | |
977 ---------->| B->2 |------>| |
978 | +-------+ | CPU 2 |
989 If, however, a read barrier were to be placed between the load of B and the
993 ======================= =======================
1002 then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
1006 | | +------+ +-------+
1007 | |------>| A=1 |------ --->| A->0 |
1008 | | +------+ \ +-------+
1009 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1010 | | +------+ | +-------+
1011 | |------>| B=2 |--- | : :
1012 | | +------+ \ | : : +-------+
1013 +-------+ : : \ | +-------+ | |
1014 ---------->| B->2 |------>| |
1015 | +-------+ | CPU 2 |
1018 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1019 barrier causes all effects \ +-------+ | |
1020 prior to the storage of B ---->| A->1 |------>| |
1021 to be perceptible to CPU 2 +-------+ | |
1025 To illustrate this more completely, consider what could happen if the code
1026 contained a load of A either side of the read barrier:
1029 ======================= =======================
1035 LOAD A [first load of A]
1037 LOAD A [second load of A]
1039 Even though the two loads of A both occur after the load of B, they may both
1040 come up with different values:
1043 | | +------+ +-------+
1044 | |------>| A=1 |------ --->| A->0 |
1045 | | +------+ \ +-------+
1046 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1047 | | +------+ | +-------+
1048 | |------>| B=2 |--- | : :
1049 | | +------+ \ | : : +-------+
1050 +-------+ : : \ | +-------+ | |
1051 ---------->| B->2 |------>| |
1052 | +-------+ | CPU 2 |
1056 | | A->0 |------>| 1st |
1058 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1059 barrier causes all effects \ +-------+ | |
1060 prior to the storage of B ---->| A->1 |------>| 2nd |
1061 to be perceptible to CPU 2 +-------+ | |
1065 But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1066 before the read barrier completes anyway:
1069 | | +------+ +-------+
1070 | |------>| A=1 |------ --->| A->0 |
1071 | | +------+ \ +-------+
1072 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1073 | | +------+ | +-------+
1074 | |------>| B=2 |--- | : :
1075 | | +------+ \ | : : +-------+
1076 +-------+ : : \ | +-------+ | |
1077 ---------->| B->2 |------>| |
1078 | +-------+ | CPU 2 |
1082 ---->| A->1 |------>| 1st |
1084 rrrrrrrrrrrrrrrrr | |
1086 | A->1 |------>| 2nd |
1091 The guarantee is that the second load will always come up with A == 1 if the
1092 load of B came up with B == 2. No such guarantee exists for the first load of
1093 A; that may come up with either A == 0 or A == 1.
1096 READ MEMORY BARRIERS VS LOAD SPECULATION
1097 ----------------------------------------
1099 Many CPUs speculate with loads: that is they see that they will need to load an
1100 item from memory, and they find a time where they're not using the bus for any
1101 other loads, and so do the load in advance - even though they haven't actually
1102 got to that point in the instruction execution flow yet. This permits the
1103 actual load instruction to potentially complete immediately because the CPU
1104 already has the value to hand.
1106 It may turn out that the CPU didn't actually need the value - perhaps because a
1107 branch circumvented the load - in which case it can discard the value or just
1108 cache it for later use.
1113 ======================= =======================
1115 DIVIDE } Divide instructions generally
1116 DIVIDE } take a long time to perform
1119 Which might appear as this:
1123 --->| B->2 |------>| |
1127 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1128 division speculates on the +-------+ ~ | |
1132 Once the divisions are complete --> : : ~-->| |
1133 the CPU can then perform the : : | |
1134 LOAD with immediate effect : : +-------+
1137 Placing a read barrier or a data dependency barrier just before the second
1141 ======================= =======================
1148 will force any value speculatively obtained to be reconsidered to an extent
1149 dependent on the type of barrier used. If there was no change made to the
1150 speculated memory location, then the speculated value will just be used:
1154 --->| B->2 |------>| |
1158 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1159 division speculates on the +-------+ ~ | |
1164 rrrrrrrrrrrrrrrr~ | |
1171 but if there was an update or an invalidation from another CPU pending, then
1172 the speculation will be cancelled and the value reloaded:
1176 --->| B->2 |------>| |
1180 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1181 division speculates on the +-------+ ~ | |
1186 rrrrrrrrrrrrrrrrr | |
1188 The speculation is discarded ---> --->| A->1 |------>| |
1189 and an updated value is +-------+ | |
1190 retrieved : : +-------+
1196 Transitivity is a deeply intuitive notion about ordering that is not
1197 always provided by real computer systems. The following example
1198 demonstrates transitivity (also called "cumulativity"):
1201 ======================= ======================= =======================
1203 STORE X=1 LOAD X STORE Y=1
1204 <general barrier> <general barrier>
1207 Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
1208 This indicates that CPU 2's load from X in some sense follows CPU 1's
1209 store to X and that CPU 2's load from Y in some sense preceded CPU 3's
1210 store to Y. The question is then "Can CPU 3's load from X return 0?"
1212 Because CPU 2's load from X in some sense came after CPU 1's store, it
1213 is natural to expect that CPU 3's load from X must therefore return 1.
1214 This expectation is an example of transitivity: if a load executing on
1215 CPU A follows a load from the same variable executing on CPU B, then
1216 CPU A's load must either return the same value that CPU B's load did,
1217 or must return some later value.
1219 In the Linux kernel, use of general memory barriers guarantees
1220 transitivity. Therefore, in the above example, if CPU 2's load from X
1221 returns 1 and its load from Y returns 0, then CPU 3's load from X must
1224 However, transitivity is -not- guaranteed for read or write barriers.
1225 For example, suppose that CPU 2's general barrier in the above example
1226 is changed to a read barrier as shown below:
1229 ======================= ======================= =======================
1231 STORE X=1 LOAD X STORE Y=1
1232 <read barrier> <general barrier>
1235 This substitution destroys transitivity: in this example, it is perfectly
1236 legal for CPU 2's load from X to return 1, its load from Y to return 0,
1237 and CPU 3's load from X to return 0.
1239 The key point is that although CPU 2's read barrier orders its pair
1240 of loads, it does not guarantee to order CPU 1's store. Therefore, if
1241 this example runs on a system where CPUs 1 and 2 share a store buffer
1242 or a level of cache, CPU 2 might have early access to CPU 1's writes.
1243 General barriers are therefore required to ensure that all CPUs agree
1244 on the combined order of CPU 1's and CPU 2's accesses.
1246 To reiterate, if your code requires transitivity, use general barriers
1250 ========================
1251 EXPLICIT KERNEL BARRIERS
1252 ========================
1254 The Linux kernel has a variety of different barriers that act at different
1257 (*) Compiler barrier.
1259 (*) CPU memory barriers.
1261 (*) MMIO write barrier.
1267 The Linux kernel has an explicit compiler barrier function that prevents the
1268 compiler from moving the memory accesses either side of it to the other side:
1272 This is a general barrier -- there are no read-read or write-write variants
1273 of barrier(). However, ACCESS_ONCE() can be thought of as a weak form
1274 for barrier() that affects only the specific accesses flagged by the
1277 The barrier() function has the following effects:
1279 (*) Prevents the compiler from reordering accesses following the
1280 barrier() to precede any accesses preceding the barrier().
1281 One example use for this property is to ease communication between
1282 interrupt-handler code and the code that was interrupted.
1284 (*) Within a loop, forces the compiler to load the variables used
1285 in that loop's conditional on each pass through that loop.
1287 The ACCESS_ONCE() function can prevent any number of optimizations that,
1288 while perfectly safe in single-threaded code, can be fatal in concurrent
1289 code. Here are some examples of these sorts of optimizations:
1291 (*) The compiler is within its rights to reorder loads and stores
1292 to the same variable, and in some cases, the CPU is within its
1293 rights to reorder loads to the same variable. This means that
1299 Might result in an older value of x stored in a[1] than in a[0].
1300 Prevent both the compiler and the CPU from doing this as follows:
1302 a[0] = ACCESS_ONCE(x);
1303 a[1] = ACCESS_ONCE(x);
1305 In short, ACCESS_ONCE() provides cache coherence for accesses from
1306 multiple CPUs to a single variable.
1308 (*) The compiler is within its rights to merge successive loads from
1309 the same variable. Such merging can cause the compiler to "optimize"
1313 do_something_with(tmp);
1315 into the following code, which, although in some sense legitimate
1316 for single-threaded code, is almost certainly not what the developer
1321 do_something_with(tmp);
1323 Use ACCESS_ONCE() to prevent the compiler from doing this to you:
1325 while (tmp = ACCESS_ONCE(a))
1326 do_something_with(tmp);
1328 (*) The compiler is within its rights to reload a variable, for example,
1329 in cases where high register pressure prevents the compiler from
1330 keeping all data of interest in registers. The compiler might
1331 therefore optimize the variable 'tmp' out of our previous example:
1334 do_something_with(tmp);
1336 This could result in the following code, which is perfectly safe in
1337 single-threaded code, but can be fatal in concurrent code:
1340 do_something_with(a);
1342 For example, the optimized version of this code could result in
1343 passing a zero to do_something_with() in the case where the variable
1344 a was modified by some other CPU between the "while" statement and
1345 the call to do_something_with().
1347 Again, use ACCESS_ONCE() to prevent the compiler from doing this:
1349 while (tmp = ACCESS_ONCE(a))
1350 do_something_with(tmp);
1352 Note that if the compiler runs short of registers, it might save
1353 tmp onto the stack. The overhead of this saving and later restoring
1354 is why compilers reload variables. Doing so is perfectly safe for
1355 single-threaded code, so you need to tell the compiler about cases
1356 where it is not safe.
1358 (*) The compiler is within its rights to omit a load entirely if it knows
1359 what the value will be. For example, if the compiler can prove that
1360 the value of variable 'a' is always zero, it can optimize this code:
1363 do_something_with(tmp);
1369 This transformation is a win for single-threaded code because it gets
1370 rid of a load and a branch. The problem is that the compiler will
1371 carry out its proof assuming that the current CPU is the only one
1372 updating variable 'a'. If variable 'a' is shared, then the compiler's
1373 proof will be erroneous. Use ACCESS_ONCE() to tell the compiler
1374 that it doesn't know as much as it thinks it does:
1376 while (tmp = ACCESS_ONCE(a))
1377 do_something_with(tmp);
1379 But please note that the compiler is also closely watching what you
1380 do with the value after the ACCESS_ONCE(). For example, suppose you
1381 do the following and MAX is a preprocessor macro with the value 1:
1383 while ((tmp = ACCESS_ONCE(a)) % MAX)
1384 do_something_with(tmp);
1386 Then the compiler knows that the result of the "%" operator applied
1387 to MAX will always be zero, again allowing the compiler to optimize
1388 the code into near-nonexistence. (It will still load from the
1391 (*) Similarly, the compiler is within its rights to omit a store entirely
1392 if it knows that the variable already has the value being stored.
1393 Again, the compiler assumes that the current CPU is the only one
1394 storing into the variable, which can cause the compiler to do the
1395 wrong thing for shared variables. For example, suppose you have
1399 /* Code that does not store to variable a. */
1402 The compiler sees that the value of variable 'a' is already zero, so
1403 it might well omit the second store. This would come as a fatal
1404 surprise if some other CPU might have stored to variable 'a' in the
1407 Use ACCESS_ONCE() to prevent the compiler from making this sort of
1411 /* Code that does not store to variable a. */
1414 (*) The compiler is within its rights to reorder memory accesses unless
1415 you tell it not to. For example, consider the following interaction
1416 between process-level code and an interrupt handler:
1418 void process_level(void)
1420 msg = get_message();
1424 void interrupt_handler(void)
1427 process_message(msg);
1430 There is nothing to prevent the compiler from transforming
1431 process_level() to the following, in fact, this might well be a
1432 win for single-threaded code:
1434 void process_level(void)
1437 msg = get_message();
1440 If the interrupt occurs between these two statement, then
1441 interrupt_handler() might be passed a garbled msg. Use ACCESS_ONCE()
1442 to prevent this as follows:
1444 void process_level(void)
1446 ACCESS_ONCE(msg) = get_message();
1447 ACCESS_ONCE(flag) = true;
1450 void interrupt_handler(void)
1452 if (ACCESS_ONCE(flag))
1453 process_message(ACCESS_ONCE(msg));
1456 Note that the ACCESS_ONCE() wrappers in interrupt_handler()
1457 are needed if this interrupt handler can itself be interrupted
1458 by something that also accesses 'flag' and 'msg', for example,
1459 a nested interrupt or an NMI. Otherwise, ACCESS_ONCE() is not
1460 needed in interrupt_handler() other than for documentation purposes.
1461 (Note also that nested interrupts do not typically occur in modern
1462 Linux kernels, in fact, if an interrupt handler returns with
1463 interrupts enabled, you will get a WARN_ONCE() splat.)
1465 You should assume that the compiler can move ACCESS_ONCE() past
1466 code not containing ACCESS_ONCE(), barrier(), or similar primitives.
1468 This effect could also be achieved using barrier(), but ACCESS_ONCE()
1469 is more selective: With ACCESS_ONCE(), the compiler need only forget
1470 the contents of the indicated memory locations, while with barrier()
1471 the compiler must discard the value of all memory locations that
1472 it has currented cached in any machine registers. Of course,
1473 the compiler must also respect the order in which the ACCESS_ONCE()s
1474 occur, though the CPU of course need not do so.
1476 (*) The compiler is within its rights to invent stores to a variable,
1477 as in the following example:
1484 The compiler might save a branch by optimizing this as follows:
1490 In single-threaded code, this is not only safe, but also saves
1491 a branch. Unfortunately, in concurrent code, this optimization
1492 could cause some other CPU to see a spurious value of 42 -- even
1493 if variable 'a' was never zero -- when loading variable 'b'.
1494 Use ACCESS_ONCE() to prevent this as follows:
1499 ACCESS_ONCE(b) = 42;
1501 The compiler can also invent loads. These are usually less
1502 damaging, but they can result in cache-line bouncing and thus in
1503 poor performance and scalability. Use ACCESS_ONCE() to prevent
1506 (*) For aligned memory locations whose size allows them to be accessed
1507 with a single memory-reference instruction, prevents "load tearing"
1508 and "store tearing," in which a single large access is replaced by
1509 multiple smaller accesses. For example, given an architecture having
1510 16-bit store instructions with 7-bit immediate fields, the compiler
1511 might be tempted to use two 16-bit store-immediate instructions to
1512 implement the following 32-bit store:
1516 Please note that GCC really does use this sort of optimization,
1517 which is not surprising given that it would likely take more
1518 than two instructions to build the constant and then store it.
1519 This optimization can therefore be a win in single-threaded code.
1520 In fact, a recent bug (since fixed) caused GCC to incorrectly use
1521 this optimization in a volatile store. In the absence of such bugs,
1522 use of ACCESS_ONCE() prevents store tearing in the following example:
1524 ACCESS_ONCE(p) = 0x00010002;
1526 Use of packed structures can also result in load and store tearing,
1529 struct __attribute__((__packed__)) foo {
1534 struct foo foo1, foo2;
1541 Because there are no ACCESS_ONCE() wrappers and no volatile markings,
1542 the compiler would be well within its rights to implement these three
1543 assignment statements as a pair of 32-bit loads followed by a pair
1544 of 32-bit stores. This would result in load tearing on 'foo1.b'
1545 and store tearing on 'foo2.b'. ACCESS_ONCE() again prevents tearing
1549 ACCESS_ONCE(foo2.b) = ACCESS_ONCE(foo1.b);
1552 All that aside, it is never necessary to use ACCESS_ONCE() on a variable
1553 that has been marked volatile. For example, because 'jiffies' is marked
1554 volatile, it is never necessary to say ACCESS_ONCE(jiffies). The reason
1555 for this is that ACCESS_ONCE() is implemented as a volatile cast, which
1556 has no effect when its argument is already marked volatile.
1558 Please note that these compiler barriers have no direct effect on the CPU,
1559 which may then reorder things however it wishes.
1565 The Linux kernel has eight basic CPU memory barriers:
1567 TYPE MANDATORY SMP CONDITIONAL
1568 =============== ======================= ===========================
1569 GENERAL mb() smp_mb()
1570 WRITE wmb() smp_wmb()
1571 READ rmb() smp_rmb()
1572 DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends()
1575 All memory barriers except the data dependency barriers imply a compiler
1576 barrier. Data dependencies do not impose any additional compiler ordering.
1578 Aside: In the case of data dependencies, the compiler would be expected to
1579 issue the loads in the correct order (eg. `a[b]` would have to load the value
1580 of b before loading a[b]), however there is no guarantee in the C specification
1581 that the compiler may not speculate the value of b (eg. is equal to 1) and load
1582 a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the
1583 problem of a compiler reloading b after having loaded a[b], thus having a newer
1584 copy of b than a[b]. A consensus has not yet been reached about these problems,
1585 however the ACCESS_ONCE macro is a good place to start looking.
1587 SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1588 systems because it is assumed that a CPU will appear to be self-consistent,
1589 and will order overlapping accesses correctly with respect to itself.
1591 [!] Note that SMP memory barriers _must_ be used to control the ordering of
1592 references to shared memory on SMP systems, though the use of locking instead
1595 Mandatory barriers should not be used to control SMP effects, since mandatory
1596 barriers unnecessarily impose overhead on UP systems. They may, however, be
1597 used to control MMIO effects on accesses through relaxed memory I/O windows.
1598 These are required even on non-SMP systems as they affect the order in which
1599 memory operations appear to a device by prohibiting both the compiler and the
1600 CPU from reordering them.
1603 There are some more advanced barrier functions:
1605 (*) set_mb(var, value)
1607 This assigns the value to the variable and then inserts a full memory
1608 barrier after it, depending on the function. It isn't guaranteed to
1609 insert anything more than a compiler barrier in a UP compilation.
1612 (*) smp_mb__before_atomic();
1613 (*) smp_mb__after_atomic();
1615 These are for use with atomic (such as add, subtract, increment and
1616 decrement) functions that don't return a value, especially when used for
1617 reference counting. These functions do not imply memory barriers.
1619 These are also used for atomic bitop functions that do not return a
1620 value (such as set_bit and clear_bit).
1622 As an example, consider a piece of code that marks an object as being dead
1623 and then decrements the object's reference count:
1626 smp_mb__before_atomic();
1627 atomic_dec(&obj->ref_count);
1629 This makes sure that the death mark on the object is perceived to be set
1630 *before* the reference counter is decremented.
1632 See Documentation/atomic_ops.txt for more information. See the "Atomic
1633 operations" subsection for information on where to use these.
1639 These are for use with consistent memory to guarantee the ordering
1640 of writes or reads of shared memory accessible to both the CPU and a
1643 For example, consider a device driver that shares memory with a device
1644 and uses a descriptor status value to indicate if the descriptor belongs
1645 to the device or the CPU, and a doorbell to notify it when new
1646 descriptors are available:
1648 if (desc->status != DEVICE_OWN) {
1649 /* do not read data until we own descriptor */
1652 /* read/modify data */
1653 read_data = desc->data;
1654 desc->data = write_data;
1656 /* flush modifications before status update */
1659 /* assign ownership */
1660 desc->status = DEVICE_OWN;
1662 /* force memory to sync before notifying device via MMIO */
1665 /* notify device of new descriptors */
1666 writel(DESC_NOTIFY, doorbell);
1669 The dma_rmb() allows us guarantee the device has released ownership
1670 before we read the data from the descriptor, and he dma_wmb() allows
1671 us to guarantee the data is written to the descriptor before the device
1672 can see it now has ownership. The wmb() is needed to guarantee that the
1673 cache coherent memory writes have completed before attempting a write to
1674 the cache incoherent MMIO region.
1676 See Documentation/DMA-API.txt for more information on consistent memory.
1681 The Linux kernel also has a special barrier for use with memory-mapped I/O
1686 This is a variation on the mandatory write barrier that causes writes to weakly
1687 ordered I/O regions to be partially ordered. Its effects may go beyond the
1688 CPU->Hardware interface and actually affect the hardware at some level.
1690 See the subsection "Locks vs I/O accesses" for more information.
1693 ===============================
1694 IMPLICIT KERNEL MEMORY BARRIERS
1695 ===============================
1697 Some of the other functions in the linux kernel imply memory barriers, amongst
1698 which are locking and scheduling functions.
1700 This specification is a _minimum_ guarantee; any particular architecture may
1701 provide more substantial guarantees, but these may not be relied upon outside
1702 of arch specific code.
1708 The Linux kernel has a number of locking constructs:
1717 In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
1718 for each construct. These operations all imply certain barriers:
1720 (1) ACQUIRE operation implication:
1722 Memory operations issued after the ACQUIRE will be completed after the
1723 ACQUIRE operation has completed.
1725 Memory operations issued before the ACQUIRE may be completed after
1726 the ACQUIRE operation has completed. An smp_mb__before_spinlock(),
1727 combined with a following ACQUIRE, orders prior loads against
1728 subsequent loads and stores and also orders prior stores against
1729 subsequent stores. Note that this is weaker than smp_mb()! The
1730 smp_mb__before_spinlock() primitive is free on many architectures.
1732 (2) RELEASE operation implication:
1734 Memory operations issued before the RELEASE will be completed before the
1735 RELEASE operation has completed.
1737 Memory operations issued after the RELEASE may be completed before the
1738 RELEASE operation has completed.
1740 (3) ACQUIRE vs ACQUIRE implication:
1742 All ACQUIRE operations issued before another ACQUIRE operation will be
1743 completed before that ACQUIRE operation.
1745 (4) ACQUIRE vs RELEASE implication:
1747 All ACQUIRE operations issued before a RELEASE operation will be
1748 completed before the RELEASE operation.
1750 (5) Failed conditional ACQUIRE implication:
1752 Certain locking variants of the ACQUIRE operation may fail, either due to
1753 being unable to get the lock immediately, or due to receiving an unblocked
1754 signal whilst asleep waiting for the lock to become available. Failed
1755 locks do not imply any sort of barrier.
1757 [!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
1758 one-way barriers is that the effects of instructions outside of a critical
1759 section may seep into the inside of the critical section.
1761 An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
1762 because it is possible for an access preceding the ACQUIRE to happen after the
1763 ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
1764 the two accesses can themselves then cross:
1773 ACQUIRE M, STORE *B, STORE *A, RELEASE M
1775 When the ACQUIRE and RELEASE are a lock acquisition and release,
1776 respectively, this same reordering can occur if the lock's ACQUIRE and
1777 RELEASE are to the same lock variable, but only from the perspective of
1778 another CPU not holding that lock. In short, a ACQUIRE followed by an
1779 RELEASE may -not- be assumed to be a full memory barrier.
1781 Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not
1782 imply a full memory barrier. If it is necessary for a RELEASE-ACQUIRE
1783 pair to produce a full barrier, the ACQUIRE can be followed by an
1784 smp_mb__after_unlock_lock() invocation. This will produce a full barrier
1785 if either (a) the RELEASE and the ACQUIRE are executed by the same
1786 CPU or task, or (b) the RELEASE and ACQUIRE act on the same variable.
1787 The smp_mb__after_unlock_lock() primitive is free on many architectures.
1788 Without smp_mb__after_unlock_lock(), the CPU's execution of the critical
1789 sections corresponding to the RELEASE and the ACQUIRE can cross, so that:
1798 ACQUIRE N, STORE *B, STORE *A, RELEASE M
1800 It might appear that this reordering could introduce a deadlock.
1801 However, this cannot happen because if such a deadlock threatened,
1802 the RELEASE would simply complete, thereby avoiding the deadlock.
1806 One key point is that we are only talking about the CPU doing
1807 the reordering, not the compiler. If the compiler (or, for
1808 that matter, the developer) switched the operations, deadlock
1811 But suppose the CPU reordered the operations. In this case,
1812 the unlock precedes the lock in the assembly code. The CPU
1813 simply elected to try executing the later lock operation first.
1814 If there is a deadlock, this lock operation will simply spin (or
1815 try to sleep, but more on that later). The CPU will eventually
1816 execute the unlock operation (which preceded the lock operation
1817 in the assembly code), which will unravel the potential deadlock,
1818 allowing the lock operation to succeed.
1820 But what if the lock is a sleeplock? In that case, the code will
1821 try to enter the scheduler, where it will eventually encounter
1822 a memory barrier, which will force the earlier unlock operation
1823 to complete, again unraveling the deadlock. There might be
1824 a sleep-unlock race, but the locking primitive needs to resolve
1825 such races properly in any case.
1827 With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
1828 For example, with the following code, the store to *A will always be
1829 seen by other CPUs before the store to *B:
1834 smp_mb__after_unlock_lock();
1837 The operations will always occur in one of the following orders:
1839 STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
1840 STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
1841 ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
1843 If the RELEASE and ACQUIRE were instead both operating on the same lock
1844 variable, only the first of these alternatives can occur. In addition,
1845 the more strongly ordered systems may rule out some of the above orders.
1846 But in any case, as noted earlier, the smp_mb__after_unlock_lock()
1847 ensures that the store to *A will always be seen as happening before
1850 Locks and semaphores may not provide any guarantee of ordering on UP compiled
1851 systems, and so cannot be counted on in such a situation to actually achieve
1852 anything at all - especially with respect to I/O accesses - unless combined
1853 with interrupt disabling operations.
1855 See also the section on "Inter-CPU locking barrier effects".
1858 As an example, consider the following:
1869 The following sequence of events is acceptable:
1871 ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
1873 [+] Note that {*F,*A} indicates a combined access.
1875 But none of the following are:
1877 {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E
1878 *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F
1879 *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F
1880 *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E
1884 INTERRUPT DISABLING FUNCTIONS
1885 -----------------------------
1887 Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
1888 (RELEASE equivalent) will act as compiler barriers only. So if memory or I/O
1889 barriers are required in such a situation, they must be provided from some
1893 SLEEP AND WAKE-UP FUNCTIONS
1894 ---------------------------
1896 Sleeping and waking on an event flagged in global data can be viewed as an
1897 interaction between two pieces of data: the task state of the task waiting for
1898 the event and the global data used to indicate the event. To make sure that
1899 these appear to happen in the right order, the primitives to begin the process
1900 of going to sleep, and the primitives to initiate a wake up imply certain
1903 Firstly, the sleeper normally follows something like this sequence of events:
1906 set_current_state(TASK_UNINTERRUPTIBLE);
1907 if (event_indicated)
1912 A general memory barrier is interpolated automatically by set_current_state()
1913 after it has altered the task state:
1916 ===============================
1917 set_current_state();
1919 STORE current->state
1921 LOAD event_indicated
1923 set_current_state() may be wrapped by:
1926 prepare_to_wait_exclusive();
1928 which therefore also imply a general memory barrier after setting the state.
1929 The whole sequence above is available in various canned forms, all of which
1930 interpolate the memory barrier in the right place:
1933 wait_event_interruptible();
1934 wait_event_interruptible_exclusive();
1935 wait_event_interruptible_timeout();
1936 wait_event_killable();
1937 wait_event_timeout();
1942 Secondly, code that performs a wake up normally follows something like this:
1944 event_indicated = 1;
1945 wake_up(&event_wait_queue);
1949 event_indicated = 1;
1950 wake_up_process(event_daemon);
1952 A write memory barrier is implied by wake_up() and co. if and only if they wake
1953 something up. The barrier occurs before the task state is cleared, and so sits
1954 between the STORE to indicate the event and the STORE to set TASK_RUNNING:
1957 =============================== ===============================
1958 set_current_state(); STORE event_indicated
1959 set_mb(); wake_up();
1960 STORE current->state <write barrier>
1961 <general barrier> STORE current->state
1962 LOAD event_indicated
1964 To repeat, this write memory barrier is present if and only if something
1965 is actually awakened. To see this, consider the following sequence of
1966 events, where X and Y are both initially zero:
1969 =============================== ===============================
1970 X = 1; STORE event_indicated
1971 smp_mb(); wake_up();
1972 Y = 1; wait_event(wq, Y == 1);
1973 wake_up(); load from Y sees 1, no memory barrier
1974 load from X might see 0
1976 In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
1979 The available waker functions include:
1985 wake_up_interruptible();
1986 wake_up_interruptible_all();
1987 wake_up_interruptible_nr();
1988 wake_up_interruptible_poll();
1989 wake_up_interruptible_sync();
1990 wake_up_interruptible_sync_poll();
1992 wake_up_locked_poll();
1998 [!] Note that the memory barriers implied by the sleeper and the waker do _not_
1999 order multiple stores before the wake-up with respect to loads of those stored
2000 values after the sleeper has called set_current_state(). For instance, if the
2003 set_current_state(TASK_INTERRUPTIBLE);
2004 if (event_indicated)
2006 __set_current_state(TASK_RUNNING);
2007 do_something(my_data);
2012 event_indicated = 1;
2013 wake_up(&event_wait_queue);
2015 there's no guarantee that the change to event_indicated will be perceived by
2016 the sleeper as coming after the change to my_data. In such a circumstance, the
2017 code on both sides must interpolate its own memory barriers between the
2018 separate data accesses. Thus the above sleeper ought to do:
2020 set_current_state(TASK_INTERRUPTIBLE);
2021 if (event_indicated) {
2023 do_something(my_data);
2026 and the waker should do:
2030 event_indicated = 1;
2031 wake_up(&event_wait_queue);
2034 MISCELLANEOUS FUNCTIONS
2035 -----------------------
2037 Other functions that imply barriers:
2039 (*) schedule() and similar imply full memory barriers.
2042 ===================================
2043 INTER-CPU ACQUIRING BARRIER EFFECTS
2044 ===================================
2046 On SMP systems locking primitives give a more substantial form of barrier: one
2047 that does affect memory access ordering on other CPUs, within the context of
2048 conflict on any particular lock.
2051 ACQUIRES VS MEMORY ACCESSES
2052 ---------------------------
2054 Consider the following: the system has a pair of spinlocks (M) and (Q), and
2055 three CPUs; then should the following sequence of events occur:
2058 =============================== ===============================
2059 ACCESS_ONCE(*A) = a; ACCESS_ONCE(*E) = e;
2061 ACCESS_ONCE(*B) = b; ACCESS_ONCE(*F) = f;
2062 ACCESS_ONCE(*C) = c; ACCESS_ONCE(*G) = g;
2064 ACCESS_ONCE(*D) = d; ACCESS_ONCE(*H) = h;
2066 Then there is no guarantee as to what order CPU 3 will see the accesses to *A
2067 through *H occur in, other than the constraints imposed by the separate locks
2068 on the separate CPUs. It might, for example, see:
2070 *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
2072 But it won't see any of:
2074 *B, *C or *D preceding ACQUIRE M
2075 *A, *B or *C following RELEASE M
2076 *F, *G or *H preceding ACQUIRE Q
2077 *E, *F or *G following RELEASE Q
2080 However, if the following occurs:
2083 =============================== ===============================
2084 ACCESS_ONCE(*A) = a;
2086 ACCESS_ONCE(*B) = b;
2087 ACCESS_ONCE(*C) = c;
2089 ACCESS_ONCE(*D) = d; ACCESS_ONCE(*E) = e;
2091 smp_mb__after_unlock_lock();
2092 ACCESS_ONCE(*F) = f;
2093 ACCESS_ONCE(*G) = g;
2095 ACCESS_ONCE(*H) = h;
2099 *E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
2100 ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
2102 But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
2104 *B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
2105 *A, *B or *C following RELEASE M [1]
2106 *F, *G or *H preceding ACQUIRE M [2]
2107 *A, *B, *C, *E, *F or *G following RELEASE M [2]
2109 Note that the smp_mb__after_unlock_lock() is critically important
2110 here: Without it CPU 3 might see some of the above orderings.
2111 Without smp_mb__after_unlock_lock(), the accesses are not guaranteed
2112 to be seen in order unless CPU 3 holds lock M.
2115 ACQUIRES VS I/O ACCESSES
2116 ------------------------
2118 Under certain circumstances (especially involving NUMA), I/O accesses within
2119 two spinlocked sections on two different CPUs may be seen as interleaved by the
2120 PCI bridge, because the PCI bridge does not necessarily participate in the
2121 cache-coherence protocol, and is therefore incapable of issuing the required
2122 read memory barriers.
2127 =============================== ===============================
2137 may be seen by the PCI bridge as follows:
2139 STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5
2141 which would probably cause the hardware to malfunction.
2144 What is necessary here is to intervene with an mmiowb() before dropping the
2145 spinlock, for example:
2148 =============================== ===============================
2160 this will ensure that the two stores issued on CPU 1 appear at the PCI bridge
2161 before either of the stores issued on CPU 2.
2164 Furthermore, following a store by a load from the same device obviates the need
2165 for the mmiowb(), because the load forces the store to complete before the load
2169 =============================== ===============================
2180 See Documentation/DocBook/deviceiobook.tmpl for more information.
2183 =================================
2184 WHERE ARE MEMORY BARRIERS NEEDED?
2185 =================================
2187 Under normal operation, memory operation reordering is generally not going to
2188 be a problem as a single-threaded linear piece of code will still appear to
2189 work correctly, even if it's in an SMP kernel. There are, however, four
2190 circumstances in which reordering definitely _could_ be a problem:
2192 (*) Interprocessor interaction.
2194 (*) Atomic operations.
2196 (*) Accessing devices.
2201 INTERPROCESSOR INTERACTION
2202 --------------------------
2204 When there's a system with more than one processor, more than one CPU in the
2205 system may be working on the same data set at the same time. This can cause
2206 synchronisation problems, and the usual way of dealing with them is to use
2207 locks. Locks, however, are quite expensive, and so it may be preferable to
2208 operate without the use of a lock if at all possible. In such a case
2209 operations that affect both CPUs may have to be carefully ordered to prevent
2212 Consider, for example, the R/W semaphore slow path. Here a waiting process is
2213 queued on the semaphore, by virtue of it having a piece of its stack linked to
2214 the semaphore's list of waiting processes:
2216 struct rw_semaphore {
2219 struct list_head waiters;
2222 struct rwsem_waiter {
2223 struct list_head list;
2224 struct task_struct *task;
2227 To wake up a particular waiter, the up_read() or up_write() functions have to:
2229 (1) read the next pointer from this waiter's record to know as to where the
2230 next waiter record is;
2232 (2) read the pointer to the waiter's task structure;
2234 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2236 (4) call wake_up_process() on the task; and
2238 (5) release the reference held on the waiter's task struct.
2240 In other words, it has to perform this sequence of events:
2242 LOAD waiter->list.next;
2248 and if any of these steps occur out of order, then the whole thing may
2251 Once it has queued itself and dropped the semaphore lock, the waiter does not
2252 get the lock again; it instead just waits for its task pointer to be cleared
2253 before proceeding. Since the record is on the waiter's stack, this means that
2254 if the task pointer is cleared _before_ the next pointer in the list is read,
2255 another CPU might start processing the waiter and might clobber the waiter's
2256 stack before the up*() function has a chance to read the next pointer.
2258 Consider then what might happen to the above sequence of events:
2261 =============================== ===============================
2268 Woken up by other event
2273 foo() clobbers *waiter
2275 LOAD waiter->list.next;
2278 This could be dealt with using the semaphore lock, but then the down_xxx()
2279 function has to needlessly get the spinlock again after being woken up.
2281 The way to deal with this is to insert a general SMP memory barrier:
2283 LOAD waiter->list.next;
2290 In this case, the barrier makes a guarantee that all memory accesses before the
2291 barrier will appear to happen before all the memory accesses after the barrier
2292 with respect to the other CPUs on the system. It does _not_ guarantee that all
2293 the memory accesses before the barrier will be complete by the time the barrier
2294 instruction itself is complete.
2296 On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2297 compiler barrier, thus making sure the compiler emits the instructions in the
2298 right order without actually intervening in the CPU. Since there's only one
2299 CPU, that CPU's dependency ordering logic will take care of everything else.
2305 Whilst they are technically interprocessor interaction considerations, atomic
2306 operations are noted specially as some of them imply full memory barriers and
2307 some don't, but they're very heavily relied on as a group throughout the
2310 Any atomic operation that modifies some state in memory and returns information
2311 about the state (old or new) implies an SMP-conditional general memory barrier
2312 (smp_mb()) on each side of the actual operation (with the exception of
2313 explicit lock operations, described later). These include:
2317 atomic_xchg(); atomic_long_xchg();
2318 atomic_cmpxchg(); atomic_long_cmpxchg();
2319 atomic_inc_return(); atomic_long_inc_return();
2320 atomic_dec_return(); atomic_long_dec_return();
2321 atomic_add_return(); atomic_long_add_return();
2322 atomic_sub_return(); atomic_long_sub_return();
2323 atomic_inc_and_test(); atomic_long_inc_and_test();
2324 atomic_dec_and_test(); atomic_long_dec_and_test();
2325 atomic_sub_and_test(); atomic_long_sub_and_test();
2326 atomic_add_negative(); atomic_long_add_negative();
2328 test_and_clear_bit();
2329 test_and_change_bit();
2331 /* when succeeds (returns 1) */
2332 atomic_add_unless(); atomic_long_add_unless();
2334 These are used for such things as implementing ACQUIRE-class and RELEASE-class
2335 operations and adjusting reference counters towards object destruction, and as
2336 such the implicit memory barrier effects are necessary.
2339 The following operations are potential problems as they do _not_ imply memory
2340 barriers, but might be used for implementing such things as RELEASE-class
2348 With these the appropriate explicit memory barrier should be used if necessary
2349 (smp_mb__before_atomic() for instance).
2352 The following also do _not_ imply memory barriers, and so may require explicit
2353 memory barriers under some circumstances (smp_mb__before_atomic() for
2361 If they're used for statistics generation, then they probably don't need memory
2362 barriers, unless there's a coupling between statistical data.
2364 If they're used for reference counting on an object to control its lifetime,
2365 they probably don't need memory barriers because either the reference count
2366 will be adjusted inside a locked section, or the caller will already hold
2367 sufficient references to make the lock, and thus a memory barrier unnecessary.
2369 If they're used for constructing a lock of some description, then they probably
2370 do need memory barriers as a lock primitive generally has to do things in a
2373 Basically, each usage case has to be carefully considered as to whether memory
2374 barriers are needed or not.
2376 The following operations are special locking primitives:
2378 test_and_set_bit_lock();
2380 __clear_bit_unlock();
2382 These implement ACQUIRE-class and RELEASE-class operations. These should be used in
2383 preference to other operations when implementing locking primitives, because
2384 their implementations can be optimised on many architectures.
2386 [!] Note that special memory barrier primitives are available for these
2387 situations because on some CPUs the atomic instructions used imply full memory
2388 barriers, and so barrier instructions are superfluous in conjunction with them,
2389 and in such cases the special barrier primitives will be no-ops.
2391 See Documentation/atomic_ops.txt for more information.
2397 Many devices can be memory mapped, and so appear to the CPU as if they're just
2398 a set of memory locations. To control such a device, the driver usually has to
2399 make the right memory accesses in exactly the right order.
2401 However, having a clever CPU or a clever compiler creates a potential problem
2402 in that the carefully sequenced accesses in the driver code won't reach the
2403 device in the requisite order if the CPU or the compiler thinks it is more
2404 efficient to reorder, combine or merge accesses - something that would cause
2405 the device to malfunction.
2407 Inside of the Linux kernel, I/O should be done through the appropriate accessor
2408 routines - such as inb() or writel() - which know how to make such accesses
2409 appropriately sequential. Whilst this, for the most part, renders the explicit
2410 use of memory barriers unnecessary, there are a couple of situations where they
2413 (1) On some systems, I/O stores are not strongly ordered across all CPUs, and
2414 so for _all_ general drivers locks should be used and mmiowb() must be
2415 issued prior to unlocking the critical section.
2417 (2) If the accessor functions are used to refer to an I/O memory window with
2418 relaxed memory access properties, then _mandatory_ memory barriers are
2419 required to enforce ordering.
2421 See Documentation/DocBook/deviceiobook.tmpl for more information.
2427 A driver may be interrupted by its own interrupt service routine, and thus the
2428 two parts of the driver may interfere with each other's attempts to control or
2431 This may be alleviated - at least in part - by disabling local interrupts (a
2432 form of locking), such that the critical operations are all contained within
2433 the interrupt-disabled section in the driver. Whilst the driver's interrupt
2434 routine is executing, the driver's core may not run on the same CPU, and its
2435 interrupt is not permitted to happen again until the current interrupt has been
2436 handled, thus the interrupt handler does not need to lock against that.
2438 However, consider a driver that was talking to an ethernet card that sports an
2439 address register and a data register. If that driver's core talks to the card
2440 under interrupt-disablement and then the driver's interrupt handler is invoked:
2451 The store to the data register might happen after the second store to the
2452 address register if ordering rules are sufficiently relaxed:
2454 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2457 If ordering rules are relaxed, it must be assumed that accesses done inside an
2458 interrupt disabled section may leak outside of it and may interleave with
2459 accesses performed in an interrupt - and vice versa - unless implicit or
2460 explicit barriers are used.
2462 Normally this won't be a problem because the I/O accesses done inside such
2463 sections will include synchronous load operations on strictly ordered I/O
2464 registers that form implicit I/O barriers. If this isn't sufficient then an
2465 mmiowb() may need to be used explicitly.
2468 A similar situation may occur between an interrupt routine and two routines
2469 running on separate CPUs that communicate with each other. If such a case is
2470 likely, then interrupt-disabling locks should be used to guarantee ordering.
2473 ==========================
2474 KERNEL I/O BARRIER EFFECTS
2475 ==========================
2477 When accessing I/O memory, drivers should use the appropriate accessor
2482 These are intended to talk to I/O space rather than memory space, but
2483 that's primarily a CPU-specific concept. The i386 and x86_64 processors do
2484 indeed have special I/O space access cycles and instructions, but many
2485 CPUs don't have such a concept.
2487 The PCI bus, amongst others, defines an I/O space concept which - on such
2488 CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O
2489 space. However, it may also be mapped as a virtual I/O space in the CPU's
2490 memory map, particularly on those CPUs that don't support alternate I/O
2493 Accesses to this space may be fully synchronous (as on i386), but
2494 intermediary bridges (such as the PCI host bridge) may not fully honour
2497 They are guaranteed to be fully ordered with respect to each other.
2499 They are not guaranteed to be fully ordered with respect to other types of
2500 memory and I/O operation.
2502 (*) readX(), writeX():
2504 Whether these are guaranteed to be fully ordered and uncombined with
2505 respect to each other on the issuing CPU depends on the characteristics
2506 defined for the memory window through which they're accessing. On later
2507 i386 architecture machines, for example, this is controlled by way of the
2510 Ordinarily, these will be guaranteed to be fully ordered and uncombined,
2511 provided they're not accessing a prefetchable device.
2513 However, intermediary hardware (such as a PCI bridge) may indulge in
2514 deferral if it so wishes; to flush a store, a load from the same location
2515 is preferred[*], but a load from the same device or from configuration
2516 space should suffice for PCI.
2518 [*] NOTE! attempting to load from the same location as was written to may
2519 cause a malfunction - consider the 16550 Rx/Tx serial registers for
2522 Used with prefetchable I/O memory, an mmiowb() barrier may be required to
2523 force stores to be ordered.
2525 Please refer to the PCI specification for more information on interactions
2526 between PCI transactions.
2528 (*) readX_relaxed(), writeX_relaxed()
2530 These are similar to readX() and writeX(), but provide weaker memory
2531 ordering guarantees. Specifically, they do not guarantee ordering with
2532 respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee
2533 ordering with respect to LOCK or UNLOCK operations. If the latter is
2534 required, an mmiowb() barrier can be used. Note that relaxed accesses to
2535 the same peripheral are guaranteed to be ordered with respect to each
2538 (*) ioreadX(), iowriteX()
2540 These will perform appropriately for the type of access they're actually
2541 doing, be it inX()/outX() or readX()/writeX().
2544 ========================================
2545 ASSUMED MINIMUM EXECUTION ORDERING MODEL
2546 ========================================
2548 It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2549 maintain the appearance of program causality with respect to itself. Some CPUs
2550 (such as i386 or x86_64) are more constrained than others (such as powerpc or
2551 frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2552 of arch-specific code.
2554 This means that it must be considered that the CPU will execute its instruction
2555 stream in any order it feels like - or even in parallel - provided that if an
2556 instruction in the stream depends on an earlier instruction, then that
2557 earlier instruction must be sufficiently complete[*] before the later
2558 instruction may proceed; in other words: provided that the appearance of
2559 causality is maintained.
2561 [*] Some instructions have more than one effect - such as changing the
2562 condition codes, changing registers or changing memory - and different
2563 instructions may depend on different effects.
2565 A CPU may also discard any instruction sequence that winds up having no
2566 ultimate effect. For example, if two adjacent instructions both load an
2567 immediate value into the same register, the first may be discarded.
2570 Similarly, it has to be assumed that compiler might reorder the instruction
2571 stream in any way it sees fit, again provided the appearance of causality is
2575 ============================
2576 THE EFFECTS OF THE CPU CACHE
2577 ============================
2579 The way cached memory operations are perceived across the system is affected to
2580 a certain extent by the caches that lie between CPUs and memory, and by the
2581 memory coherence system that maintains the consistency of state in the system.
2583 As far as the way a CPU interacts with another part of the system through the
2584 caches goes, the memory system has to include the CPU's caches, and memory
2585 barriers for the most part act at the interface between the CPU and its cache
2586 (memory barriers logically act on the dotted line in the following diagram):
2588 <--- CPU ---> : <----------- Memory ----------->
2590 +--------+ +--------+ : +--------+ +-----------+
2591 | | | | : | | | | +--------+
2592 | CPU | | Memory | : | CPU | | | | |
2593 | Core |--->| Access |----->| Cache |<-->| | | |
2594 | | | Queue | : | | | |--->| Memory |
2595 | | | | : | | | | | |
2596 +--------+ +--------+ : +--------+ | | | |
2597 : | Cache | +--------+
2599 : | Mechanism | +--------+
2600 +--------+ +--------+ : +--------+ | | | |
2601 | | | | : | | | | | |
2602 | CPU | | Memory | : | CPU | | |--->| Device |
2603 | Core |--->| Access |----->| Cache |<-->| | | |
2604 | | | Queue | : | | | | | |
2605 | | | | : | | | | +--------+
2606 +--------+ +--------+ : +--------+ +-----------+
2610 Although any particular load or store may not actually appear outside of the
2611 CPU that issued it since it may have been satisfied within the CPU's own cache,
2612 it will still appear as if the full memory access had taken place as far as the
2613 other CPUs are concerned since the cache coherency mechanisms will migrate the
2614 cacheline over to the accessing CPU and propagate the effects upon conflict.
2616 The CPU core may execute instructions in any order it deems fit, provided the
2617 expected program causality appears to be maintained. Some of the instructions
2618 generate load and store operations which then go into the queue of memory
2619 accesses to be performed. The core may place these in the queue in any order
2620 it wishes, and continue execution until it is forced to wait for an instruction
2623 What memory barriers are concerned with is controlling the order in which
2624 accesses cross from the CPU side of things to the memory side of things, and
2625 the order in which the effects are perceived to happen by the other observers
2628 [!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2629 their own loads and stores as if they had happened in program order.
2631 [!] MMIO or other device accesses may bypass the cache system. This depends on
2632 the properties of the memory window through which devices are accessed and/or
2633 the use of any special device communication instructions the CPU may have.
2639 Life isn't quite as simple as it may appear above, however: for while the
2640 caches are expected to be coherent, there's no guarantee that that coherency
2641 will be ordered. This means that whilst changes made on one CPU will
2642 eventually become visible on all CPUs, there's no guarantee that they will
2643 become apparent in the same order on those other CPUs.
2646 Consider dealing with a system that has a pair of CPUs (1 & 2), each of which
2647 has a pair of parallel data caches (CPU 1 has A/B, and CPU 2 has C/D):
2652 +--------+ : +--->| Cache A |<------->| |
2653 | | : | +---------+ | |
2655 | | : | +---------+ | |
2656 +--------+ : +--->| Cache B |<------->| |
2659 : +---------+ | System |
2660 +--------+ : +--->| Cache C |<------->| |
2661 | | : | +---------+ | |
2663 | | : | +---------+ | |
2664 +--------+ : +--->| Cache D |<------->| |
2669 Imagine the system has the following properties:
2671 (*) an odd-numbered cache line may be in cache A, cache C or it may still be
2674 (*) an even-numbered cache line may be in cache B, cache D or it may still be
2677 (*) whilst the CPU core is interrogating one cache, the other cache may be
2678 making use of the bus to access the rest of the system - perhaps to
2679 displace a dirty cacheline or to do a speculative load;
2681 (*) each cache has a queue of operations that need to be applied to that cache
2682 to maintain coherency with the rest of the system;
2684 (*) the coherency queue is not flushed by normal loads to lines already
2685 present in the cache, even though the contents of the queue may
2686 potentially affect those loads.
2688 Imagine, then, that two writes are made on the first CPU, with a write barrier
2689 between them to guarantee that they will appear to reach that CPU's caches in
2690 the requisite order:
2693 =============== =============== =======================================
2694 u == 0, v == 1 and p == &u, q == &u
2696 smp_wmb(); Make sure change to v is visible before
2698 <A:modify v=2> v is now in cache A exclusively
2700 <B:modify p=&v> p is now in cache B exclusively
2702 The write memory barrier forces the other CPUs in the system to perceive that
2703 the local CPU's caches have apparently been updated in the correct order. But
2704 now imagine that the second CPU wants to read those values:
2707 =============== =============== =======================================
2712 The above pair of reads may then fail to happen in the expected order, as the
2713 cacheline holding p may get updated in one of the second CPU's caches whilst
2714 the update to the cacheline holding v is delayed in the other of the second
2715 CPU's caches by some other cache event:
2718 =============== =============== =======================================
2719 u == 0, v == 1 and p == &u, q == &u
2722 <A:modify v=2> <C:busy>
2726 <B:modify p=&v> <D:commit p=&v>
2729 <C:read *q> Reads from v before v updated in cache
2733 Basically, whilst both cachelines will be updated on CPU 2 eventually, there's
2734 no guarantee that, without intervention, the order of update will be the same
2735 as that committed on CPU 1.
2738 To intervene, we need to interpolate a data dependency barrier or a read
2739 barrier between the loads. This will force the cache to commit its coherency
2740 queue before processing any further requests:
2743 =============== =============== =======================================
2744 u == 0, v == 1 and p == &u, q == &u
2747 <A:modify v=2> <C:busy>
2751 <B:modify p=&v> <D:commit p=&v>
2753 smp_read_barrier_depends()
2757 <C:read *q> Reads from v after v updated in cache
2760 This sort of problem can be encountered on DEC Alpha processors as they have a
2761 split cache that improves performance by making better use of the data bus.
2762 Whilst most CPUs do imply a data dependency barrier on the read when a memory
2763 access depends on a read, not all do, so it may not be relied on.
2765 Other CPUs may also have split caches, but must coordinate between the various
2766 cachelets for normal memory accesses. The semantics of the Alpha removes the
2767 need for coordination in the absence of memory barriers.
2770 CACHE COHERENCY VS DMA
2771 ----------------------
2773 Not all systems maintain cache coherency with respect to devices doing DMA. In
2774 such cases, a device attempting DMA may obtain stale data from RAM because
2775 dirty cache lines may be resident in the caches of various CPUs, and may not
2776 have been written back to RAM yet. To deal with this, the appropriate part of
2777 the kernel must flush the overlapping bits of cache on each CPU (and maybe
2778 invalidate them as well).
2780 In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2781 cache lines being written back to RAM from a CPU's cache after the device has
2782 installed its own data, or cache lines present in the CPU's cache may simply
2783 obscure the fact that RAM has been updated, until at such time as the cacheline
2784 is discarded from the CPU's cache and reloaded. To deal with this, the
2785 appropriate part of the kernel must invalidate the overlapping bits of the
2788 See Documentation/cachetlb.txt for more information on cache management.
2791 CACHE COHERENCY VS MMIO
2792 -----------------------
2794 Memory mapped I/O usually takes place through memory locations that are part of
2795 a window in the CPU's memory space that has different properties assigned than
2796 the usual RAM directed window.
2798 Amongst these properties is usually the fact that such accesses bypass the
2799 caching entirely and go directly to the device buses. This means MMIO accesses
2800 may, in effect, overtake accesses to cached memory that were emitted earlier.
2801 A memory barrier isn't sufficient in such a case, but rather the cache must be
2802 flushed between the cached memory write and the MMIO access if the two are in
2806 =========================
2807 THE THINGS CPUS GET UP TO
2808 =========================
2810 A programmer might take it for granted that the CPU will perform memory
2811 operations in exactly the order specified, so that if the CPU is, for example,
2812 given the following piece of code to execute:
2814 a = ACCESS_ONCE(*A);
2815 ACCESS_ONCE(*B) = b;
2816 c = ACCESS_ONCE(*C);
2817 d = ACCESS_ONCE(*D);
2818 ACCESS_ONCE(*E) = e;
2820 they would then expect that the CPU will complete the memory operation for each
2821 instruction before moving on to the next one, leading to a definite sequence of
2822 operations as seen by external observers in the system:
2824 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2827 Reality is, of course, much messier. With many CPUs and compilers, the above
2828 assumption doesn't hold because:
2830 (*) loads are more likely to need to be completed immediately to permit
2831 execution progress, whereas stores can often be deferred without a
2834 (*) loads may be done speculatively, and the result discarded should it prove
2835 to have been unnecessary;
2837 (*) loads may be done speculatively, leading to the result having been fetched
2838 at the wrong time in the expected sequence of events;
2840 (*) the order of the memory accesses may be rearranged to promote better use
2841 of the CPU buses and caches;
2843 (*) loads and stores may be combined to improve performance when talking to
2844 memory or I/O hardware that can do batched accesses of adjacent locations,
2845 thus cutting down on transaction setup costs (memory and PCI devices may
2846 both be able to do this); and
2848 (*) the CPU's data cache may affect the ordering, and whilst cache-coherency
2849 mechanisms may alleviate this - once the store has actually hit the cache
2850 - there's no guarantee that the coherency management will be propagated in
2851 order to other CPUs.
2853 So what another CPU, say, might actually observe from the above piece of code
2856 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2858 (Where "LOAD {*C,*D}" is a combined load)
2861 However, it is guaranteed that a CPU will be self-consistent: it will see its
2862 _own_ accesses appear to be correctly ordered, without the need for a memory
2863 barrier. For instance with the following code:
2865 U = ACCESS_ONCE(*A);
2866 ACCESS_ONCE(*A) = V;
2867 ACCESS_ONCE(*A) = W;
2868 X = ACCESS_ONCE(*A);
2869 ACCESS_ONCE(*A) = Y;
2870 Z = ACCESS_ONCE(*A);
2872 and assuming no intervention by an external influence, it can be assumed that
2873 the final result will appear to be:
2875 U == the original value of *A
2880 The code above may cause the CPU to generate the full sequence of memory
2883 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2885 in that order, but, without intervention, the sequence may have almost any
2886 combination of elements combined or discarded, provided the program's view of
2887 the world remains consistent. Note that ACCESS_ONCE() is -not- optional
2888 in the above example, as there are architectures where a given CPU might
2889 reorder successive loads to the same location. On such architectures,
2890 ACCESS_ONCE() does whatever is necessary to prevent this, for example, on
2891 Itanium the volatile casts used by ACCESS_ONCE() cause GCC to emit the
2892 special ld.acq and st.rel instructions that prevent such reordering.
2894 The compiler may also combine, discard or defer elements of the sequence before
2895 the CPU even sees them.
2906 since, without either a write barrier or an ACCESS_ONCE(), it can be
2907 assumed that the effect of the storage of V to *A is lost. Similarly:
2912 may, without a memory barrier or an ACCESS_ONCE(), be reduced to:
2917 and the LOAD operation never appear outside of the CPU.
2920 AND THEN THERE'S THE ALPHA
2921 --------------------------
2923 The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
2924 some versions of the Alpha CPU have a split data cache, permitting them to have
2925 two semantically-related cache lines updated at separate times. This is where
2926 the data dependency barrier really becomes necessary as this synchronises both
2927 caches with the memory coherence system, thus making it seem like pointer
2928 changes vs new data occur in the right order.
2930 The Alpha defines the Linux kernel's memory barrier model.
2932 See the subsection on "Cache Coherency" above.
2942 Memory barriers can be used to implement circular buffering without the need
2943 of a lock to serialise the producer with the consumer. See:
2945 Documentation/circular-buffers.txt
2954 Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2956 Chapter 5.2: Physical Address Space Characteristics
2957 Chapter 5.4: Caches and Write Buffers
2958 Chapter 5.5: Data Sharing
2959 Chapter 5.6: Read/Write Ordering
2961 AMD64 Architecture Programmer's Manual Volume 2: System Programming
2962 Chapter 7.1: Memory-Access Ordering
2963 Chapter 7.4: Buffering and Combining Memory Writes
2965 IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2966 System Programming Guide
2967 Chapter 7.1: Locked Atomic Operations
2968 Chapter 7.2: Memory Ordering
2969 Chapter 7.4: Serializing Instructions
2971 The SPARC Architecture Manual, Version 9
2972 Chapter 8: Memory Models
2973 Appendix D: Formal Specification of the Memory Models
2974 Appendix J: Programming with the Memory Models
2976 UltraSPARC Programmer Reference Manual
2977 Chapter 5: Memory Accesses and Cacheability
2978 Chapter 15: Sparc-V9 Memory Models
2980 UltraSPARC III Cu User's Manual
2981 Chapter 9: Memory Models
2983 UltraSPARC IIIi Processor User's Manual
2984 Chapter 8: Memory Models
2986 UltraSPARC Architecture 2005
2988 Appendix D: Formal Specifications of the Memory Models
2990 UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
2991 Chapter 8: Memory Models
2992 Appendix F: Caches and Cache Coherency
2994 Solaris Internals, Core Kernel Architecture, p63-68:
2995 Chapter 3.3: Hardware Considerations for Locks and
2998 Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
2999 for Kernel Programmers:
3000 Chapter 13: Other Memory Models
3002 Intel Itanium Architecture Software Developer's Manual: Volume 1:
3003 Section 2.6: Speculation
3004 Section 4.4: Memory Access
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