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, x=LOAD A->3, y=LOAD B->4
125 STORE A=3, STORE B=4, y=LOAD B->4, x=LOAD A->3
126 STORE A=3, x=LOAD A->3, STORE B=4, y=LOAD B->4
127 STORE A=3, x=LOAD A->3, y=LOAD B->2, STORE B=4
128 STORE A=3, y=LOAD B->2, STORE B=4, x=LOAD A->3
129 STORE A=3, y=LOAD B->2, x=LOAD A->3, STORE B=4
130 STORE B=4, STORE A=3, x=LOAD A->3, y=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:
577 if (ACCESS_ONCE(q)) {
581 Please note that ACCESS_ONCE() is not optional! Without the ACCESS_ONCE(),
582 the compiler is within its rights to transform this example:
586 b = p; /* BUG: Compiler can reorder!!! */
589 b = p; /* BUG: Compiler can reorder!!! */
593 into this, which of course defeats the ordering:
602 Worse yet, if the compiler is able to prove (say) that the value of
603 variable 'a' is always non-zero, it would be well within its rights
604 to optimize the original example by eliminating the "if" statement
608 b = p; /* BUG: Compiler can reorder!!! */
611 The solution is again ACCESS_ONCE() and barrier(), which preserves the
612 ordering between the load from variable 'a' and the store to variable 'b':
625 The initial ACCESS_ONCE() is required to prevent the compiler from
626 proving the value of 'a', and the pair of barrier() invocations are
627 required to prevent the compiler from pulling the two identical stores
628 to 'b' out from the legs of the "if" statement.
630 It is important to note that control dependencies absolutely require a
631 a conditional. For example, the following "optimized" version of
632 the above example breaks ordering, which is why the barrier() invocations
633 are absolutely required if you have identical stores in both legs of
637 ACCESS_ONCE(b) = p; /* BUG: No ordering vs. load from a!!! */
639 /* ACCESS_ONCE(b) = p; -- moved up, BUG!!! */
642 /* ACCESS_ONCE(b) = p; -- moved up, BUG!!! */
646 It is of course legal for the prior load to be part of the conditional,
647 for example, as follows:
649 if (ACCESS_ONCE(a) > 0) {
651 ACCESS_ONCE(b) = q / 2;
655 ACCESS_ONCE(b) = q / 3;
659 This will again ensure that the load from variable 'a' is ordered before the
660 stores to variable 'b'.
662 In addition, you need to be careful what you do with the local variable 'q',
663 otherwise the compiler might be able to guess the value and again remove
664 the needed conditional. For example:
677 If MAX is defined to be 1, then the compiler knows that (q % MAX) is
678 equal to zero, in which case the compiler is within its rights to
679 transform the above code into the following:
685 This transformation loses the ordering between the load from variable 'a'
686 and the store to variable 'b'. If you are relying on this ordering, you
687 should do something like the following:
690 BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
699 Finally, control dependencies do -not- provide transitivity. This is
700 demonstrated by two related examples:
703 ===================== =====================
704 r1 = ACCESS_ONCE(x); r2 = ACCESS_ONCE(y);
705 if (r1 >= 0) if (r2 >= 0)
706 ACCESS_ONCE(y) = 1; ACCESS_ONCE(x) = 1;
708 assert(!(r1 == 1 && r2 == 1));
710 The above two-CPU example will never trigger the assert(). However,
711 if control dependencies guaranteed transitivity (which they do not),
712 then adding the following two CPUs would guarantee a related assertion:
715 ===================== =====================
716 ACCESS_ONCE(x) = 2; ACCESS_ONCE(y) = 2;
718 assert(!(r1 == 2 && r2 == 2 && x == 1 && y == 1)); /* FAILS!!! */
720 But because control dependencies do -not- provide transitivity, the
721 above assertion can fail after the combined four-CPU example completes.
722 If you need the four-CPU example to provide ordering, you will need
723 smp_mb() between the loads and stores in the CPU 0 and CPU 1 code fragments.
727 (*) Control dependencies can order prior loads against later stores.
728 However, they do -not- guarantee any other sort of ordering:
729 Not prior loads against later loads, nor prior stores against
730 later anything. If you need these other forms of ordering,
731 use smb_rmb(), smp_wmb(), or, in the case of prior stores and
732 later loads, smp_mb().
734 (*) If both legs of the "if" statement begin with identical stores
735 to the same variable, a barrier() statement is required at the
736 beginning of each leg of the "if" statement.
738 (*) Control dependencies require at least one run-time conditional
739 between the prior load and the subsequent store, and this
740 conditional must involve the prior load. If the compiler
741 is able to optimize the conditional away, it will have also
742 optimized away the ordering. Careful use of ACCESS_ONCE() can
743 help to preserve the needed conditional.
745 (*) Control dependencies require that the compiler avoid reordering the
746 dependency into nonexistence. Careful use of ACCESS_ONCE() or
747 barrier() can help to preserve your control dependency. Please
748 see the Compiler Barrier section for more information.
750 (*) Control dependencies do -not- provide transitivity. If you
751 need transitivity, use smp_mb().
757 When dealing with CPU-CPU interactions, certain types of memory barrier should
758 always be paired. A lack of appropriate pairing is almost certainly an error.
760 General barriers pair with each other, though they also pair with
761 most other types of barriers, albeit without transitivity. An acquire
762 barrier pairs with a release barrier, but both may also pair with other
763 barriers, including of course general barriers. A write barrier pairs
764 with a data dependency barrier, an acquire barrier, a release barrier,
765 a read barrier, or a general barrier. Similarly a read barrier or a
766 data dependency barrier pairs with a write barrier, an acquire barrier,
767 a release barrier, or a general barrier:
770 =============== ===============
773 ACCESS_ONCE(b) = 2; x = ACCESS_ONCE(b);
780 =============== ===============================
783 ACCESS_ONCE(b) = &a; x = ACCESS_ONCE(b);
784 <data dependency barrier>
787 Basically, the read barrier always has to be there, even though it can be of
790 [!] Note that the stores before the write barrier would normally be expected to
791 match the loads after the read barrier or the data dependency barrier, and vice
795 =================== ===================
796 ACCESS_ONCE(a) = 1; }---- --->{ v = ACCESS_ONCE(c);
797 ACCESS_ONCE(b) = 2; } \ / { w = ACCESS_ONCE(d);
798 <write barrier> \ <read barrier>
799 ACCESS_ONCE(c) = 3; } / \ { x = ACCESS_ONCE(a);
800 ACCESS_ONCE(d) = 4; }---- --->{ y = ACCESS_ONCE(b);
803 EXAMPLES OF MEMORY BARRIER SEQUENCES
804 ------------------------------------
806 Firstly, write barriers act as partial orderings on store operations.
807 Consider the following sequence of events:
810 =======================
818 This sequence of events is committed to the memory coherence system in an order
819 that the rest of the system might perceive as the unordered set of { STORE A,
820 STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
825 | |------>| C=3 | } /\
826 | | : +------+ }----- \ -----> Events perceptible to
827 | | : | A=1 | } \/ the rest of the system
829 | CPU 1 | : | B=2 | }
831 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier
832 | | +------+ } requires all stores prior to the
833 | | : | E=5 | } barrier to be committed before
834 | | : +------+ } further stores may take place
839 | Sequence in which stores are committed to the
840 | memory system by CPU 1
844 Secondly, data dependency barriers act as partial orderings on data-dependent
845 loads. Consider the following sequence of events:
848 ======================= =======================
849 { B = 7; X = 9; Y = 8; C = &Y }
854 STORE D = 4 LOAD C (gets &B)
857 Without intervention, CPU 2 may perceive the events on CPU 1 in some
858 effectively random order, despite the write barrier issued by CPU 1:
861 | | +------+ +-------+ | Sequence of update
862 | |------>| B=2 |----- --->| Y->8 | | of perception on
863 | | : +------+ \ +-------+ | CPU 2
864 | CPU 1 | : | A=1 | \ --->| C->&Y | V
865 | | +------+ | +-------+
866 | | wwwwwwwwwwwwwwww | : :
868 | | : | C=&B |--- | : : +-------+
869 | | : +------+ \ | +-------+ | |
870 | |------>| D=4 | ----------->| C->&B |------>| |
871 | | +------+ | +-------+ | |
872 +-------+ : : | : : | |
876 Apparently incorrect ---> | | B->7 |------>| |
877 perception of B (!) | +-------+ | |
880 The load of X holds ---> \ | X->9 |------>| |
881 up the maintenance \ +-------+ | |
882 of coherence of B ----->| B->2 | +-------+
887 In the above example, CPU 2 perceives that B is 7, despite the load of *C
888 (which would be B) coming after the LOAD of C.
890 If, however, a data dependency barrier were to be placed between the load of C
891 and the load of *C (ie: B) on CPU 2:
894 ======================= =======================
895 { B = 7; X = 9; Y = 8; C = &Y }
900 STORE D = 4 LOAD C (gets &B)
901 <data dependency barrier>
904 then the following will occur:
907 | | +------+ +-------+
908 | |------>| B=2 |----- --->| Y->8 |
909 | | : +------+ \ +-------+
910 | CPU 1 | : | A=1 | \ --->| C->&Y |
911 | | +------+ | +-------+
912 | | wwwwwwwwwwwwwwww | : :
914 | | : | C=&B |--- | : : +-------+
915 | | : +------+ \ | +-------+ | |
916 | |------>| D=4 | ----------->| C->&B |------>| |
917 | | +------+ | +-------+ | |
918 +-------+ : : | : : | |
924 Makes sure all effects ---> \ ddddddddddddddddd | |
925 prior to the store of C \ +-------+ | |
926 are perceptible to ----->| B->2 |------>| |
927 subsequent loads +-------+ | |
931 And thirdly, a read barrier acts as a partial order on loads. Consider the
932 following sequence of events:
935 ======================= =======================
943 Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
944 some effectively random order, despite the write barrier issued by CPU 1:
947 | | +------+ +-------+
948 | |------>| A=1 |------ --->| A->0 |
949 | | +------+ \ +-------+
950 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
951 | | +------+ | +-------+
952 | |------>| B=2 |--- | : :
953 | | +------+ \ | : : +-------+
954 +-------+ : : \ | +-------+ | |
955 ---------->| B->2 |------>| |
956 | +-------+ | CPU 2 |
967 If, however, a read barrier were to be placed between the load of B and the
971 ======================= =======================
980 then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
984 | | +------+ +-------+
985 | |------>| A=1 |------ --->| A->0 |
986 | | +------+ \ +-------+
987 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
988 | | +------+ | +-------+
989 | |------>| B=2 |--- | : :
990 | | +------+ \ | : : +-------+
991 +-------+ : : \ | +-------+ | |
992 ---------->| B->2 |------>| |
993 | +-------+ | CPU 2 |
996 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
997 barrier causes all effects \ +-------+ | |
998 prior to the storage of B ---->| A->1 |------>| |
999 to be perceptible to CPU 2 +-------+ | |
1003 To illustrate this more completely, consider what could happen if the code
1004 contained a load of A either side of the read barrier:
1007 ======================= =======================
1013 LOAD A [first load of A]
1015 LOAD A [second load of A]
1017 Even though the two loads of A both occur after the load of B, they may both
1018 come up with different values:
1021 | | +------+ +-------+
1022 | |------>| A=1 |------ --->| A->0 |
1023 | | +------+ \ +-------+
1024 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1025 | | +------+ | +-------+
1026 | |------>| B=2 |--- | : :
1027 | | +------+ \ | : : +-------+
1028 +-------+ : : \ | +-------+ | |
1029 ---------->| B->2 |------>| |
1030 | +-------+ | CPU 2 |
1034 | | A->0 |------>| 1st |
1036 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1037 barrier causes all effects \ +-------+ | |
1038 prior to the storage of B ---->| A->1 |------>| 2nd |
1039 to be perceptible to CPU 2 +-------+ | |
1043 But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1044 before the read barrier completes anyway:
1047 | | +------+ +-------+
1048 | |------>| A=1 |------ --->| A->0 |
1049 | | +------+ \ +-------+
1050 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1051 | | +------+ | +-------+
1052 | |------>| B=2 |--- | : :
1053 | | +------+ \ | : : +-------+
1054 +-------+ : : \ | +-------+ | |
1055 ---------->| B->2 |------>| |
1056 | +-------+ | CPU 2 |
1060 ---->| A->1 |------>| 1st |
1062 rrrrrrrrrrrrrrrrr | |
1064 | A->1 |------>| 2nd |
1069 The guarantee is that the second load will always come up with A == 1 if the
1070 load of B came up with B == 2. No such guarantee exists for the first load of
1071 A; that may come up with either A == 0 or A == 1.
1074 READ MEMORY BARRIERS VS LOAD SPECULATION
1075 ----------------------------------------
1077 Many CPUs speculate with loads: that is they see that they will need to load an
1078 item from memory, and they find a time where they're not using the bus for any
1079 other loads, and so do the load in advance - even though they haven't actually
1080 got to that point in the instruction execution flow yet. This permits the
1081 actual load instruction to potentially complete immediately because the CPU
1082 already has the value to hand.
1084 It may turn out that the CPU didn't actually need the value - perhaps because a
1085 branch circumvented the load - in which case it can discard the value or just
1086 cache it for later use.
1091 ======================= =======================
1093 DIVIDE } Divide instructions generally
1094 DIVIDE } take a long time to perform
1097 Which might appear as this:
1101 --->| B->2 |------>| |
1105 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1106 division speculates on the +-------+ ~ | |
1110 Once the divisions are complete --> : : ~-->| |
1111 the CPU can then perform the : : | |
1112 LOAD with immediate effect : : +-------+
1115 Placing a read barrier or a data dependency barrier just before the second
1119 ======================= =======================
1126 will force any value speculatively obtained to be reconsidered to an extent
1127 dependent on the type of barrier used. If there was no change made to the
1128 speculated memory location, then the speculated value will just be used:
1132 --->| B->2 |------>| |
1136 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1137 division speculates on the +-------+ ~ | |
1142 rrrrrrrrrrrrrrrr~ | |
1149 but if there was an update or an invalidation from another CPU pending, then
1150 the speculation will be cancelled and the value reloaded:
1154 --->| B->2 |------>| |
1158 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1159 division speculates on the +-------+ ~ | |
1164 rrrrrrrrrrrrrrrrr | |
1166 The speculation is discarded ---> --->| A->1 |------>| |
1167 and an updated value is +-------+ | |
1168 retrieved : : +-------+
1174 Transitivity is a deeply intuitive notion about ordering that is not
1175 always provided by real computer systems. The following example
1176 demonstrates transitivity (also called "cumulativity"):
1179 ======================= ======================= =======================
1181 STORE X=1 LOAD X STORE Y=1
1182 <general barrier> <general barrier>
1185 Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
1186 This indicates that CPU 2's load from X in some sense follows CPU 1's
1187 store to X and that CPU 2's load from Y in some sense preceded CPU 3's
1188 store to Y. The question is then "Can CPU 3's load from X return 0?"
1190 Because CPU 2's load from X in some sense came after CPU 1's store, it
1191 is natural to expect that CPU 3's load from X must therefore return 1.
1192 This expectation is an example of transitivity: if a load executing on
1193 CPU A follows a load from the same variable executing on CPU B, then
1194 CPU A's load must either return the same value that CPU B's load did,
1195 or must return some later value.
1197 In the Linux kernel, use of general memory barriers guarantees
1198 transitivity. Therefore, in the above example, if CPU 2's load from X
1199 returns 1 and its load from Y returns 0, then CPU 3's load from X must
1202 However, transitivity is -not- guaranteed for read or write barriers.
1203 For example, suppose that CPU 2's general barrier in the above example
1204 is changed to a read barrier as shown below:
1207 ======================= ======================= =======================
1209 STORE X=1 LOAD X STORE Y=1
1210 <read barrier> <general barrier>
1213 This substitution destroys transitivity: in this example, it is perfectly
1214 legal for CPU 2's load from X to return 1, its load from Y to return 0,
1215 and CPU 3's load from X to return 0.
1217 The key point is that although CPU 2's read barrier orders its pair
1218 of loads, it does not guarantee to order CPU 1's store. Therefore, if
1219 this example runs on a system where CPUs 1 and 2 share a store buffer
1220 or a level of cache, CPU 2 might have early access to CPU 1's writes.
1221 General barriers are therefore required to ensure that all CPUs agree
1222 on the combined order of CPU 1's and CPU 2's accesses.
1224 To reiterate, if your code requires transitivity, use general barriers
1228 ========================
1229 EXPLICIT KERNEL BARRIERS
1230 ========================
1232 The Linux kernel has a variety of different barriers that act at different
1235 (*) Compiler barrier.
1237 (*) CPU memory barriers.
1239 (*) MMIO write barrier.
1245 The Linux kernel has an explicit compiler barrier function that prevents the
1246 compiler from moving the memory accesses either side of it to the other side:
1250 This is a general barrier -- there are no read-read or write-write variants
1251 of barrier(). However, ACCESS_ONCE() can be thought of as a weak form
1252 for barrier() that affects only the specific accesses flagged by the
1255 The barrier() function has the following effects:
1257 (*) Prevents the compiler from reordering accesses following the
1258 barrier() to precede any accesses preceding the barrier().
1259 One example use for this property is to ease communication between
1260 interrupt-handler code and the code that was interrupted.
1262 (*) Within a loop, forces the compiler to load the variables used
1263 in that loop's conditional on each pass through that loop.
1265 The ACCESS_ONCE() function can prevent any number of optimizations that,
1266 while perfectly safe in single-threaded code, can be fatal in concurrent
1267 code. Here are some examples of these sorts of optimizations:
1269 (*) The compiler is within its rights to reorder loads and stores
1270 to the same variable, and in some cases, the CPU is within its
1271 rights to reorder loads to the same variable. This means that
1277 Might result in an older value of x stored in a[1] than in a[0].
1278 Prevent both the compiler and the CPU from doing this as follows:
1280 a[0] = ACCESS_ONCE(x);
1281 a[1] = ACCESS_ONCE(x);
1283 In short, ACCESS_ONCE() provides cache coherence for accesses from
1284 multiple CPUs to a single variable.
1286 (*) The compiler is within its rights to merge successive loads from
1287 the same variable. Such merging can cause the compiler to "optimize"
1291 do_something_with(tmp);
1293 into the following code, which, although in some sense legitimate
1294 for single-threaded code, is almost certainly not what the developer
1299 do_something_with(tmp);
1301 Use ACCESS_ONCE() to prevent the compiler from doing this to you:
1303 while (tmp = ACCESS_ONCE(a))
1304 do_something_with(tmp);
1306 (*) The compiler is within its rights to reload a variable, for example,
1307 in cases where high register pressure prevents the compiler from
1308 keeping all data of interest in registers. The compiler might
1309 therefore optimize the variable 'tmp' out of our previous example:
1312 do_something_with(tmp);
1314 This could result in the following code, which is perfectly safe in
1315 single-threaded code, but can be fatal in concurrent code:
1318 do_something_with(a);
1320 For example, the optimized version of this code could result in
1321 passing a zero to do_something_with() in the case where the variable
1322 a was modified by some other CPU between the "while" statement and
1323 the call to do_something_with().
1325 Again, use ACCESS_ONCE() to prevent the compiler from doing this:
1327 while (tmp = ACCESS_ONCE(a))
1328 do_something_with(tmp);
1330 Note that if the compiler runs short of registers, it might save
1331 tmp onto the stack. The overhead of this saving and later restoring
1332 is why compilers reload variables. Doing so is perfectly safe for
1333 single-threaded code, so you need to tell the compiler about cases
1334 where it is not safe.
1336 (*) The compiler is within its rights to omit a load entirely if it knows
1337 what the value will be. For example, if the compiler can prove that
1338 the value of variable 'a' is always zero, it can optimize this code:
1341 do_something_with(tmp);
1347 This transformation is a win for single-threaded code because it gets
1348 rid of a load and a branch. The problem is that the compiler will
1349 carry out its proof assuming that the current CPU is the only one
1350 updating variable 'a'. If variable 'a' is shared, then the compiler's
1351 proof will be erroneous. Use ACCESS_ONCE() to tell the compiler
1352 that it doesn't know as much as it thinks it does:
1354 while (tmp = ACCESS_ONCE(a))
1355 do_something_with(tmp);
1357 But please note that the compiler is also closely watching what you
1358 do with the value after the ACCESS_ONCE(). For example, suppose you
1359 do the following and MAX is a preprocessor macro with the value 1:
1361 while ((tmp = ACCESS_ONCE(a)) % MAX)
1362 do_something_with(tmp);
1364 Then the compiler knows that the result of the "%" operator applied
1365 to MAX will always be zero, again allowing the compiler to optimize
1366 the code into near-nonexistence. (It will still load from the
1369 (*) Similarly, the compiler is within its rights to omit a store entirely
1370 if it knows that the variable already has the value being stored.
1371 Again, the compiler assumes that the current CPU is the only one
1372 storing into the variable, which can cause the compiler to do the
1373 wrong thing for shared variables. For example, suppose you have
1377 /* Code that does not store to variable a. */
1380 The compiler sees that the value of variable 'a' is already zero, so
1381 it might well omit the second store. This would come as a fatal
1382 surprise if some other CPU might have stored to variable 'a' in the
1385 Use ACCESS_ONCE() to prevent the compiler from making this sort of
1389 /* Code that does not store to variable a. */
1392 (*) The compiler is within its rights to reorder memory accesses unless
1393 you tell it not to. For example, consider the following interaction
1394 between process-level code and an interrupt handler:
1396 void process_level(void)
1398 msg = get_message();
1402 void interrupt_handler(void)
1405 process_message(msg);
1408 There is nothing to prevent the compiler from transforming
1409 process_level() to the following, in fact, this might well be a
1410 win for single-threaded code:
1412 void process_level(void)
1415 msg = get_message();
1418 If the interrupt occurs between these two statement, then
1419 interrupt_handler() might be passed a garbled msg. Use ACCESS_ONCE()
1420 to prevent this as follows:
1422 void process_level(void)
1424 ACCESS_ONCE(msg) = get_message();
1425 ACCESS_ONCE(flag) = true;
1428 void interrupt_handler(void)
1430 if (ACCESS_ONCE(flag))
1431 process_message(ACCESS_ONCE(msg));
1434 Note that the ACCESS_ONCE() wrappers in interrupt_handler()
1435 are needed if this interrupt handler can itself be interrupted
1436 by something that also accesses 'flag' and 'msg', for example,
1437 a nested interrupt or an NMI. Otherwise, ACCESS_ONCE() is not
1438 needed in interrupt_handler() other than for documentation purposes.
1439 (Note also that nested interrupts do not typically occur in modern
1440 Linux kernels, in fact, if an interrupt handler returns with
1441 interrupts enabled, you will get a WARN_ONCE() splat.)
1443 You should assume that the compiler can move ACCESS_ONCE() past
1444 code not containing ACCESS_ONCE(), barrier(), or similar primitives.
1446 This effect could also be achieved using barrier(), but ACCESS_ONCE()
1447 is more selective: With ACCESS_ONCE(), the compiler need only forget
1448 the contents of the indicated memory locations, while with barrier()
1449 the compiler must discard the value of all memory locations that
1450 it has currented cached in any machine registers. Of course,
1451 the compiler must also respect the order in which the ACCESS_ONCE()s
1452 occur, though the CPU of course need not do so.
1454 (*) The compiler is within its rights to invent stores to a variable,
1455 as in the following example:
1462 The compiler might save a branch by optimizing this as follows:
1468 In single-threaded code, this is not only safe, but also saves
1469 a branch. Unfortunately, in concurrent code, this optimization
1470 could cause some other CPU to see a spurious value of 42 -- even
1471 if variable 'a' was never zero -- when loading variable 'b'.
1472 Use ACCESS_ONCE() to prevent this as follows:
1477 ACCESS_ONCE(b) = 42;
1479 The compiler can also invent loads. These are usually less
1480 damaging, but they can result in cache-line bouncing and thus in
1481 poor performance and scalability. Use ACCESS_ONCE() to prevent
1484 (*) For aligned memory locations whose size allows them to be accessed
1485 with a single memory-reference instruction, prevents "load tearing"
1486 and "store tearing," in which a single large access is replaced by
1487 multiple smaller accesses. For example, given an architecture having
1488 16-bit store instructions with 7-bit immediate fields, the compiler
1489 might be tempted to use two 16-bit store-immediate instructions to
1490 implement the following 32-bit store:
1494 Please note that GCC really does use this sort of optimization,
1495 which is not surprising given that it would likely take more
1496 than two instructions to build the constant and then store it.
1497 This optimization can therefore be a win in single-threaded code.
1498 In fact, a recent bug (since fixed) caused GCC to incorrectly use
1499 this optimization in a volatile store. In the absence of such bugs,
1500 use of ACCESS_ONCE() prevents store tearing in the following example:
1502 ACCESS_ONCE(p) = 0x00010002;
1504 Use of packed structures can also result in load and store tearing,
1507 struct __attribute__((__packed__)) foo {
1512 struct foo foo1, foo2;
1519 Because there are no ACCESS_ONCE() wrappers and no volatile markings,
1520 the compiler would be well within its rights to implement these three
1521 assignment statements as a pair of 32-bit loads followed by a pair
1522 of 32-bit stores. This would result in load tearing on 'foo1.b'
1523 and store tearing on 'foo2.b'. ACCESS_ONCE() again prevents tearing
1527 ACCESS_ONCE(foo2.b) = ACCESS_ONCE(foo1.b);
1530 All that aside, it is never necessary to use ACCESS_ONCE() on a variable
1531 that has been marked volatile. For example, because 'jiffies' is marked
1532 volatile, it is never necessary to say ACCESS_ONCE(jiffies). The reason
1533 for this is that ACCESS_ONCE() is implemented as a volatile cast, which
1534 has no effect when its argument is already marked volatile.
1536 Please note that these compiler barriers have no direct effect on the CPU,
1537 which may then reorder things however it wishes.
1543 The Linux kernel has eight basic CPU memory barriers:
1545 TYPE MANDATORY SMP CONDITIONAL
1546 =============== ======================= ===========================
1547 GENERAL mb() smp_mb()
1548 WRITE wmb() smp_wmb()
1549 READ rmb() smp_rmb()
1550 DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends()
1553 All memory barriers except the data dependency barriers imply a compiler
1554 barrier. Data dependencies do not impose any additional compiler ordering.
1556 Aside: In the case of data dependencies, the compiler would be expected to
1557 issue the loads in the correct order (eg. `a[b]` would have to load the value
1558 of b before loading a[b]), however there is no guarantee in the C specification
1559 that the compiler may not speculate the value of b (eg. is equal to 1) and load
1560 a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the
1561 problem of a compiler reloading b after having loaded a[b], thus having a newer
1562 copy of b than a[b]. A consensus has not yet been reached about these problems,
1563 however the ACCESS_ONCE macro is a good place to start looking.
1565 SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1566 systems because it is assumed that a CPU will appear to be self-consistent,
1567 and will order overlapping accesses correctly with respect to itself.
1569 [!] Note that SMP memory barriers _must_ be used to control the ordering of
1570 references to shared memory on SMP systems, though the use of locking instead
1573 Mandatory barriers should not be used to control SMP effects, since mandatory
1574 barriers unnecessarily impose overhead on UP systems. They may, however, be
1575 used to control MMIO effects on accesses through relaxed memory I/O windows.
1576 These are required even on non-SMP systems as they affect the order in which
1577 memory operations appear to a device by prohibiting both the compiler and the
1578 CPU from reordering them.
1581 There are some more advanced barrier functions:
1583 (*) set_mb(var, value)
1585 This assigns the value to the variable and then inserts a full memory
1586 barrier after it, depending on the function. It isn't guaranteed to
1587 insert anything more than a compiler barrier in a UP compilation.
1590 (*) smp_mb__before_atomic();
1591 (*) smp_mb__after_atomic();
1593 These are for use with atomic (such as add, subtract, increment and
1594 decrement) functions that don't return a value, especially when used for
1595 reference counting. These functions do not imply memory barriers.
1597 These are also used for atomic bitop functions that do not return a
1598 value (such as set_bit and clear_bit).
1600 As an example, consider a piece of code that marks an object as being dead
1601 and then decrements the object's reference count:
1604 smp_mb__before_atomic();
1605 atomic_dec(&obj->ref_count);
1607 This makes sure that the death mark on the object is perceived to be set
1608 *before* the reference counter is decremented.
1610 See Documentation/atomic_ops.txt for more information. See the "Atomic
1611 operations" subsection for information on where to use these.
1617 The Linux kernel also has a special barrier for use with memory-mapped I/O
1622 This is a variation on the mandatory write barrier that causes writes to weakly
1623 ordered I/O regions to be partially ordered. Its effects may go beyond the
1624 CPU->Hardware interface and actually affect the hardware at some level.
1626 See the subsection "Locks vs I/O accesses" for more information.
1629 ===============================
1630 IMPLICIT KERNEL MEMORY BARRIERS
1631 ===============================
1633 Some of the other functions in the linux kernel imply memory barriers, amongst
1634 which are locking and scheduling functions.
1636 This specification is a _minimum_ guarantee; any particular architecture may
1637 provide more substantial guarantees, but these may not be relied upon outside
1638 of arch specific code.
1644 The Linux kernel has a number of locking constructs:
1653 In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
1654 for each construct. These operations all imply certain barriers:
1656 (1) ACQUIRE operation implication:
1658 Memory operations issued after the ACQUIRE will be completed after the
1659 ACQUIRE operation has completed.
1661 Memory operations issued before the ACQUIRE may be completed after
1662 the ACQUIRE operation has completed. An smp_mb__before_spinlock(),
1663 combined with a following ACQUIRE, orders prior loads against
1664 subsequent loads and stores and also orders prior stores against
1665 subsequent stores. Note that this is weaker than smp_mb()! The
1666 smp_mb__before_spinlock() primitive is free on many architectures.
1668 (2) RELEASE operation implication:
1670 Memory operations issued before the RELEASE will be completed before the
1671 RELEASE operation has completed.
1673 Memory operations issued after the RELEASE may be completed before the
1674 RELEASE operation has completed.
1676 (3) ACQUIRE vs ACQUIRE implication:
1678 All ACQUIRE operations issued before another ACQUIRE operation will be
1679 completed before that ACQUIRE operation.
1681 (4) ACQUIRE vs RELEASE implication:
1683 All ACQUIRE operations issued before a RELEASE operation will be
1684 completed before the RELEASE operation.
1686 (5) Failed conditional ACQUIRE implication:
1688 Certain locking variants of the ACQUIRE operation may fail, either due to
1689 being unable to get the lock immediately, or due to receiving an unblocked
1690 signal whilst asleep waiting for the lock to become available. Failed
1691 locks do not imply any sort of barrier.
1693 [!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
1694 one-way barriers is that the effects of instructions outside of a critical
1695 section may seep into the inside of the critical section.
1697 An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
1698 because it is possible for an access preceding the ACQUIRE to happen after the
1699 ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
1700 the two accesses can themselves then cross:
1709 ACQUIRE M, STORE *B, STORE *A, RELEASE M
1711 When the ACQUIRE and RELEASE are a lock acquisition and release,
1712 respectively, this same reordering can occur if the lock's ACQUIRE and
1713 RELEASE are to the same lock variable, but only from the perspective of
1714 another CPU not holding that lock. In short, a ACQUIRE followed by an
1715 RELEASE may -not- be assumed to be a full memory barrier.
1717 Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not
1718 imply a full memory barrier. If it is necessary for a RELEASE-ACQUIRE
1719 pair to produce a full barrier, the ACQUIRE can be followed by an
1720 smp_mb__after_unlock_lock() invocation. This will produce a full barrier
1721 if either (a) the RELEASE and the ACQUIRE are executed by the same
1722 CPU or task, or (b) the RELEASE and ACQUIRE act on the same variable.
1723 The smp_mb__after_unlock_lock() primitive is free on many architectures.
1724 Without smp_mb__after_unlock_lock(), the CPU's execution of the critical
1725 sections corresponding to the RELEASE and the ACQUIRE can cross, so that:
1734 ACQUIRE N, STORE *B, STORE *A, RELEASE M
1736 It might appear that this reordering could introduce a deadlock.
1737 However, this cannot happen because if such a deadlock threatened,
1738 the RELEASE would simply complete, thereby avoiding the deadlock.
1742 One key point is that we are only talking about the CPU doing
1743 the reordering, not the compiler. If the compiler (or, for
1744 that matter, the developer) switched the operations, deadlock
1747 But suppose the CPU reordered the operations. In this case,
1748 the unlock precedes the lock in the assembly code. The CPU
1749 simply elected to try executing the later lock operation first.
1750 If there is a deadlock, this lock operation will simply spin (or
1751 try to sleep, but more on that later). The CPU will eventually
1752 execute the unlock operation (which preceded the lock operation
1753 in the assembly code), which will unravel the potential deadlock,
1754 allowing the lock operation to succeed.
1756 But what if the lock is a sleeplock? In that case, the code will
1757 try to enter the scheduler, where it will eventually encounter
1758 a memory barrier, which will force the earlier unlock operation
1759 to complete, again unraveling the deadlock. There might be
1760 a sleep-unlock race, but the locking primitive needs to resolve
1761 such races properly in any case.
1763 With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
1764 For example, with the following code, the store to *A will always be
1765 seen by other CPUs before the store to *B:
1770 smp_mb__after_unlock_lock();
1773 The operations will always occur in one of the following orders:
1775 STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
1776 STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
1777 ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
1779 If the RELEASE and ACQUIRE were instead both operating on the same lock
1780 variable, only the first of these alternatives can occur. In addition,
1781 the more strongly ordered systems may rule out some of the above orders.
1782 But in any case, as noted earlier, the smp_mb__after_unlock_lock()
1783 ensures that the store to *A will always be seen as happening before
1786 Locks and semaphores may not provide any guarantee of ordering on UP compiled
1787 systems, and so cannot be counted on in such a situation to actually achieve
1788 anything at all - especially with respect to I/O accesses - unless combined
1789 with interrupt disabling operations.
1791 See also the section on "Inter-CPU locking barrier effects".
1794 As an example, consider the following:
1805 The following sequence of events is acceptable:
1807 ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
1809 [+] Note that {*F,*A} indicates a combined access.
1811 But none of the following are:
1813 {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E
1814 *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F
1815 *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F
1816 *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E
1820 INTERRUPT DISABLING FUNCTIONS
1821 -----------------------------
1823 Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
1824 (RELEASE equivalent) will act as compiler barriers only. So if memory or I/O
1825 barriers are required in such a situation, they must be provided from some
1829 SLEEP AND WAKE-UP FUNCTIONS
1830 ---------------------------
1832 Sleeping and waking on an event flagged in global data can be viewed as an
1833 interaction between two pieces of data: the task state of the task waiting for
1834 the event and the global data used to indicate the event. To make sure that
1835 these appear to happen in the right order, the primitives to begin the process
1836 of going to sleep, and the primitives to initiate a wake up imply certain
1839 Firstly, the sleeper normally follows something like this sequence of events:
1842 set_current_state(TASK_UNINTERRUPTIBLE);
1843 if (event_indicated)
1848 A general memory barrier is interpolated automatically by set_current_state()
1849 after it has altered the task state:
1852 ===============================
1853 set_current_state();
1855 STORE current->state
1857 LOAD event_indicated
1859 set_current_state() may be wrapped by:
1862 prepare_to_wait_exclusive();
1864 which therefore also imply a general memory barrier after setting the state.
1865 The whole sequence above is available in various canned forms, all of which
1866 interpolate the memory barrier in the right place:
1869 wait_event_interruptible();
1870 wait_event_interruptible_exclusive();
1871 wait_event_interruptible_timeout();
1872 wait_event_killable();
1873 wait_event_timeout();
1878 Secondly, code that performs a wake up normally follows something like this:
1880 event_indicated = 1;
1881 wake_up(&event_wait_queue);
1885 event_indicated = 1;
1886 wake_up_process(event_daemon);
1888 A write memory barrier is implied by wake_up() and co. if and only if they wake
1889 something up. The barrier occurs before the task state is cleared, and so sits
1890 between the STORE to indicate the event and the STORE to set TASK_RUNNING:
1893 =============================== ===============================
1894 set_current_state(); STORE event_indicated
1895 set_mb(); wake_up();
1896 STORE current->state <write barrier>
1897 <general barrier> STORE current->state
1898 LOAD event_indicated
1900 To repeat, this write memory barrier is present if and only if something
1901 is actually awakened. To see this, consider the following sequence of
1902 events, where X and Y are both initially zero:
1905 =============================== ===============================
1906 X = 1; STORE event_indicated
1907 smp_mb(); wake_up();
1908 Y = 1; wait_event(wq, Y == 1);
1909 wake_up(); load from Y sees 1, no memory barrier
1910 load from X might see 0
1912 In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
1915 The available waker functions include:
1921 wake_up_interruptible();
1922 wake_up_interruptible_all();
1923 wake_up_interruptible_nr();
1924 wake_up_interruptible_poll();
1925 wake_up_interruptible_sync();
1926 wake_up_interruptible_sync_poll();
1928 wake_up_locked_poll();
1934 [!] Note that the memory barriers implied by the sleeper and the waker do _not_
1935 order multiple stores before the wake-up with respect to loads of those stored
1936 values after the sleeper has called set_current_state(). For instance, if the
1939 set_current_state(TASK_INTERRUPTIBLE);
1940 if (event_indicated)
1942 __set_current_state(TASK_RUNNING);
1943 do_something(my_data);
1948 event_indicated = 1;
1949 wake_up(&event_wait_queue);
1951 there's no guarantee that the change to event_indicated will be perceived by
1952 the sleeper as coming after the change to my_data. In such a circumstance, the
1953 code on both sides must interpolate its own memory barriers between the
1954 separate data accesses. Thus the above sleeper ought to do:
1956 set_current_state(TASK_INTERRUPTIBLE);
1957 if (event_indicated) {
1959 do_something(my_data);
1962 and the waker should do:
1966 event_indicated = 1;
1967 wake_up(&event_wait_queue);
1970 MISCELLANEOUS FUNCTIONS
1971 -----------------------
1973 Other functions that imply barriers:
1975 (*) schedule() and similar imply full memory barriers.
1978 ===================================
1979 INTER-CPU ACQUIRING BARRIER EFFECTS
1980 ===================================
1982 On SMP systems locking primitives give a more substantial form of barrier: one
1983 that does affect memory access ordering on other CPUs, within the context of
1984 conflict on any particular lock.
1987 ACQUIRES VS MEMORY ACCESSES
1988 ---------------------------
1990 Consider the following: the system has a pair of spinlocks (M) and (Q), and
1991 three CPUs; then should the following sequence of events occur:
1994 =============================== ===============================
1995 ACCESS_ONCE(*A) = a; ACCESS_ONCE(*E) = e;
1997 ACCESS_ONCE(*B) = b; ACCESS_ONCE(*F) = f;
1998 ACCESS_ONCE(*C) = c; ACCESS_ONCE(*G) = g;
2000 ACCESS_ONCE(*D) = d; ACCESS_ONCE(*H) = h;
2002 Then there is no guarantee as to what order CPU 3 will see the accesses to *A
2003 through *H occur in, other than the constraints imposed by the separate locks
2004 on the separate CPUs. It might, for example, see:
2006 *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
2008 But it won't see any of:
2010 *B, *C or *D preceding ACQUIRE M
2011 *A, *B or *C following RELEASE M
2012 *F, *G or *H preceding ACQUIRE Q
2013 *E, *F or *G following RELEASE Q
2016 However, if the following occurs:
2019 =============================== ===============================
2020 ACCESS_ONCE(*A) = a;
2022 ACCESS_ONCE(*B) = b;
2023 ACCESS_ONCE(*C) = c;
2025 ACCESS_ONCE(*D) = d; ACCESS_ONCE(*E) = e;
2027 smp_mb__after_unlock_lock();
2028 ACCESS_ONCE(*F) = f;
2029 ACCESS_ONCE(*G) = g;
2031 ACCESS_ONCE(*H) = h;
2035 *E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
2036 ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
2038 But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
2040 *B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
2041 *A, *B or *C following RELEASE M [1]
2042 *F, *G or *H preceding ACQUIRE M [2]
2043 *A, *B, *C, *E, *F or *G following RELEASE M [2]
2045 Note that the smp_mb__after_unlock_lock() is critically important
2046 here: Without it CPU 3 might see some of the above orderings.
2047 Without smp_mb__after_unlock_lock(), the accesses are not guaranteed
2048 to be seen in order unless CPU 3 holds lock M.
2051 ACQUIRES VS I/O ACCESSES
2052 ------------------------
2054 Under certain circumstances (especially involving NUMA), I/O accesses within
2055 two spinlocked sections on two different CPUs may be seen as interleaved by the
2056 PCI bridge, because the PCI bridge does not necessarily participate in the
2057 cache-coherence protocol, and is therefore incapable of issuing the required
2058 read memory barriers.
2063 =============================== ===============================
2073 may be seen by the PCI bridge as follows:
2075 STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5
2077 which would probably cause the hardware to malfunction.
2080 What is necessary here is to intervene with an mmiowb() before dropping the
2081 spinlock, for example:
2084 =============================== ===============================
2096 this will ensure that the two stores issued on CPU 1 appear at the PCI bridge
2097 before either of the stores issued on CPU 2.
2100 Furthermore, following a store by a load from the same device obviates the need
2101 for the mmiowb(), because the load forces the store to complete before the load
2105 =============================== ===============================
2116 See Documentation/DocBook/deviceiobook.tmpl for more information.
2119 =================================
2120 WHERE ARE MEMORY BARRIERS NEEDED?
2121 =================================
2123 Under normal operation, memory operation reordering is generally not going to
2124 be a problem as a single-threaded linear piece of code will still appear to
2125 work correctly, even if it's in an SMP kernel. There are, however, four
2126 circumstances in which reordering definitely _could_ be a problem:
2128 (*) Interprocessor interaction.
2130 (*) Atomic operations.
2132 (*) Accessing devices.
2137 INTERPROCESSOR INTERACTION
2138 --------------------------
2140 When there's a system with more than one processor, more than one CPU in the
2141 system may be working on the same data set at the same time. This can cause
2142 synchronisation problems, and the usual way of dealing with them is to use
2143 locks. Locks, however, are quite expensive, and so it may be preferable to
2144 operate without the use of a lock if at all possible. In such a case
2145 operations that affect both CPUs may have to be carefully ordered to prevent
2148 Consider, for example, the R/W semaphore slow path. Here a waiting process is
2149 queued on the semaphore, by virtue of it having a piece of its stack linked to
2150 the semaphore's list of waiting processes:
2152 struct rw_semaphore {
2155 struct list_head waiters;
2158 struct rwsem_waiter {
2159 struct list_head list;
2160 struct task_struct *task;
2163 To wake up a particular waiter, the up_read() or up_write() functions have to:
2165 (1) read the next pointer from this waiter's record to know as to where the
2166 next waiter record is;
2168 (2) read the pointer to the waiter's task structure;
2170 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2172 (4) call wake_up_process() on the task; and
2174 (5) release the reference held on the waiter's task struct.
2176 In other words, it has to perform this sequence of events:
2178 LOAD waiter->list.next;
2184 and if any of these steps occur out of order, then the whole thing may
2187 Once it has queued itself and dropped the semaphore lock, the waiter does not
2188 get the lock again; it instead just waits for its task pointer to be cleared
2189 before proceeding. Since the record is on the waiter's stack, this means that
2190 if the task pointer is cleared _before_ the next pointer in the list is read,
2191 another CPU might start processing the waiter and might clobber the waiter's
2192 stack before the up*() function has a chance to read the next pointer.
2194 Consider then what might happen to the above sequence of events:
2197 =============================== ===============================
2204 Woken up by other event
2209 foo() clobbers *waiter
2211 LOAD waiter->list.next;
2214 This could be dealt with using the semaphore lock, but then the down_xxx()
2215 function has to needlessly get the spinlock again after being woken up.
2217 The way to deal with this is to insert a general SMP memory barrier:
2219 LOAD waiter->list.next;
2226 In this case, the barrier makes a guarantee that all memory accesses before the
2227 barrier will appear to happen before all the memory accesses after the barrier
2228 with respect to the other CPUs on the system. It does _not_ guarantee that all
2229 the memory accesses before the barrier will be complete by the time the barrier
2230 instruction itself is complete.
2232 On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2233 compiler barrier, thus making sure the compiler emits the instructions in the
2234 right order without actually intervening in the CPU. Since there's only one
2235 CPU, that CPU's dependency ordering logic will take care of everything else.
2241 Whilst they are technically interprocessor interaction considerations, atomic
2242 operations are noted specially as some of them imply full memory barriers and
2243 some don't, but they're very heavily relied on as a group throughout the
2246 Any atomic operation that modifies some state in memory and returns information
2247 about the state (old or new) implies an SMP-conditional general memory barrier
2248 (smp_mb()) on each side of the actual operation (with the exception of
2249 explicit lock operations, described later). These include:
2253 atomic_xchg(); atomic_long_xchg();
2254 atomic_cmpxchg(); atomic_long_cmpxchg();
2255 atomic_inc_return(); atomic_long_inc_return();
2256 atomic_dec_return(); atomic_long_dec_return();
2257 atomic_add_return(); atomic_long_add_return();
2258 atomic_sub_return(); atomic_long_sub_return();
2259 atomic_inc_and_test(); atomic_long_inc_and_test();
2260 atomic_dec_and_test(); atomic_long_dec_and_test();
2261 atomic_sub_and_test(); atomic_long_sub_and_test();
2262 atomic_add_negative(); atomic_long_add_negative();
2264 test_and_clear_bit();
2265 test_and_change_bit();
2267 /* when succeeds (returns 1) */
2268 atomic_add_unless(); atomic_long_add_unless();
2270 These are used for such things as implementing ACQUIRE-class and RELEASE-class
2271 operations and adjusting reference counters towards object destruction, and as
2272 such the implicit memory barrier effects are necessary.
2275 The following operations are potential problems as they do _not_ imply memory
2276 barriers, but might be used for implementing such things as RELEASE-class
2284 With these the appropriate explicit memory barrier should be used if necessary
2285 (smp_mb__before_atomic() for instance).
2288 The following also do _not_ imply memory barriers, and so may require explicit
2289 memory barriers under some circumstances (smp_mb__before_atomic() for
2297 If they're used for statistics generation, then they probably don't need memory
2298 barriers, unless there's a coupling between statistical data.
2300 If they're used for reference counting on an object to control its lifetime,
2301 they probably don't need memory barriers because either the reference count
2302 will be adjusted inside a locked section, or the caller will already hold
2303 sufficient references to make the lock, and thus a memory barrier unnecessary.
2305 If they're used for constructing a lock of some description, then they probably
2306 do need memory barriers as a lock primitive generally has to do things in a
2309 Basically, each usage case has to be carefully considered as to whether memory
2310 barriers are needed or not.
2312 The following operations are special locking primitives:
2314 test_and_set_bit_lock();
2316 __clear_bit_unlock();
2318 These implement ACQUIRE-class and RELEASE-class operations. These should be used in
2319 preference to other operations when implementing locking primitives, because
2320 their implementations can be optimised on many architectures.
2322 [!] Note that special memory barrier primitives are available for these
2323 situations because on some CPUs the atomic instructions used imply full memory
2324 barriers, and so barrier instructions are superfluous in conjunction with them,
2325 and in such cases the special barrier primitives will be no-ops.
2327 See Documentation/atomic_ops.txt for more information.
2333 Many devices can be memory mapped, and so appear to the CPU as if they're just
2334 a set of memory locations. To control such a device, the driver usually has to
2335 make the right memory accesses in exactly the right order.
2337 However, having a clever CPU or a clever compiler creates a potential problem
2338 in that the carefully sequenced accesses in the driver code won't reach the
2339 device in the requisite order if the CPU or the compiler thinks it is more
2340 efficient to reorder, combine or merge accesses - something that would cause
2341 the device to malfunction.
2343 Inside of the Linux kernel, I/O should be done through the appropriate accessor
2344 routines - such as inb() or writel() - which know how to make such accesses
2345 appropriately sequential. Whilst this, for the most part, renders the explicit
2346 use of memory barriers unnecessary, there are a couple of situations where they
2349 (1) On some systems, I/O stores are not strongly ordered across all CPUs, and
2350 so for _all_ general drivers locks should be used and mmiowb() must be
2351 issued prior to unlocking the critical section.
2353 (2) If the accessor functions are used to refer to an I/O memory window with
2354 relaxed memory access properties, then _mandatory_ memory barriers are
2355 required to enforce ordering.
2357 See Documentation/DocBook/deviceiobook.tmpl for more information.
2363 A driver may be interrupted by its own interrupt service routine, and thus the
2364 two parts of the driver may interfere with each other's attempts to control or
2367 This may be alleviated - at least in part - by disabling local interrupts (a
2368 form of locking), such that the critical operations are all contained within
2369 the interrupt-disabled section in the driver. Whilst the driver's interrupt
2370 routine is executing, the driver's core may not run on the same CPU, and its
2371 interrupt is not permitted to happen again until the current interrupt has been
2372 handled, thus the interrupt handler does not need to lock against that.
2374 However, consider a driver that was talking to an ethernet card that sports an
2375 address register and a data register. If that driver's core talks to the card
2376 under interrupt-disablement and then the driver's interrupt handler is invoked:
2387 The store to the data register might happen after the second store to the
2388 address register if ordering rules are sufficiently relaxed:
2390 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2393 If ordering rules are relaxed, it must be assumed that accesses done inside an
2394 interrupt disabled section may leak outside of it and may interleave with
2395 accesses performed in an interrupt - and vice versa - unless implicit or
2396 explicit barriers are used.
2398 Normally this won't be a problem because the I/O accesses done inside such
2399 sections will include synchronous load operations on strictly ordered I/O
2400 registers that form implicit I/O barriers. If this isn't sufficient then an
2401 mmiowb() may need to be used explicitly.
2404 A similar situation may occur between an interrupt routine and two routines
2405 running on separate CPUs that communicate with each other. If such a case is
2406 likely, then interrupt-disabling locks should be used to guarantee ordering.
2409 ==========================
2410 KERNEL I/O BARRIER EFFECTS
2411 ==========================
2413 When accessing I/O memory, drivers should use the appropriate accessor
2418 These are intended to talk to I/O space rather than memory space, but
2419 that's primarily a CPU-specific concept. The i386 and x86_64 processors do
2420 indeed have special I/O space access cycles and instructions, but many
2421 CPUs don't have such a concept.
2423 The PCI bus, amongst others, defines an I/O space concept which - on such
2424 CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O
2425 space. However, it may also be mapped as a virtual I/O space in the CPU's
2426 memory map, particularly on those CPUs that don't support alternate I/O
2429 Accesses to this space may be fully synchronous (as on i386), but
2430 intermediary bridges (such as the PCI host bridge) may not fully honour
2433 They are guaranteed to be fully ordered with respect to each other.
2435 They are not guaranteed to be fully ordered with respect to other types of
2436 memory and I/O operation.
2438 (*) readX(), writeX():
2440 Whether these are guaranteed to be fully ordered and uncombined with
2441 respect to each other on the issuing CPU depends on the characteristics
2442 defined for the memory window through which they're accessing. On later
2443 i386 architecture machines, for example, this is controlled by way of the
2446 Ordinarily, these will be guaranteed to be fully ordered and uncombined,
2447 provided they're not accessing a prefetchable device.
2449 However, intermediary hardware (such as a PCI bridge) may indulge in
2450 deferral if it so wishes; to flush a store, a load from the same location
2451 is preferred[*], but a load from the same device or from configuration
2452 space should suffice for PCI.
2454 [*] NOTE! attempting to load from the same location as was written to may
2455 cause a malfunction - consider the 16550 Rx/Tx serial registers for
2458 Used with prefetchable I/O memory, an mmiowb() barrier may be required to
2459 force stores to be ordered.
2461 Please refer to the PCI specification for more information on interactions
2462 between PCI transactions.
2466 These are similar to readX(), but are not guaranteed to be ordered in any
2467 way. Be aware that there is no I/O read barrier available.
2469 (*) ioreadX(), iowriteX()
2471 These will perform appropriately for the type of access they're actually
2472 doing, be it inX()/outX() or readX()/writeX().
2475 ========================================
2476 ASSUMED MINIMUM EXECUTION ORDERING MODEL
2477 ========================================
2479 It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2480 maintain the appearance of program causality with respect to itself. Some CPUs
2481 (such as i386 or x86_64) are more constrained than others (such as powerpc or
2482 frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2483 of arch-specific code.
2485 This means that it must be considered that the CPU will execute its instruction
2486 stream in any order it feels like - or even in parallel - provided that if an
2487 instruction in the stream depends on an earlier instruction, then that
2488 earlier instruction must be sufficiently complete[*] before the later
2489 instruction may proceed; in other words: provided that the appearance of
2490 causality is maintained.
2492 [*] Some instructions have more than one effect - such as changing the
2493 condition codes, changing registers or changing memory - and different
2494 instructions may depend on different effects.
2496 A CPU may also discard any instruction sequence that winds up having no
2497 ultimate effect. For example, if two adjacent instructions both load an
2498 immediate value into the same register, the first may be discarded.
2501 Similarly, it has to be assumed that compiler might reorder the instruction
2502 stream in any way it sees fit, again provided the appearance of causality is
2506 ============================
2507 THE EFFECTS OF THE CPU CACHE
2508 ============================
2510 The way cached memory operations are perceived across the system is affected to
2511 a certain extent by the caches that lie between CPUs and memory, and by the
2512 memory coherence system that maintains the consistency of state in the system.
2514 As far as the way a CPU interacts with another part of the system through the
2515 caches goes, the memory system has to include the CPU's caches, and memory
2516 barriers for the most part act at the interface between the CPU and its cache
2517 (memory barriers logically act on the dotted line in the following diagram):
2519 <--- CPU ---> : <----------- Memory ----------->
2521 +--------+ +--------+ : +--------+ +-----------+
2522 | | | | : | | | | +--------+
2523 | CPU | | Memory | : | CPU | | | | |
2524 | Core |--->| Access |----->| Cache |<-->| | | |
2525 | | | Queue | : | | | |--->| Memory |
2526 | | | | : | | | | | |
2527 +--------+ +--------+ : +--------+ | | | |
2528 : | Cache | +--------+
2530 : | Mechanism | +--------+
2531 +--------+ +--------+ : +--------+ | | | |
2532 | | | | : | | | | | |
2533 | CPU | | Memory | : | CPU | | |--->| Device |
2534 | Core |--->| Access |----->| Cache |<-->| | | |
2535 | | | Queue | : | | | | | |
2536 | | | | : | | | | +--------+
2537 +--------+ +--------+ : +--------+ +-----------+
2541 Although any particular load or store may not actually appear outside of the
2542 CPU that issued it since it may have been satisfied within the CPU's own cache,
2543 it will still appear as if the full memory access had taken place as far as the
2544 other CPUs are concerned since the cache coherency mechanisms will migrate the
2545 cacheline over to the accessing CPU and propagate the effects upon conflict.
2547 The CPU core may execute instructions in any order it deems fit, provided the
2548 expected program causality appears to be maintained. Some of the instructions
2549 generate load and store operations which then go into the queue of memory
2550 accesses to be performed. The core may place these in the queue in any order
2551 it wishes, and continue execution until it is forced to wait for an instruction
2554 What memory barriers are concerned with is controlling the order in which
2555 accesses cross from the CPU side of things to the memory side of things, and
2556 the order in which the effects are perceived to happen by the other observers
2559 [!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2560 their own loads and stores as if they had happened in program order.
2562 [!] MMIO or other device accesses may bypass the cache system. This depends on
2563 the properties of the memory window through which devices are accessed and/or
2564 the use of any special device communication instructions the CPU may have.
2570 Life isn't quite as simple as it may appear above, however: for while the
2571 caches are expected to be coherent, there's no guarantee that that coherency
2572 will be ordered. This means that whilst changes made on one CPU will
2573 eventually become visible on all CPUs, there's no guarantee that they will
2574 become apparent in the same order on those other CPUs.
2577 Consider dealing with a system that has a pair of CPUs (1 & 2), each of which
2578 has a pair of parallel data caches (CPU 1 has A/B, and CPU 2 has C/D):
2583 +--------+ : +--->| Cache A |<------->| |
2584 | | : | +---------+ | |
2586 | | : | +---------+ | |
2587 +--------+ : +--->| Cache B |<------->| |
2590 : +---------+ | System |
2591 +--------+ : +--->| Cache C |<------->| |
2592 | | : | +---------+ | |
2594 | | : | +---------+ | |
2595 +--------+ : +--->| Cache D |<------->| |
2600 Imagine the system has the following properties:
2602 (*) an odd-numbered cache line may be in cache A, cache C or it may still be
2605 (*) an even-numbered cache line may be in cache B, cache D or it may still be
2608 (*) whilst the CPU core is interrogating one cache, the other cache may be
2609 making use of the bus to access the rest of the system - perhaps to
2610 displace a dirty cacheline or to do a speculative load;
2612 (*) each cache has a queue of operations that need to be applied to that cache
2613 to maintain coherency with the rest of the system;
2615 (*) the coherency queue is not flushed by normal loads to lines already
2616 present in the cache, even though the contents of the queue may
2617 potentially affect those loads.
2619 Imagine, then, that two writes are made on the first CPU, with a write barrier
2620 between them to guarantee that they will appear to reach that CPU's caches in
2621 the requisite order:
2624 =============== =============== =======================================
2625 u == 0, v == 1 and p == &u, q == &u
2627 smp_wmb(); Make sure change to v is visible before
2629 <A:modify v=2> v is now in cache A exclusively
2631 <B:modify p=&v> p is now in cache B exclusively
2633 The write memory barrier forces the other CPUs in the system to perceive that
2634 the local CPU's caches have apparently been updated in the correct order. But
2635 now imagine that the second CPU wants to read those values:
2638 =============== =============== =======================================
2643 The above pair of reads may then fail to happen in the expected order, as the
2644 cacheline holding p may get updated in one of the second CPU's caches whilst
2645 the update to the cacheline holding v is delayed in the other of the second
2646 CPU's caches by some other cache event:
2649 =============== =============== =======================================
2650 u == 0, v == 1 and p == &u, q == &u
2653 <A:modify v=2> <C:busy>
2657 <B:modify p=&v> <D:commit p=&v>
2660 <C:read *q> Reads from v before v updated in cache
2664 Basically, whilst both cachelines will be updated on CPU 2 eventually, there's
2665 no guarantee that, without intervention, the order of update will be the same
2666 as that committed on CPU 1.
2669 To intervene, we need to interpolate a data dependency barrier or a read
2670 barrier between the loads. This will force the cache to commit its coherency
2671 queue before processing any further requests:
2674 =============== =============== =======================================
2675 u == 0, v == 1 and p == &u, q == &u
2678 <A:modify v=2> <C:busy>
2682 <B:modify p=&v> <D:commit p=&v>
2684 smp_read_barrier_depends()
2688 <C:read *q> Reads from v after v updated in cache
2691 This sort of problem can be encountered on DEC Alpha processors as they have a
2692 split cache that improves performance by making better use of the data bus.
2693 Whilst most CPUs do imply a data dependency barrier on the read when a memory
2694 access depends on a read, not all do, so it may not be relied on.
2696 Other CPUs may also have split caches, but must coordinate between the various
2697 cachelets for normal memory accesses. The semantics of the Alpha removes the
2698 need for coordination in the absence of memory barriers.
2701 CACHE COHERENCY VS DMA
2702 ----------------------
2704 Not all systems maintain cache coherency with respect to devices doing DMA. In
2705 such cases, a device attempting DMA may obtain stale data from RAM because
2706 dirty cache lines may be resident in the caches of various CPUs, and may not
2707 have been written back to RAM yet. To deal with this, the appropriate part of
2708 the kernel must flush the overlapping bits of cache on each CPU (and maybe
2709 invalidate them as well).
2711 In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2712 cache lines being written back to RAM from a CPU's cache after the device has
2713 installed its own data, or cache lines present in the CPU's cache may simply
2714 obscure the fact that RAM has been updated, until at such time as the cacheline
2715 is discarded from the CPU's cache and reloaded. To deal with this, the
2716 appropriate part of the kernel must invalidate the overlapping bits of the
2719 See Documentation/cachetlb.txt for more information on cache management.
2722 CACHE COHERENCY VS MMIO
2723 -----------------------
2725 Memory mapped I/O usually takes place through memory locations that are part of
2726 a window in the CPU's memory space that has different properties assigned than
2727 the usual RAM directed window.
2729 Amongst these properties is usually the fact that such accesses bypass the
2730 caching entirely and go directly to the device buses. This means MMIO accesses
2731 may, in effect, overtake accesses to cached memory that were emitted earlier.
2732 A memory barrier isn't sufficient in such a case, but rather the cache must be
2733 flushed between the cached memory write and the MMIO access if the two are in
2737 =========================
2738 THE THINGS CPUS GET UP TO
2739 =========================
2741 A programmer might take it for granted that the CPU will perform memory
2742 operations in exactly the order specified, so that if the CPU is, for example,
2743 given the following piece of code to execute:
2745 a = ACCESS_ONCE(*A);
2746 ACCESS_ONCE(*B) = b;
2747 c = ACCESS_ONCE(*C);
2748 d = ACCESS_ONCE(*D);
2749 ACCESS_ONCE(*E) = e;
2751 they would then expect that the CPU will complete the memory operation for each
2752 instruction before moving on to the next one, leading to a definite sequence of
2753 operations as seen by external observers in the system:
2755 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2758 Reality is, of course, much messier. With many CPUs and compilers, the above
2759 assumption doesn't hold because:
2761 (*) loads are more likely to need to be completed immediately to permit
2762 execution progress, whereas stores can often be deferred without a
2765 (*) loads may be done speculatively, and the result discarded should it prove
2766 to have been unnecessary;
2768 (*) loads may be done speculatively, leading to the result having been fetched
2769 at the wrong time in the expected sequence of events;
2771 (*) the order of the memory accesses may be rearranged to promote better use
2772 of the CPU buses and caches;
2774 (*) loads and stores may be combined to improve performance when talking to
2775 memory or I/O hardware that can do batched accesses of adjacent locations,
2776 thus cutting down on transaction setup costs (memory and PCI devices may
2777 both be able to do this); and
2779 (*) the CPU's data cache may affect the ordering, and whilst cache-coherency
2780 mechanisms may alleviate this - once the store has actually hit the cache
2781 - there's no guarantee that the coherency management will be propagated in
2782 order to other CPUs.
2784 So what another CPU, say, might actually observe from the above piece of code
2787 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2789 (Where "LOAD {*C,*D}" is a combined load)
2792 However, it is guaranteed that a CPU will be self-consistent: it will see its
2793 _own_ accesses appear to be correctly ordered, without the need for a memory
2794 barrier. For instance with the following code:
2796 U = ACCESS_ONCE(*A);
2797 ACCESS_ONCE(*A) = V;
2798 ACCESS_ONCE(*A) = W;
2799 X = ACCESS_ONCE(*A);
2800 ACCESS_ONCE(*A) = Y;
2801 Z = ACCESS_ONCE(*A);
2803 and assuming no intervention by an external influence, it can be assumed that
2804 the final result will appear to be:
2806 U == the original value of *A
2811 The code above may cause the CPU to generate the full sequence of memory
2814 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2816 in that order, but, without intervention, the sequence may have almost any
2817 combination of elements combined or discarded, provided the program's view of
2818 the world remains consistent. Note that ACCESS_ONCE() is -not- optional
2819 in the above example, as there are architectures where a given CPU might
2820 reorder successive loads to the same location. On such architectures,
2821 ACCESS_ONCE() does whatever is necessary to prevent this, for example, on
2822 Itanium the volatile casts used by ACCESS_ONCE() cause GCC to emit the
2823 special ld.acq and st.rel instructions that prevent such reordering.
2825 The compiler may also combine, discard or defer elements of the sequence before
2826 the CPU even sees them.
2837 since, without either a write barrier or an ACCESS_ONCE(), it can be
2838 assumed that the effect of the storage of V to *A is lost. Similarly:
2843 may, without a memory barrier or an ACCESS_ONCE(), be reduced to:
2848 and the LOAD operation never appear outside of the CPU.
2851 AND THEN THERE'S THE ALPHA
2852 --------------------------
2854 The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
2855 some versions of the Alpha CPU have a split data cache, permitting them to have
2856 two semantically-related cache lines updated at separate times. This is where
2857 the data dependency barrier really becomes necessary as this synchronises both
2858 caches with the memory coherence system, thus making it seem like pointer
2859 changes vs new data occur in the right order.
2861 The Alpha defines the Linux kernel's memory barrier model.
2863 See the subsection on "Cache Coherency" above.
2873 Memory barriers can be used to implement circular buffering without the need
2874 of a lock to serialise the producer with the consumer. See:
2876 Documentation/circular-buffers.txt
2885 Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2887 Chapter 5.2: Physical Address Space Characteristics
2888 Chapter 5.4: Caches and Write Buffers
2889 Chapter 5.5: Data Sharing
2890 Chapter 5.6: Read/Write Ordering
2892 AMD64 Architecture Programmer's Manual Volume 2: System Programming
2893 Chapter 7.1: Memory-Access Ordering
2894 Chapter 7.4: Buffering and Combining Memory Writes
2896 IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2897 System Programming Guide
2898 Chapter 7.1: Locked Atomic Operations
2899 Chapter 7.2: Memory Ordering
2900 Chapter 7.4: Serializing Instructions
2902 The SPARC Architecture Manual, Version 9
2903 Chapter 8: Memory Models
2904 Appendix D: Formal Specification of the Memory Models
2905 Appendix J: Programming with the Memory Models
2907 UltraSPARC Programmer Reference Manual
2908 Chapter 5: Memory Accesses and Cacheability
2909 Chapter 15: Sparc-V9 Memory Models
2911 UltraSPARC III Cu User's Manual
2912 Chapter 9: Memory Models
2914 UltraSPARC IIIi Processor User's Manual
2915 Chapter 8: Memory Models
2917 UltraSPARC Architecture 2005
2919 Appendix D: Formal Specifications of the Memory Models
2921 UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
2922 Chapter 8: Memory Models
2923 Appendix F: Caches and Cache Coherency
2925 Solaris Internals, Core Kernel Architecture, p63-68:
2926 Chapter 3.3: Hardware Considerations for Locks and
2929 Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
2930 for Kernel Programmers:
2931 Chapter 13: Other Memory Models
2933 Intel Itanium Architecture Software Developer's Manual: Volume 1:
2934 Section 2.6: Speculation
2935 Section 4.4: Memory Access
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