1 ============================
2 LINUX KERNEL MEMORY BARRIERS
3 ============================
5 By: David Howells <dhowells@redhat.com>
9 (*) Abstract memory access model.
14 (*) What are memory barriers?
16 - Varieties of memory barrier.
17 - What may not be assumed about memory barriers?
18 - Data dependency barriers.
19 - Control dependencies.
20 - SMP barrier pairing.
21 - Examples of memory barrier sequences.
22 - Read memory barriers vs load speculation.
24 (*) Explicit kernel barriers.
27 - CPU memory barriers.
30 (*) Implicit kernel memory barriers.
33 - Interrupt disabling functions.
34 - Miscellaneous functions.
36 (*) Inter-CPU locking barrier effects.
38 - Locks vs memory accesses.
39 - Locks vs I/O accesses.
41 (*) Where are memory barriers needed?
43 - Interprocessor interaction.
48 (*) Kernel I/O barrier effects.
50 (*) Assumed minimum execution ordering model.
52 (*) The effects of the cpu cache.
55 - Cache coherency vs DMA.
56 - Cache coherency vs MMIO.
58 (*) The things CPUs get up to.
60 - And then there's the Alpha.
65 ============================
66 ABSTRACT MEMORY ACCESS MODEL
67 ============================
69 Consider the following abstract model of the system:
74 +-------+ : +--------+ : +-------+
77 | CPU 1 |<----->| Memory |<----->| CPU 2 |
80 +-------+ : +--------+ : +-------+
88 +---------->| Device |<----------+
94 Each CPU executes a program that generates memory access operations. In the
95 abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
96 perform the memory operations in any order it likes, provided program causality
97 appears to be maintained. Similarly, the compiler may also arrange the
98 instructions it emits in any order it likes, provided it doesn't affect the
99 apparent operation of the program.
101 So in the above diagram, the effects of the memory operations performed by a
102 CPU are perceived by the rest of the system as the operations cross the
103 interface between the CPU and rest of the system (the dotted lines).
106 For example, consider the following sequence of events:
109 =============== ===============
114 The set of accesses as seen by the memory system in the middle can be arranged
115 in 24 different combinations:
117 STORE A=3, STORE B=4, x=LOAD A->3, y=LOAD B->4
118 STORE A=3, STORE B=4, y=LOAD B->4, x=LOAD A->3
119 STORE A=3, x=LOAD A->3, STORE B=4, y=LOAD B->4
120 STORE A=3, x=LOAD A->3, y=LOAD B->2, STORE B=4
121 STORE A=3, y=LOAD B->2, STORE B=4, x=LOAD A->3
122 STORE A=3, y=LOAD B->2, x=LOAD A->3, STORE B=4
123 STORE B=4, STORE A=3, x=LOAD A->3, y=LOAD B->4
127 and can thus result in four different combinations of values:
135 Furthermore, the stores committed by a CPU to the memory system may not be
136 perceived by the loads made by another CPU in the same order as the stores were
140 As a further example, consider this sequence of events:
143 =============== ===============
144 { A == 1, B == 2, C = 3, P == &A, Q == &C }
148 There is an obvious data dependency here, as the value loaded into D depends on
149 the address retrieved from P by CPU 2. At the end of the sequence, any of the
150 following results are possible:
152 (Q == &A) and (D == 1)
153 (Q == &B) and (D == 2)
154 (Q == &B) and (D == 4)
156 Note that CPU 2 will never try and load C into D because the CPU will load P
157 into Q before issuing the load of *Q.
163 Some devices present their control interfaces as collections of memory
164 locations, but the order in which the control registers are accessed is very
165 important. For instance, imagine an ethernet card with a set of internal
166 registers that are accessed through an address port register (A) and a data
167 port register (D). To read internal register 5, the following code might then
173 but this might show up as either of the following two sequences:
175 STORE *A = 5, x = LOAD *D
176 x = LOAD *D, STORE *A = 5
178 the second of which will almost certainly result in a malfunction, since it set
179 the address _after_ attempting to read the register.
185 There are some minimal guarantees that may be expected of a CPU:
187 (*) On any given CPU, dependent memory accesses will be issued in order, with
188 respect to itself. This means that for:
192 the CPU will issue the following memory operations:
194 Q = LOAD P, D = LOAD *Q
196 and always in that order.
198 (*) Overlapping loads and stores within a particular CPU will appear to be
199 ordered within that CPU. This means that for:
203 the CPU will only issue the following sequence of memory operations:
205 a = LOAD *X, STORE *X = b
211 the CPU will only issue:
213 STORE *X = c, d = LOAD *X
215 (Loads and stores overlap if they are targeted at overlapping pieces of
218 And there are a number of things that _must_ or _must_not_ be assumed:
220 (*) It _must_not_ be assumed that independent loads and stores will be issued
221 in the order given. This means that for:
223 X = *A; Y = *B; *D = Z;
225 we may get any of the following sequences:
227 X = LOAD *A, Y = LOAD *B, STORE *D = Z
228 X = LOAD *A, STORE *D = Z, Y = LOAD *B
229 Y = LOAD *B, X = LOAD *A, STORE *D = Z
230 Y = LOAD *B, STORE *D = Z, X = LOAD *A
231 STORE *D = Z, X = LOAD *A, Y = LOAD *B
232 STORE *D = Z, Y = LOAD *B, X = LOAD *A
234 (*) It _must_ be assumed that overlapping memory accesses may be merged or
235 discarded. This means that for:
237 X = *A; Y = *(A + 4);
239 we may get any one of the following sequences:
241 X = LOAD *A; Y = LOAD *(A + 4);
242 Y = LOAD *(A + 4); X = LOAD *A;
243 {X, Y} = LOAD {*A, *(A + 4) };
249 we may get either of:
251 STORE *A = X; Y = LOAD *A;
255 =========================
256 WHAT ARE MEMORY BARRIERS?
257 =========================
259 As can be seen above, independent memory operations are effectively performed
260 in random order, but this can be a problem for CPU-CPU interaction and for I/O.
261 What is required is some way of intervening to instruct the compiler and the
262 CPU to restrict the order.
264 Memory barriers are such interventions. They impose a perceived partial
265 ordering over the memory operations on either side of the barrier.
267 Such enforcement is important because the CPUs and other devices in a system
268 can use a variety of tricks to improve performance, including reordering,
269 deferral and combination of memory operations; speculative loads; speculative
270 branch prediction and various types of caching. Memory barriers are used to
271 override or suppress these tricks, allowing the code to sanely control the
272 interaction of multiple CPUs and/or devices.
275 VARIETIES OF MEMORY BARRIER
276 ---------------------------
278 Memory barriers come in four basic varieties:
280 (1) Write (or store) memory barriers.
282 A write memory barrier gives a guarantee that all the STORE operations
283 specified before the barrier will appear to happen before all the STORE
284 operations specified after the barrier with respect to the other
285 components of the system.
287 A write barrier is a partial ordering on stores only; it is not required
288 to have any effect on loads.
290 A CPU can be viewed as committing a sequence of store operations to the
291 memory system as time progresses. All stores before a write barrier will
292 occur in the sequence _before_ all the stores after the write barrier.
294 [!] Note that write barriers should normally be paired with read or data
295 dependency barriers; see the "SMP barrier pairing" subsection.
298 (2) Data dependency barriers.
300 A data dependency barrier is a weaker form of read barrier. In the case
301 where two loads are performed such that the second depends on the result
302 of the first (eg: the first load retrieves the address to which the second
303 load will be directed), a data dependency barrier would be required to
304 make sure that the target of the second load is updated before the address
305 obtained by the first load is accessed.
307 A data dependency barrier is a partial ordering on interdependent loads
308 only; it is not required to have any effect on stores, independent loads
309 or overlapping loads.
311 As mentioned in (1), the other CPUs in the system can be viewed as
312 committing sequences of stores to the memory system that the CPU being
313 considered can then perceive. A data dependency barrier issued by the CPU
314 under consideration guarantees that for any load preceding it, if that
315 load touches one of a sequence of stores from another CPU, then by the
316 time the barrier completes, the effects of all the stores prior to that
317 touched by the load will be perceptible to any loads issued after the data
320 See the "Examples of memory barrier sequences" subsection for diagrams
321 showing the ordering constraints.
323 [!] Note that the first load really has to have a _data_ dependency and
324 not a control dependency. If the address for the second load is dependent
325 on the first load, but the dependency is through a conditional rather than
326 actually loading the address itself, then it's a _control_ dependency and
327 a full read barrier or better is required. See the "Control dependencies"
328 subsection for more information.
330 [!] Note that data dependency barriers should normally be paired with
331 write barriers; see the "SMP barrier pairing" subsection.
334 (3) Read (or load) memory barriers.
336 A read barrier is a data dependency barrier plus a guarantee that all the
337 LOAD operations specified before the barrier will appear to happen before
338 all the LOAD operations specified after the barrier with respect to the
339 other components of the system.
341 A read barrier is a partial ordering on loads only; it is not required to
342 have any effect on stores.
344 Read memory barriers imply data dependency barriers, and so can substitute
347 [!] Note that read barriers should normally be paired with write barriers;
348 see the "SMP barrier pairing" subsection.
351 (4) General memory barriers.
353 A general memory barrier gives a guarantee that all the LOAD and STORE
354 operations specified before the barrier will appear to happen before all
355 the LOAD and STORE operations specified after the barrier with respect to
356 the other components of the system.
358 A general memory barrier is a partial ordering over both loads and stores.
360 General memory barriers imply both read and write memory barriers, and so
361 can substitute for either.
364 And a couple of implicit varieties:
368 This acts as a one-way permeable barrier. It guarantees that all memory
369 operations after the LOCK operation will appear to happen after the LOCK
370 operation with respect to the other components of the system.
372 Memory operations that occur before a LOCK operation may appear to happen
375 A LOCK operation should almost always be paired with an UNLOCK operation.
378 (6) UNLOCK operations.
380 This also acts as a one-way permeable barrier. It guarantees that all
381 memory operations before the UNLOCK operation will appear to happen before
382 the UNLOCK operation with respect to the other components of the system.
384 Memory operations that occur after an UNLOCK operation may appear to
385 happen before it completes.
387 LOCK and UNLOCK operations are guaranteed to appear with respect to each
388 other strictly in the order specified.
390 The use of LOCK and UNLOCK operations generally precludes the need for
391 other sorts of memory barrier (but note the exceptions mentioned in the
392 subsection "MMIO write barrier").
395 Memory barriers are only required where there's a possibility of interaction
396 between two CPUs or between a CPU and a device. If it can be guaranteed that
397 there won't be any such interaction in any particular piece of code, then
398 memory barriers are unnecessary in that piece of code.
401 Note that these are the _minimum_ guarantees. Different architectures may give
402 more substantial guarantees, but they may _not_ be relied upon outside of arch
406 WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
407 ----------------------------------------------
409 There are certain things that the Linux kernel memory barriers do not guarantee:
411 (*) There is no guarantee that any of the memory accesses specified before a
412 memory barrier will be _complete_ by the completion of a memory barrier
413 instruction; the barrier can be considered to draw a line in that CPU's
414 access queue that accesses of the appropriate type may not cross.
416 (*) There is no guarantee that issuing a memory barrier on one CPU will have
417 any direct effect on another CPU or any other hardware in the system. The
418 indirect effect will be the order in which the second CPU sees the effects
419 of the first CPU's accesses occur, but see the next point:
421 (*) There is no guarantee that a CPU will see the correct order of effects
422 from a second CPU's accesses, even _if_ the second CPU uses a memory
423 barrier, unless the first CPU _also_ uses a matching memory barrier (see
424 the subsection on "SMP Barrier Pairing").
426 (*) There is no guarantee that some intervening piece of off-the-CPU
427 hardware[*] will not reorder the memory accesses. CPU cache coherency
428 mechanisms should propagate the indirect effects of a memory barrier
429 between CPUs, but might not do so in order.
431 [*] For information on bus mastering DMA and coherency please read:
433 Documentation/PCI/pci.txt
434 Documentation/PCI/PCI-DMA-mapping.txt
435 Documentation/DMA-API.txt
438 DATA DEPENDENCY BARRIERS
439 ------------------------
441 The usage requirements of data dependency barriers are a little subtle, and
442 it's not always obvious that they're needed. To illustrate, consider the
443 following sequence of events:
446 =============== ===============
447 { A == 1, B == 2, C = 3, P == &A, Q == &C }
454 There's a clear data dependency here, and it would seem that by the end of the
455 sequence, Q must be either &A or &B, and that:
457 (Q == &A) implies (D == 1)
458 (Q == &B) implies (D == 4)
460 But! CPU 2's perception of P may be updated _before_ its perception of B, thus
461 leading to the following situation:
463 (Q == &B) and (D == 2) ????
465 Whilst this may seem like a failure of coherency or causality maintenance, it
466 isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
469 To deal with this, a data dependency barrier or better must be inserted
470 between the address load and the data load:
473 =============== ===============
474 { A == 1, B == 2, C = 3, P == &A, Q == &C }
479 <data dependency barrier>
482 This enforces the occurrence of one of the two implications, and prevents the
483 third possibility from arising.
485 [!] Note that this extremely counterintuitive situation arises most easily on
486 machines with split caches, so that, for example, one cache bank processes
487 even-numbered cache lines and the other bank processes odd-numbered cache
488 lines. The pointer P might be stored in an odd-numbered cache line, and the
489 variable B might be stored in an even-numbered cache line. Then, if the
490 even-numbered bank of the reading CPU's cache is extremely busy while the
491 odd-numbered bank is idle, one can see the new value of the pointer P (&B),
492 but the old value of the variable B (2).
495 Another example of where data dependency barriers might by required is where a
496 number is read from memory and then used to calculate the index for an array
500 =============== ===============
501 { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
506 <data dependency barrier>
510 The data dependency barrier is very important to the RCU system, for example.
511 See rcu_dereference() in include/linux/rcupdate.h. This permits the current
512 target of an RCU'd pointer to be replaced with a new modified target, without
513 the replacement target appearing to be incompletely initialised.
515 See also the subsection on "Cache Coherency" for a more thorough example.
521 A control dependency requires a full read memory barrier, not simply a data
522 dependency barrier to make it work correctly. Consider the following bit of
528 <data dependency barrier>
531 This will not have the desired effect because there is no actual data
532 dependency, but rather a control dependency that the CPU may short-circuit by
533 attempting to predict the outcome in advance. In such a case what's actually
546 When dealing with CPU-CPU interactions, certain types of memory barrier should
547 always be paired. A lack of appropriate pairing is almost certainly an error.
549 A write barrier should always be paired with a data dependency barrier or read
550 barrier, though a general barrier would also be viable. Similarly a read
551 barrier or a data dependency barrier should always be paired with at least an
552 write barrier, though, again, a general barrier is viable:
555 =============== ===============
565 =============== ===============================
569 <data dependency barrier>
572 Basically, the read barrier always has to be there, even though it can be of
575 [!] Note that the stores before the write barrier would normally be expected to
576 match the loads after the read barrier or the data dependency barrier, and vice
580 =============== ===============
581 a = 1; }---- --->{ v = c
583 <write barrier> \ <read barrier>
584 c = 3; } / \ { x = a;
585 d = 4; }---- --->{ y = b;
588 EXAMPLES OF MEMORY BARRIER SEQUENCES
589 ------------------------------------
591 Firstly, write barriers act as partial orderings on store operations.
592 Consider the following sequence of events:
595 =======================
603 This sequence of events is committed to the memory coherence system in an order
604 that the rest of the system might perceive as the unordered set of { STORE A,
605 STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
610 | |------>| C=3 | } /\
611 | | : +------+ }----- \ -----> Events perceptible to
612 | | : | A=1 | } \/ the rest of the system
614 | CPU 1 | : | B=2 | }
616 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier
617 | | +------+ } requires all stores prior to the
618 | | : | E=5 | } barrier to be committed before
619 | | : +------+ } further stores may take place
624 | Sequence in which stores are committed to the
625 | memory system by CPU 1
629 Secondly, data dependency barriers act as partial orderings on data-dependent
630 loads. Consider the following sequence of events:
633 ======================= =======================
634 { B = 7; X = 9; Y = 8; C = &Y }
639 STORE D = 4 LOAD C (gets &B)
642 Without intervention, CPU 2 may perceive the events on CPU 1 in some
643 effectively random order, despite the write barrier issued by CPU 1:
646 | | +------+ +-------+ | Sequence of update
647 | |------>| B=2 |----- --->| Y->8 | | of perception on
648 | | : +------+ \ +-------+ | CPU 2
649 | CPU 1 | : | A=1 | \ --->| C->&Y | V
650 | | +------+ | +-------+
651 | | wwwwwwwwwwwwwwww | : :
653 | | : | C=&B |--- | : : +-------+
654 | | : +------+ \ | +-------+ | |
655 | |------>| D=4 | ----------->| C->&B |------>| |
656 | | +------+ | +-------+ | |
657 +-------+ : : | : : | |
661 Apparently incorrect ---> | | B->7 |------>| |
662 perception of B (!) | +-------+ | |
665 The load of X holds ---> \ | X->9 |------>| |
666 up the maintenance \ +-------+ | |
667 of coherence of B ----->| B->2 | +-------+
672 In the above example, CPU 2 perceives that B is 7, despite the load of *C
673 (which would be B) coming after the LOAD of C.
675 If, however, a data dependency barrier were to be placed between the load of C
676 and the load of *C (ie: B) on CPU 2:
679 ======================= =======================
680 { B = 7; X = 9; Y = 8; C = &Y }
685 STORE D = 4 LOAD C (gets &B)
686 <data dependency barrier>
689 then the following will occur:
692 | | +------+ +-------+
693 | |------>| B=2 |----- --->| Y->8 |
694 | | : +------+ \ +-------+
695 | CPU 1 | : | A=1 | \ --->| C->&Y |
696 | | +------+ | +-------+
697 | | wwwwwwwwwwwwwwww | : :
699 | | : | C=&B |--- | : : +-------+
700 | | : +------+ \ | +-------+ | |
701 | |------>| D=4 | ----------->| C->&B |------>| |
702 | | +------+ | +-------+ | |
703 +-------+ : : | : : | |
709 Makes sure all effects ---> \ ddddddddddddddddd | |
710 prior to the store of C \ +-------+ | |
711 are perceptible to ----->| B->2 |------>| |
712 subsequent loads +-------+ | |
716 And thirdly, a read barrier acts as a partial order on loads. Consider the
717 following sequence of events:
720 ======================= =======================
728 Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
729 some effectively random order, despite the write barrier issued by CPU 1:
732 | | +------+ +-------+
733 | |------>| A=1 |------ --->| A->0 |
734 | | +------+ \ +-------+
735 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
736 | | +------+ | +-------+
737 | |------>| B=2 |--- | : :
738 | | +------+ \ | : : +-------+
739 +-------+ : : \ | +-------+ | |
740 ---------->| B->2 |------>| |
741 | +-------+ | CPU 2 |
752 If, however, a read barrier were to be placed between the load of B and the
756 ======================= =======================
765 then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
769 | | +------+ +-------+
770 | |------>| A=1 |------ --->| A->0 |
771 | | +------+ \ +-------+
772 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
773 | | +------+ | +-------+
774 | |------>| B=2 |--- | : :
775 | | +------+ \ | : : +-------+
776 +-------+ : : \ | +-------+ | |
777 ---------->| B->2 |------>| |
778 | +-------+ | CPU 2 |
781 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
782 barrier causes all effects \ +-------+ | |
783 prior to the storage of B ---->| A->1 |------>| |
784 to be perceptible to CPU 2 +-------+ | |
788 To illustrate this more completely, consider what could happen if the code
789 contained a load of A either side of the read barrier:
792 ======================= =======================
798 LOAD A [first load of A]
800 LOAD A [second load of A]
802 Even though the two loads of A both occur after the load of B, they may both
803 come up with different values:
806 | | +------+ +-------+
807 | |------>| A=1 |------ --->| A->0 |
808 | | +------+ \ +-------+
809 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
810 | | +------+ | +-------+
811 | |------>| B=2 |--- | : :
812 | | +------+ \ | : : +-------+
813 +-------+ : : \ | +-------+ | |
814 ---------->| B->2 |------>| |
815 | +-------+ | CPU 2 |
819 | | A->0 |------>| 1st |
821 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
822 barrier causes all effects \ +-------+ | |
823 prior to the storage of B ---->| A->1 |------>| 2nd |
824 to be perceptible to CPU 2 +-------+ | |
828 But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
829 before the read barrier completes anyway:
832 | | +------+ +-------+
833 | |------>| A=1 |------ --->| A->0 |
834 | | +------+ \ +-------+
835 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
836 | | +------+ | +-------+
837 | |------>| B=2 |--- | : :
838 | | +------+ \ | : : +-------+
839 +-------+ : : \ | +-------+ | |
840 ---------->| B->2 |------>| |
841 | +-------+ | CPU 2 |
845 ---->| A->1 |------>| 1st |
847 rrrrrrrrrrrrrrrrr | |
849 | A->1 |------>| 2nd |
854 The guarantee is that the second load will always come up with A == 1 if the
855 load of B came up with B == 2. No such guarantee exists for the first load of
856 A; that may come up with either A == 0 or A == 1.
859 READ MEMORY BARRIERS VS LOAD SPECULATION
860 ----------------------------------------
862 Many CPUs speculate with loads: that is they see that they will need to load an
863 item from memory, and they find a time where they're not using the bus for any
864 other loads, and so do the load in advance - even though they haven't actually
865 got to that point in the instruction execution flow yet. This permits the
866 actual load instruction to potentially complete immediately because the CPU
867 already has the value to hand.
869 It may turn out that the CPU didn't actually need the value - perhaps because a
870 branch circumvented the load - in which case it can discard the value or just
871 cache it for later use.
876 ======================= =======================
878 DIVIDE } Divide instructions generally
879 DIVIDE } take a long time to perform
882 Which might appear as this:
886 --->| B->2 |------>| |
890 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
891 division speculates on the +-------+ ~ | |
895 Once the divisions are complete --> : : ~-->| |
896 the CPU can then perform the : : | |
897 LOAD with immediate effect : : +-------+
900 Placing a read barrier or a data dependency barrier just before the second
904 ======================= =======================
911 will force any value speculatively obtained to be reconsidered to an extent
912 dependent on the type of barrier used. If there was no change made to the
913 speculated memory location, then the speculated value will just be used:
917 --->| B->2 |------>| |
921 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
922 division speculates on the +-------+ ~ | |
927 rrrrrrrrrrrrrrrr~ | |
934 but if there was an update or an invalidation from another CPU pending, then
935 the speculation will be cancelled and the value reloaded:
939 --->| B->2 |------>| |
943 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
944 division speculates on the +-------+ ~ | |
949 rrrrrrrrrrrrrrrrr | |
951 The speculation is discarded ---> --->| A->1 |------>| |
952 and an updated value is +-------+ | |
953 retrieved : : +-------+
956 ========================
957 EXPLICIT KERNEL BARRIERS
958 ========================
960 The Linux kernel has a variety of different barriers that act at different
963 (*) Compiler barrier.
965 (*) CPU memory barriers.
967 (*) MMIO write barrier.
973 The Linux kernel has an explicit compiler barrier function that prevents the
974 compiler from moving the memory accesses either side of it to the other side:
978 This is a general barrier - lesser varieties of compiler barrier do not exist.
980 The compiler barrier has no direct effect on the CPU, which may then reorder
981 things however it wishes.
987 The Linux kernel has eight basic CPU memory barriers:
989 TYPE MANDATORY SMP CONDITIONAL
990 =============== ======================= ===========================
991 GENERAL mb() smp_mb()
992 WRITE wmb() smp_wmb()
994 DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends()
997 All memory barriers except the data dependency barriers imply a compiler
998 barrier. Data dependencies do not impose any additional compiler ordering.
1000 Aside: In the case of data dependencies, the compiler would be expected to
1001 issue the loads in the correct order (eg. `a[b]` would have to load the value
1002 of b before loading a[b]), however there is no guarantee in the C specification
1003 that the compiler may not speculate the value of b (eg. is equal to 1) and load
1004 a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the
1005 problem of a compiler reloading b after having loaded a[b], thus having a newer
1006 copy of b than a[b]. A consensus has not yet been reached about these problems,
1007 however the ACCESS_ONCE macro is a good place to start looking.
1009 SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1010 systems because it is assumed that a CPU will appear to be self-consistent,
1011 and will order overlapping accesses correctly with respect to itself.
1013 [!] Note that SMP memory barriers _must_ be used to control the ordering of
1014 references to shared memory on SMP systems, though the use of locking instead
1017 Mandatory barriers should not be used to control SMP effects, since mandatory
1018 barriers unnecessarily impose overhead on UP systems. They may, however, be
1019 used to control MMIO effects on accesses through relaxed memory I/O windows.
1020 These are required even on non-SMP systems as they affect the order in which
1021 memory operations appear to a device by prohibiting both the compiler and the
1022 CPU from reordering them.
1025 There are some more advanced barrier functions:
1027 (*) set_mb(var, value)
1029 This assigns the value to the variable and then inserts a full memory
1030 barrier after it, depending on the function. It isn't guaranteed to
1031 insert anything more than a compiler barrier in a UP compilation.
1034 (*) smp_mb__before_atomic_dec();
1035 (*) smp_mb__after_atomic_dec();
1036 (*) smp_mb__before_atomic_inc();
1037 (*) smp_mb__after_atomic_inc();
1039 These are for use with atomic add, subtract, increment and decrement
1040 functions that don't return a value, especially when used for reference
1041 counting. These functions do not imply memory barriers.
1043 As an example, consider a piece of code that marks an object as being dead
1044 and then decrements the object's reference count:
1047 smp_mb__before_atomic_dec();
1048 atomic_dec(&obj->ref_count);
1050 This makes sure that the death mark on the object is perceived to be set
1051 *before* the reference counter is decremented.
1053 See Documentation/atomic_ops.txt for more information. See the "Atomic
1054 operations" subsection for information on where to use these.
1057 (*) smp_mb__before_clear_bit(void);
1058 (*) smp_mb__after_clear_bit(void);
1060 These are for use similar to the atomic inc/dec barriers. These are
1061 typically used for bitwise unlocking operations, so care must be taken as
1062 there are no implicit memory barriers here either.
1064 Consider implementing an unlock operation of some nature by clearing a
1065 locking bit. The clear_bit() would then need to be barriered like this:
1067 smp_mb__before_clear_bit();
1070 This prevents memory operations before the clear leaking to after it. See
1071 the subsection on "Locking Functions" with reference to UNLOCK operation
1074 See Documentation/atomic_ops.txt for more information. See the "Atomic
1075 operations" subsection for information on where to use these.
1081 The Linux kernel also has a special barrier for use with memory-mapped I/O
1086 This is a variation on the mandatory write barrier that causes writes to weakly
1087 ordered I/O regions to be partially ordered. Its effects may go beyond the
1088 CPU->Hardware interface and actually affect the hardware at some level.
1090 See the subsection "Locks vs I/O accesses" for more information.
1093 ===============================
1094 IMPLICIT KERNEL MEMORY BARRIERS
1095 ===============================
1097 Some of the other functions in the linux kernel imply memory barriers, amongst
1098 which are locking and scheduling functions.
1100 This specification is a _minimum_ guarantee; any particular architecture may
1101 provide more substantial guarantees, but these may not be relied upon outside
1102 of arch specific code.
1108 The Linux kernel has a number of locking constructs:
1117 In all cases there are variants on "LOCK" operations and "UNLOCK" operations
1118 for each construct. These operations all imply certain barriers:
1120 (1) LOCK operation implication:
1122 Memory operations issued after the LOCK will be completed after the LOCK
1123 operation has completed.
1125 Memory operations issued before the LOCK may be completed after the LOCK
1126 operation has completed.
1128 (2) UNLOCK operation implication:
1130 Memory operations issued before the UNLOCK will be completed before the
1131 UNLOCK operation has completed.
1133 Memory operations issued after the UNLOCK may be completed before the
1134 UNLOCK operation has completed.
1136 (3) LOCK vs LOCK implication:
1138 All LOCK operations issued before another LOCK operation will be completed
1139 before that LOCK operation.
1141 (4) LOCK vs UNLOCK implication:
1143 All LOCK operations issued before an UNLOCK operation will be completed
1144 before the UNLOCK operation.
1146 All UNLOCK operations issued before a LOCK operation will be completed
1147 before the LOCK operation.
1149 (5) Failed conditional LOCK implication:
1151 Certain variants of the LOCK operation may fail, either due to being
1152 unable to get the lock immediately, or due to receiving an unblocked
1153 signal whilst asleep waiting for the lock to become available. Failed
1154 locks do not imply any sort of barrier.
1156 Therefore, from (1), (2) and (4) an UNLOCK followed by an unconditional LOCK is
1157 equivalent to a full barrier, but a LOCK followed by an UNLOCK is not.
1159 [!] Note: one of the consequences of LOCKs and UNLOCKs being only one-way
1160 barriers is that the effects of instructions outside of a critical section
1161 may seep into the inside of the critical section.
1163 A LOCK followed by an UNLOCK may not be assumed to be full memory barrier
1164 because it is possible for an access preceding the LOCK to happen after the
1165 LOCK, and an access following the UNLOCK to happen before the UNLOCK, and the
1166 two accesses can themselves then cross:
1175 LOCK, STORE *B, STORE *A, UNLOCK
1177 Locks and semaphores may not provide any guarantee of ordering on UP compiled
1178 systems, and so cannot be counted on in such a situation to actually achieve
1179 anything at all - especially with respect to I/O accesses - unless combined
1180 with interrupt disabling operations.
1182 See also the section on "Inter-CPU locking barrier effects".
1185 As an example, consider the following:
1196 The following sequence of events is acceptable:
1198 LOCK, {*F,*A}, *E, {*C,*D}, *B, UNLOCK
1200 [+] Note that {*F,*A} indicates a combined access.
1202 But none of the following are:
1204 {*F,*A}, *B, LOCK, *C, *D, UNLOCK, *E
1205 *A, *B, *C, LOCK, *D, UNLOCK, *E, *F
1206 *A, *B, LOCK, *C, UNLOCK, *D, *E, *F
1207 *B, LOCK, *C, *D, UNLOCK, {*F,*A}, *E
1211 INTERRUPT DISABLING FUNCTIONS
1212 -----------------------------
1214 Functions that disable interrupts (LOCK equivalent) and enable interrupts
1215 (UNLOCK equivalent) will act as compiler barriers only. So if memory or I/O
1216 barriers are required in such a situation, they must be provided from some
1220 MISCELLANEOUS FUNCTIONS
1221 -----------------------
1223 Other functions that imply barriers:
1225 (*) schedule() and similar imply full memory barriers.
1228 =================================
1229 INTER-CPU LOCKING BARRIER EFFECTS
1230 =================================
1232 On SMP systems locking primitives give a more substantial form of barrier: one
1233 that does affect memory access ordering on other CPUs, within the context of
1234 conflict on any particular lock.
1237 LOCKS VS MEMORY ACCESSES
1238 ------------------------
1240 Consider the following: the system has a pair of spinlocks (M) and (Q), and
1241 three CPUs; then should the following sequence of events occur:
1244 =============================== ===============================
1252 Then there is no guarantee as to what order CPU 3 will see the accesses to *A
1253 through *H occur in, other than the constraints imposed by the separate locks
1254 on the separate CPUs. It might, for example, see:
1256 *E, LOCK M, LOCK Q, *G, *C, *F, *A, *B, UNLOCK Q, *D, *H, UNLOCK M
1258 But it won't see any of:
1260 *B, *C or *D preceding LOCK M
1261 *A, *B or *C following UNLOCK M
1262 *F, *G or *H preceding LOCK Q
1263 *E, *F or *G following UNLOCK Q
1266 However, if the following occurs:
1269 =============================== ===============================
1284 *E, LOCK M [1], *C, *B, *A, UNLOCK M [1],
1285 LOCK M [2], *H, *F, *G, UNLOCK M [2], *D
1287 But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
1289 *B, *C, *D, *F, *G or *H preceding LOCK M [1]
1290 *A, *B or *C following UNLOCK M [1]
1291 *F, *G or *H preceding LOCK M [2]
1292 *A, *B, *C, *E, *F or *G following UNLOCK M [2]
1295 LOCKS VS I/O ACCESSES
1296 ---------------------
1298 Under certain circumstances (especially involving NUMA), I/O accesses within
1299 two spinlocked sections on two different CPUs may be seen as interleaved by the
1300 PCI bridge, because the PCI bridge does not necessarily participate in the
1301 cache-coherence protocol, and is therefore incapable of issuing the required
1302 read memory barriers.
1307 =============================== ===============================
1317 may be seen by the PCI bridge as follows:
1319 STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5
1321 which would probably cause the hardware to malfunction.
1324 What is necessary here is to intervene with an mmiowb() before dropping the
1325 spinlock, for example:
1328 =============================== ===============================
1340 this will ensure that the two stores issued on CPU 1 appear at the PCI bridge
1341 before either of the stores issued on CPU 2.
1344 Furthermore, following a store by a load from the same device obviates the need
1345 for the mmiowb(), because the load forces the store to complete before the load
1349 =============================== ===============================
1360 See Documentation/DocBook/deviceiobook.tmpl for more information.
1363 =================================
1364 WHERE ARE MEMORY BARRIERS NEEDED?
1365 =================================
1367 Under normal operation, memory operation reordering is generally not going to
1368 be a problem as a single-threaded linear piece of code will still appear to
1369 work correctly, even if it's in an SMP kernel. There are, however, three
1370 circumstances in which reordering definitely _could_ be a problem:
1372 (*) Interprocessor interaction.
1374 (*) Atomic operations.
1376 (*) Accessing devices.
1381 INTERPROCESSOR INTERACTION
1382 --------------------------
1384 When there's a system with more than one processor, more than one CPU in the
1385 system may be working on the same data set at the same time. This can cause
1386 synchronisation problems, and the usual way of dealing with them is to use
1387 locks. Locks, however, are quite expensive, and so it may be preferable to
1388 operate without the use of a lock if at all possible. In such a case
1389 operations that affect both CPUs may have to be carefully ordered to prevent
1392 Consider, for example, the R/W semaphore slow path. Here a waiting process is
1393 queued on the semaphore, by virtue of it having a piece of its stack linked to
1394 the semaphore's list of waiting processes:
1396 struct rw_semaphore {
1399 struct list_head waiters;
1402 struct rwsem_waiter {
1403 struct list_head list;
1404 struct task_struct *task;
1407 To wake up a particular waiter, the up_read() or up_write() functions have to:
1409 (1) read the next pointer from this waiter's record to know as to where the
1410 next waiter record is;
1412 (2) read the pointer to the waiter's task structure;
1414 (3) clear the task pointer to tell the waiter it has been given the semaphore;
1416 (4) call wake_up_process() on the task; and
1418 (5) release the reference held on the waiter's task struct.
1420 In other words, it has to perform this sequence of events:
1422 LOAD waiter->list.next;
1428 and if any of these steps occur out of order, then the whole thing may
1431 Once it has queued itself and dropped the semaphore lock, the waiter does not
1432 get the lock again; it instead just waits for its task pointer to be cleared
1433 before proceeding. Since the record is on the waiter's stack, this means that
1434 if the task pointer is cleared _before_ the next pointer in the list is read,
1435 another CPU might start processing the waiter and might clobber the waiter's
1436 stack before the up*() function has a chance to read the next pointer.
1438 Consider then what might happen to the above sequence of events:
1441 =============================== ===============================
1448 Woken up by other event
1453 foo() clobbers *waiter
1455 LOAD waiter->list.next;
1458 This could be dealt with using the semaphore lock, but then the down_xxx()
1459 function has to needlessly get the spinlock again after being woken up.
1461 The way to deal with this is to insert a general SMP memory barrier:
1463 LOAD waiter->list.next;
1470 In this case, the barrier makes a guarantee that all memory accesses before the
1471 barrier will appear to happen before all the memory accesses after the barrier
1472 with respect to the other CPUs on the system. It does _not_ guarantee that all
1473 the memory accesses before the barrier will be complete by the time the barrier
1474 instruction itself is complete.
1476 On a UP system - where this wouldn't be a problem - the smp_mb() is just a
1477 compiler barrier, thus making sure the compiler emits the instructions in the
1478 right order without actually intervening in the CPU. Since there's only one
1479 CPU, that CPU's dependency ordering logic will take care of everything else.
1485 Whilst they are technically interprocessor interaction considerations, atomic
1486 operations are noted specially as some of them imply full memory barriers and
1487 some don't, but they're very heavily relied on as a group throughout the
1490 Any atomic operation that modifies some state in memory and returns information
1491 about the state (old or new) implies an SMP-conditional general memory barrier
1492 (smp_mb()) on each side of the actual operation (with the exception of
1493 explicit lock operations, described later). These include:
1498 atomic_inc_return();
1499 atomic_dec_return();
1500 atomic_add_return();
1501 atomic_sub_return();
1502 atomic_inc_and_test();
1503 atomic_dec_and_test();
1504 atomic_sub_and_test();
1505 atomic_add_negative();
1506 atomic_add_unless(); /* when succeeds (returns 1) */
1508 test_and_clear_bit();
1509 test_and_change_bit();
1511 These are used for such things as implementing LOCK-class and UNLOCK-class
1512 operations and adjusting reference counters towards object destruction, and as
1513 such the implicit memory barrier effects are necessary.
1516 The following operations are potential problems as they do _not_ imply memory
1517 barriers, but might be used for implementing such things as UNLOCK-class
1525 With these the appropriate explicit memory barrier should be used if necessary
1526 (smp_mb__before_clear_bit() for instance).
1529 The following also do _not_ imply memory barriers, and so may require explicit
1530 memory barriers under some circumstances (smp_mb__before_atomic_dec() for
1538 If they're used for statistics generation, then they probably don't need memory
1539 barriers, unless there's a coupling between statistical data.
1541 If they're used for reference counting on an object to control its lifetime,
1542 they probably don't need memory barriers because either the reference count
1543 will be adjusted inside a locked section, or the caller will already hold
1544 sufficient references to make the lock, and thus a memory barrier unnecessary.
1546 If they're used for constructing a lock of some description, then they probably
1547 do need memory barriers as a lock primitive generally has to do things in a
1550 Basically, each usage case has to be carefully considered as to whether memory
1551 barriers are needed or not.
1553 The following operations are special locking primitives:
1555 test_and_set_bit_lock();
1557 __clear_bit_unlock();
1559 These implement LOCK-class and UNLOCK-class operations. These should be used in
1560 preference to other operations when implementing locking primitives, because
1561 their implementations can be optimised on many architectures.
1563 [!] Note that special memory barrier primitives are available for these
1564 situations because on some CPUs the atomic instructions used imply full memory
1565 barriers, and so barrier instructions are superfluous in conjunction with them,
1566 and in such cases the special barrier primitives will be no-ops.
1568 See Documentation/atomic_ops.txt for more information.
1574 Many devices can be memory mapped, and so appear to the CPU as if they're just
1575 a set of memory locations. To control such a device, the driver usually has to
1576 make the right memory accesses in exactly the right order.
1578 However, having a clever CPU or a clever compiler creates a potential problem
1579 in that the carefully sequenced accesses in the driver code won't reach the
1580 device in the requisite order if the CPU or the compiler thinks it is more
1581 efficient to reorder, combine or merge accesses - something that would cause
1582 the device to malfunction.
1584 Inside of the Linux kernel, I/O should be done through the appropriate accessor
1585 routines - such as inb() or writel() - which know how to make such accesses
1586 appropriately sequential. Whilst this, for the most part, renders the explicit
1587 use of memory barriers unnecessary, there are a couple of situations where they
1590 (1) On some systems, I/O stores are not strongly ordered across all CPUs, and
1591 so for _all_ general drivers locks should be used and mmiowb() must be
1592 issued prior to unlocking the critical section.
1594 (2) If the accessor functions are used to refer to an I/O memory window with
1595 relaxed memory access properties, then _mandatory_ memory barriers are
1596 required to enforce ordering.
1598 See Documentation/DocBook/deviceiobook.tmpl for more information.
1604 A driver may be interrupted by its own interrupt service routine, and thus the
1605 two parts of the driver may interfere with each other's attempts to control or
1608 This may be alleviated - at least in part - by disabling local interrupts (a
1609 form of locking), such that the critical operations are all contained within
1610 the interrupt-disabled section in the driver. Whilst the driver's interrupt
1611 routine is executing, the driver's core may not run on the same CPU, and its
1612 interrupt is not permitted to happen again until the current interrupt has been
1613 handled, thus the interrupt handler does not need to lock against that.
1615 However, consider a driver that was talking to an ethernet card that sports an
1616 address register and a data register. If that driver's core talks to the card
1617 under interrupt-disablement and then the driver's interrupt handler is invoked:
1628 The store to the data register might happen after the second store to the
1629 address register if ordering rules are sufficiently relaxed:
1631 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
1634 If ordering rules are relaxed, it must be assumed that accesses done inside an
1635 interrupt disabled section may leak outside of it and may interleave with
1636 accesses performed in an interrupt - and vice versa - unless implicit or
1637 explicit barriers are used.
1639 Normally this won't be a problem because the I/O accesses done inside such
1640 sections will include synchronous load operations on strictly ordered I/O
1641 registers that form implicit I/O barriers. If this isn't sufficient then an
1642 mmiowb() may need to be used explicitly.
1645 A similar situation may occur between an interrupt routine and two routines
1646 running on separate CPUs that communicate with each other. If such a case is
1647 likely, then interrupt-disabling locks should be used to guarantee ordering.
1650 ==========================
1651 KERNEL I/O BARRIER EFFECTS
1652 ==========================
1654 When accessing I/O memory, drivers should use the appropriate accessor
1659 These are intended to talk to I/O space rather than memory space, but
1660 that's primarily a CPU-specific concept. The i386 and x86_64 processors do
1661 indeed have special I/O space access cycles and instructions, but many
1662 CPUs don't have such a concept.
1664 The PCI bus, amongst others, defines an I/O space concept which - on such
1665 CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O
1666 space. However, it may also be mapped as a virtual I/O space in the CPU's
1667 memory map, particularly on those CPUs that don't support alternate I/O
1670 Accesses to this space may be fully synchronous (as on i386), but
1671 intermediary bridges (such as the PCI host bridge) may not fully honour
1674 They are guaranteed to be fully ordered with respect to each other.
1676 They are not guaranteed to be fully ordered with respect to other types of
1677 memory and I/O operation.
1679 (*) readX(), writeX():
1681 Whether these are guaranteed to be fully ordered and uncombined with
1682 respect to each other on the issuing CPU depends on the characteristics
1683 defined for the memory window through which they're accessing. On later
1684 i386 architecture machines, for example, this is controlled by way of the
1687 Ordinarily, these will be guaranteed to be fully ordered and uncombined,
1688 provided they're not accessing a prefetchable device.
1690 However, intermediary hardware (such as a PCI bridge) may indulge in
1691 deferral if it so wishes; to flush a store, a load from the same location
1692 is preferred[*], but a load from the same device or from configuration
1693 space should suffice for PCI.
1695 [*] NOTE! attempting to load from the same location as was written to may
1696 cause a malfunction - consider the 16550 Rx/Tx serial registers for
1699 Used with prefetchable I/O memory, an mmiowb() barrier may be required to
1700 force stores to be ordered.
1702 Please refer to the PCI specification for more information on interactions
1703 between PCI transactions.
1707 These are similar to readX(), but are not guaranteed to be ordered in any
1708 way. Be aware that there is no I/O read barrier available.
1710 (*) ioreadX(), iowriteX()
1712 These will perform appropriately for the type of access they're actually
1713 doing, be it inX()/outX() or readX()/writeX().
1716 ========================================
1717 ASSUMED MINIMUM EXECUTION ORDERING MODEL
1718 ========================================
1720 It has to be assumed that the conceptual CPU is weakly-ordered but that it will
1721 maintain the appearance of program causality with respect to itself. Some CPUs
1722 (such as i386 or x86_64) are more constrained than others (such as powerpc or
1723 frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
1724 of arch-specific code.
1726 This means that it must be considered that the CPU will execute its instruction
1727 stream in any order it feels like - or even in parallel - provided that if an
1728 instruction in the stream depends on an earlier instruction, then that
1729 earlier instruction must be sufficiently complete[*] before the later
1730 instruction may proceed; in other words: provided that the appearance of
1731 causality is maintained.
1733 [*] Some instructions have more than one effect - such as changing the
1734 condition codes, changing registers or changing memory - and different
1735 instructions may depend on different effects.
1737 A CPU may also discard any instruction sequence that winds up having no
1738 ultimate effect. For example, if two adjacent instructions both load an
1739 immediate value into the same register, the first may be discarded.
1742 Similarly, it has to be assumed that compiler might reorder the instruction
1743 stream in any way it sees fit, again provided the appearance of causality is
1747 ============================
1748 THE EFFECTS OF THE CPU CACHE
1749 ============================
1751 The way cached memory operations are perceived across the system is affected to
1752 a certain extent by the caches that lie between CPUs and memory, and by the
1753 memory coherence system that maintains the consistency of state in the system.
1755 As far as the way a CPU interacts with another part of the system through the
1756 caches goes, the memory system has to include the CPU's caches, and memory
1757 barriers for the most part act at the interface between the CPU and its cache
1758 (memory barriers logically act on the dotted line in the following diagram):
1760 <--- CPU ---> : <----------- Memory ----------->
1762 +--------+ +--------+ : +--------+ +-----------+
1763 | | | | : | | | | +--------+
1764 | CPU | | Memory | : | CPU | | | | |
1765 | Core |--->| Access |----->| Cache |<-->| | | |
1766 | | | Queue | : | | | |--->| Memory |
1767 | | | | : | | | | | |
1768 +--------+ +--------+ : +--------+ | | | |
1769 : | Cache | +--------+
1771 : | Mechanism | +--------+
1772 +--------+ +--------+ : +--------+ | | | |
1773 | | | | : | | | | | |
1774 | CPU | | Memory | : | CPU | | |--->| Device |
1775 | Core |--->| Access |----->| Cache |<-->| | | |
1776 | | | Queue | : | | | | | |
1777 | | | | : | | | | +--------+
1778 +--------+ +--------+ : +--------+ +-----------+
1782 Although any particular load or store may not actually appear outside of the
1783 CPU that issued it since it may have been satisfied within the CPU's own cache,
1784 it will still appear as if the full memory access had taken place as far as the
1785 other CPUs are concerned since the cache coherency mechanisms will migrate the
1786 cacheline over to the accessing CPU and propagate the effects upon conflict.
1788 The CPU core may execute instructions in any order it deems fit, provided the
1789 expected program causality appears to be maintained. Some of the instructions
1790 generate load and store operations which then go into the queue of memory
1791 accesses to be performed. The core may place these in the queue in any order
1792 it wishes, and continue execution until it is forced to wait for an instruction
1795 What memory barriers are concerned with is controlling the order in which
1796 accesses cross from the CPU side of things to the memory side of things, and
1797 the order in which the effects are perceived to happen by the other observers
1800 [!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
1801 their own loads and stores as if they had happened in program order.
1803 [!] MMIO or other device accesses may bypass the cache system. This depends on
1804 the properties of the memory window through which devices are accessed and/or
1805 the use of any special device communication instructions the CPU may have.
1811 Life isn't quite as simple as it may appear above, however: for while the
1812 caches are expected to be coherent, there's no guarantee that that coherency
1813 will be ordered. This means that whilst changes made on one CPU will
1814 eventually become visible on all CPUs, there's no guarantee that they will
1815 become apparent in the same order on those other CPUs.
1818 Consider dealing with a system that has a pair of CPUs (1 & 2), each of which
1819 has a pair of parallel data caches (CPU 1 has A/B, and CPU 2 has C/D):
1824 +--------+ : +--->| Cache A |<------->| |
1825 | | : | +---------+ | |
1827 | | : | +---------+ | |
1828 +--------+ : +--->| Cache B |<------->| |
1831 : +---------+ | System |
1832 +--------+ : +--->| Cache C |<------->| |
1833 | | : | +---------+ | |
1835 | | : | +---------+ | |
1836 +--------+ : +--->| Cache D |<------->| |
1841 Imagine the system has the following properties:
1843 (*) an odd-numbered cache line may be in cache A, cache C or it may still be
1846 (*) an even-numbered cache line may be in cache B, cache D or it may still be
1849 (*) whilst the CPU core is interrogating one cache, the other cache may be
1850 making use of the bus to access the rest of the system - perhaps to
1851 displace a dirty cacheline or to do a speculative load;
1853 (*) each cache has a queue of operations that need to be applied to that cache
1854 to maintain coherency with the rest of the system;
1856 (*) the coherency queue is not flushed by normal loads to lines already
1857 present in the cache, even though the contents of the queue may
1858 potentially affect those loads.
1860 Imagine, then, that two writes are made on the first CPU, with a write barrier
1861 between them to guarantee that they will appear to reach that CPU's caches in
1862 the requisite order:
1865 =============== =============== =======================================
1866 u == 0, v == 1 and p == &u, q == &u
1868 smp_wmb(); Make sure change to v is visible before
1870 <A:modify v=2> v is now in cache A exclusively
1872 <B:modify p=&v> p is now in cache B exclusively
1874 The write memory barrier forces the other CPUs in the system to perceive that
1875 the local CPU's caches have apparently been updated in the correct order. But
1876 now imagine that the second CPU wants to read those values:
1879 =============== =============== =======================================
1884 The above pair of reads may then fail to happen in the expected order, as the
1885 cacheline holding p may get updated in one of the second CPU's caches whilst
1886 the update to the cacheline holding v is delayed in the other of the second
1887 CPU's caches by some other cache event:
1890 =============== =============== =======================================
1891 u == 0, v == 1 and p == &u, q == &u
1894 <A:modify v=2> <C:busy>
1898 <B:modify p=&v> <D:commit p=&v>
1901 <C:read *q> Reads from v before v updated in cache
1905 Basically, whilst both cachelines will be updated on CPU 2 eventually, there's
1906 no guarantee that, without intervention, the order of update will be the same
1907 as that committed on CPU 1.
1910 To intervene, we need to interpolate a data dependency barrier or a read
1911 barrier between the loads. This will force the cache to commit its coherency
1912 queue before processing any further requests:
1915 =============== =============== =======================================
1916 u == 0, v == 1 and p == &u, q == &u
1919 <A:modify v=2> <C:busy>
1923 <B:modify p=&v> <D:commit p=&v>
1925 smp_read_barrier_depends()
1929 <C:read *q> Reads from v after v updated in cache
1932 This sort of problem can be encountered on DEC Alpha processors as they have a
1933 split cache that improves performance by making better use of the data bus.
1934 Whilst most CPUs do imply a data dependency barrier on the read when a memory
1935 access depends on a read, not all do, so it may not be relied on.
1937 Other CPUs may also have split caches, but must coordinate between the various
1938 cachelets for normal memory accesses. The semantics of the Alpha removes the
1939 need for coordination in the absence of memory barriers.
1942 CACHE COHERENCY VS DMA
1943 ----------------------
1945 Not all systems maintain cache coherency with respect to devices doing DMA. In
1946 such cases, a device attempting DMA may obtain stale data from RAM because
1947 dirty cache lines may be resident in the caches of various CPUs, and may not
1948 have been written back to RAM yet. To deal with this, the appropriate part of
1949 the kernel must flush the overlapping bits of cache on each CPU (and maybe
1950 invalidate them as well).
1952 In addition, the data DMA'd to RAM by a device may be overwritten by dirty
1953 cache lines being written back to RAM from a CPU's cache after the device has
1954 installed its own data, or cache lines present in the CPU's cache may simply
1955 obscure the fact that RAM has been updated, until at such time as the cacheline
1956 is discarded from the CPU's cache and reloaded. To deal with this, the
1957 appropriate part of the kernel must invalidate the overlapping bits of the
1960 See Documentation/cachetlb.txt for more information on cache management.
1963 CACHE COHERENCY VS MMIO
1964 -----------------------
1966 Memory mapped I/O usually takes place through memory locations that are part of
1967 a window in the CPU's memory space that has different properties assigned than
1968 the usual RAM directed window.
1970 Amongst these properties is usually the fact that such accesses bypass the
1971 caching entirely and go directly to the device buses. This means MMIO accesses
1972 may, in effect, overtake accesses to cached memory that were emitted earlier.
1973 A memory barrier isn't sufficient in such a case, but rather the cache must be
1974 flushed between the cached memory write and the MMIO access if the two are in
1978 =========================
1979 THE THINGS CPUS GET UP TO
1980 =========================
1982 A programmer might take it for granted that the CPU will perform memory
1983 operations in exactly the order specified, so that if the CPU is, for example,
1984 given the following piece of code to execute:
1992 they would then expect that the CPU will complete the memory operation for each
1993 instruction before moving on to the next one, leading to a definite sequence of
1994 operations as seen by external observers in the system:
1996 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
1999 Reality is, of course, much messier. With many CPUs and compilers, the above
2000 assumption doesn't hold because:
2002 (*) loads are more likely to need to be completed immediately to permit
2003 execution progress, whereas stores can often be deferred without a
2006 (*) loads may be done speculatively, and the result discarded should it prove
2007 to have been unnecessary;
2009 (*) loads may be done speculatively, leading to the result having been fetched
2010 at the wrong time in the expected sequence of events;
2012 (*) the order of the memory accesses may be rearranged to promote better use
2013 of the CPU buses and caches;
2015 (*) loads and stores may be combined to improve performance when talking to
2016 memory or I/O hardware that can do batched accesses of adjacent locations,
2017 thus cutting down on transaction setup costs (memory and PCI devices may
2018 both be able to do this); and
2020 (*) the CPU's data cache may affect the ordering, and whilst cache-coherency
2021 mechanisms may alleviate this - once the store has actually hit the cache
2022 - there's no guarantee that the coherency management will be propagated in
2023 order to other CPUs.
2025 So what another CPU, say, might actually observe from the above piece of code
2028 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2030 (Where "LOAD {*C,*D}" is a combined load)
2033 However, it is guaranteed that a CPU will be self-consistent: it will see its
2034 _own_ accesses appear to be correctly ordered, without the need for a memory
2035 barrier. For instance with the following code:
2044 and assuming no intervention by an external influence, it can be assumed that
2045 the final result will appear to be:
2047 U == the original value of *A
2052 The code above may cause the CPU to generate the full sequence of memory
2055 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2057 in that order, but, without intervention, the sequence may have almost any
2058 combination of elements combined or discarded, provided the program's view of
2059 the world remains consistent.
2061 The compiler may also combine, discard or defer elements of the sequence before
2062 the CPU even sees them.
2073 since, without a write barrier, it can be assumed that the effect of the
2074 storage of V to *A is lost. Similarly:
2079 may, without a memory barrier, be reduced to:
2084 and the LOAD operation never appear outside of the CPU.
2087 AND THEN THERE'S THE ALPHA
2088 --------------------------
2090 The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
2091 some versions of the Alpha CPU have a split data cache, permitting them to have
2092 two semantically-related cache lines updated at separate times. This is where
2093 the data dependency barrier really becomes necessary as this synchronises both
2094 caches with the memory coherence system, thus making it seem like pointer
2095 changes vs new data occur in the right order.
2097 The Alpha defines the Linux kernel's memory barrier model.
2099 See the subsection on "Cache Coherency" above.
2106 Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2108 Chapter 5.2: Physical Address Space Characteristics
2109 Chapter 5.4: Caches and Write Buffers
2110 Chapter 5.5: Data Sharing
2111 Chapter 5.6: Read/Write Ordering
2113 AMD64 Architecture Programmer's Manual Volume 2: System Programming
2114 Chapter 7.1: Memory-Access Ordering
2115 Chapter 7.4: Buffering and Combining Memory Writes
2117 IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2118 System Programming Guide
2119 Chapter 7.1: Locked Atomic Operations
2120 Chapter 7.2: Memory Ordering
2121 Chapter 7.4: Serializing Instructions
2123 The SPARC Architecture Manual, Version 9
2124 Chapter 8: Memory Models
2125 Appendix D: Formal Specification of the Memory Models
2126 Appendix J: Programming with the Memory Models
2128 UltraSPARC Programmer Reference Manual
2129 Chapter 5: Memory Accesses and Cacheability
2130 Chapter 15: Sparc-V9 Memory Models
2132 UltraSPARC III Cu User's Manual
2133 Chapter 9: Memory Models
2135 UltraSPARC IIIi Processor User's Manual
2136 Chapter 8: Memory Models
2138 UltraSPARC Architecture 2005
2140 Appendix D: Formal Specifications of the Memory Models
2142 UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
2143 Chapter 8: Memory Models
2144 Appendix F: Caches and Cache Coherency
2146 Solaris Internals, Core Kernel Architecture, p63-68:
2147 Chapter 3.3: Hardware Considerations for Locks and
2150 Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
2151 for Kernel Programmers:
2152 Chapter 13: Other Memory Models
2154 Intel Itanium Architecture Software Developer's Manual: Volume 1:
2155 Section 2.6: Speculation
2156 Section 4.4: Memory Access
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