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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-11 08:27:49 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-11 08:27:49 +0000 |
| commit | ace9429bb58fd418f0c81d4c2835699bddf6bde6 (patch) | |
| tree | b2d64bc10158fdd5497876388cd68142ca374ed3 /Documentation/memory-barriers.txt | |
| parent | Initial commit. (diff) | |
| download | linux-upstream/6.6.15.tar.xz linux-upstream/6.6.15.zip | |
Adding upstream version 6.6.15.upstream/6.6.15
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 3007 |
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diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt new file mode 100644 index 0000000000..06e14efd86 --- /dev/null +++ b/Documentation/memory-barriers.txt @@ -0,0 +1,3007 @@ + ============================ + LINUX KERNEL MEMORY BARRIERS + ============================ + +By: David Howells <dhowells@redhat.com> + Paul E. McKenney <paulmck@linux.ibm.com> + Will Deacon <will.deacon@arm.com> + Peter Zijlstra <peterz@infradead.org> + +========== +DISCLAIMER +========== + +This document is not a specification; it is intentionally (for the sake of +brevity) and unintentionally (due to being human) incomplete. This document is +meant as a guide to using the various memory barriers provided by Linux, but +in case of any doubt (and there are many) please ask. Some doubts may be +resolved by referring to the formal memory consistency model and related +documentation at tools/memory-model/. Nevertheless, even this memory +model should be viewed as the collective opinion of its maintainers rather +than as an infallible oracle. + +To repeat, this document is not a specification of what Linux expects from +hardware. + +The purpose of this document is twofold: + + (1) to specify the minimum functionality that one can rely on for any + particular barrier, and + + (2) to provide a guide as to how to use the barriers that are available. + +Note that an architecture can provide more than the minimum requirement +for any particular barrier, but if the architecture provides less than +that, that architecture is incorrect. + +Note also that it is possible that a barrier may be a no-op for an +architecture because the way that arch works renders an explicit barrier +unnecessary in that case. + + +======== +CONTENTS +======== + + (*) Abstract memory access model. + + - Device operations. + - Guarantees. + + (*) What are memory barriers? + + - Varieties of memory barrier. + - What may not be assumed about memory barriers? + - Address-dependency barriers (historical). + - Control dependencies. + - SMP barrier pairing. + - Examples of memory barrier sequences. + - Read memory barriers vs load speculation. + - Multicopy atomicity. + + (*) Explicit kernel barriers. + + - Compiler barrier. + - CPU memory barriers. + + (*) Implicit kernel memory barriers. + + - Lock acquisition functions. + - Interrupt disabling functions. + - Sleep and wake-up functions. + - Miscellaneous functions. + + (*) Inter-CPU acquiring barrier effects. + + - Acquires vs memory accesses. + + (*) Where are memory barriers needed? + + - Interprocessor interaction. + - Atomic operations. + - Accessing devices. + - Interrupts. + + (*) Kernel I/O barrier effects. + + (*) Assumed minimum execution ordering model. + + (*) The effects of the cpu cache. + + - Cache coherency. + - Cache coherency vs DMA. + - Cache coherency vs MMIO. + + (*) The things CPUs get up to. + + - And then there's the Alpha. + - Virtual Machine Guests. + + (*) Example uses. + + - Circular buffers. + + (*) References. + + +============================ +ABSTRACT MEMORY ACCESS MODEL +============================ + +Consider the following abstract model of the system: + + : : + : : + : : + +-------+ : +--------+ : +-------+ + | | : | | : | | + | | : | | : | | + | CPU 1 |<----->| Memory |<----->| CPU 2 | + | | : | | : | | + | | : | | : | | + +-------+ : +--------+ : +-------+ + ^ : ^ : ^ + | : | : | + | : | : | + | : v : | + | : +--------+ : | + | : | | : | + | : | | : | + +---------->| Device |<----------+ + : | | : + : | | : + : +--------+ : + : : + +Each CPU executes a program that generates memory access operations. In the +abstract CPU, memory operation ordering is very relaxed, and a CPU may actually +perform the memory operations in any order it likes, provided program causality +appears to be maintained. Similarly, the compiler may also arrange the +instructions it emits in any order it likes, provided it doesn't affect the +apparent operation of the program. + +So in the above diagram, the effects of the memory operations performed by a +CPU are perceived by the rest of the system as the operations cross the +interface between the CPU and rest of the system (the dotted lines). + + +For example, consider the following sequence of events: + + CPU 1 CPU 2 + =============== =============== + { A == 1; B == 2 } + A = 3; x = B; + B = 4; y = A; + +The set of accesses as seen by the memory system in the middle can be arranged +in 24 different combinations: + + STORE A=3, STORE B=4, y=LOAD A->3, x=LOAD B->4 + STORE A=3, STORE B=4, x=LOAD B->4, y=LOAD A->3 + STORE A=3, y=LOAD A->3, STORE B=4, x=LOAD B->4 + STORE A=3, y=LOAD A->3, x=LOAD B->2, STORE B=4 + STORE A=3, x=LOAD B->2, STORE B=4, y=LOAD A->3 + STORE A=3, x=LOAD B->2, y=LOAD A->3, STORE B=4 + STORE B=4, STORE A=3, y=LOAD A->3, x=LOAD B->4 + STORE B=4, ... + ... + +and can thus result in four different combinations of values: + + x == 2, y == 1 + x == 2, y == 3 + x == 4, y == 1 + x == 4, y == 3 + + +Furthermore, the stores committed by a CPU to the memory system may not be +perceived by the loads made by another CPU in the same order as the stores were +committed. + + +As a further example, consider this sequence of events: + + CPU 1 CPU 2 + =============== =============== + { A == 1, B == 2, C == 3, P == &A, Q == &C } + B = 4; Q = P; + P = &B; D = *Q; + +There is an obvious address dependency here, as the value loaded into D depends +on the address retrieved from P by CPU 2. At the end of the sequence, any of +the following results are possible: + + (Q == &A) and (D == 1) + (Q == &B) and (D == 2) + (Q == &B) and (D == 4) + +Note that CPU 2 will never try and load C into D because the CPU will load P +into Q before issuing the load of *Q. + + +DEVICE OPERATIONS +----------------- + +Some devices present their control interfaces as collections of memory +locations, but the order in which the control registers are accessed is very +important. For instance, imagine an ethernet card with a set of internal +registers that are accessed through an address port register (A) and a data +port register (D). To read internal register 5, the following code might then +be used: + + *A = 5; + x = *D; + +but this might show up as either of the following two sequences: + + STORE *A = 5, x = LOAD *D + x = LOAD *D, STORE *A = 5 + +the second of which will almost certainly result in a malfunction, since it set +the address _after_ attempting to read the register. + + +GUARANTEES +---------- + +There are some minimal guarantees that may be expected of a CPU: + + (*) On any given CPU, dependent memory accesses will be issued in order, with + respect to itself. This means that for: + + Q = READ_ONCE(P); D = READ_ONCE(*Q); + + the CPU will issue the following memory operations: + + Q = LOAD P, D = LOAD *Q + + and always in that order. However, on DEC Alpha, READ_ONCE() also + emits a memory-barrier instruction, so that a DEC Alpha CPU will + instead issue the following memory operations: + + Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER + + Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler + mischief. + + (*) Overlapping loads and stores within a particular CPU will appear to be + ordered within that CPU. This means that for: + + a = READ_ONCE(*X); WRITE_ONCE(*X, b); + + the CPU will only issue the following sequence of memory operations: + + a = LOAD *X, STORE *X = b + + And for: + + WRITE_ONCE(*X, c); d = READ_ONCE(*X); + + the CPU will only issue: + + STORE *X = c, d = LOAD *X + + (Loads and stores overlap if they are targeted at overlapping pieces of + memory). + +And there are a number of things that _must_ or _must_not_ be assumed: + + (*) It _must_not_ be assumed that the compiler will do what you want + with memory references that are not protected by READ_ONCE() and + WRITE_ONCE(). Without them, the compiler is within its rights to + do all sorts of "creative" transformations, which are covered in + the COMPILER BARRIER section. + + (*) It _must_not_ be assumed that independent loads and stores will be issued + in the order given. This means that for: + + X = *A; Y = *B; *D = Z; + + we may get any of the following sequences: + + X = LOAD *A, Y = LOAD *B, STORE *D = Z + X = LOAD *A, STORE *D = Z, Y = LOAD *B + Y = LOAD *B, X = LOAD *A, STORE *D = Z + Y = LOAD *B, STORE *D = Z, X = LOAD *A + STORE *D = Z, X = LOAD *A, Y = LOAD *B + STORE *D = Z, Y = LOAD *B, X = LOAD *A + + (*) It _must_ be assumed that overlapping memory accesses may be merged or + discarded. This means that for: + + X = *A; Y = *(A + 4); + + we may get any one of the following sequences: + + X = LOAD *A; Y = LOAD *(A + 4); + Y = LOAD *(A + 4); X = LOAD *A; + {X, Y} = LOAD {*A, *(A + 4) }; + + And for: + + *A = X; *(A + 4) = Y; + + we may get any of: + + STORE *A = X; STORE *(A + 4) = Y; + STORE *(A + 4) = Y; STORE *A = X; + STORE {*A, *(A + 4) } = {X, Y}; + +And there are anti-guarantees: + + (*) These guarantees do not apply to bitfields, because compilers often + generate code to modify these using non-atomic read-modify-write + sequences. Do not attempt to use bitfields to synchronize parallel + algorithms. + + (*) Even in cases where bitfields are protected by locks, all fields + in a given bitfield must be protected by one lock. If two fields + in a given bitfield are protected by different locks, the compiler's + non-atomic read-modify-write sequences can cause an update to one + field to corrupt the value of an adjacent field. + + (*) These guarantees apply only to properly aligned and sized scalar + variables. "Properly sized" currently means variables that are + the same size as "char", "short", "int" and "long". "Properly + aligned" means the natural alignment, thus no constraints for + "char", two-byte alignment for "short", four-byte alignment for + "int", and either four-byte or eight-byte alignment for "long", + on 32-bit and 64-bit systems, respectively. Note that these + guarantees were introduced into the C11 standard, so beware when + using older pre-C11 compilers (for example, gcc 4.6). The portion + of the standard containing this guarantee is Section 3.14, which + defines "memory location" as follows: + + memory location + either an object of scalar type, or a maximal sequence + of adjacent bit-fields all having nonzero width + + NOTE 1: Two threads of execution can update and access + separate memory locations without interfering with + each other. + + NOTE 2: A bit-field and an adjacent non-bit-field member + are in separate memory locations. The same applies + to two bit-fields, if one is declared inside a nested + structure declaration and the other is not, or if the two + are separated by a zero-length bit-field declaration, + or if they are separated by a non-bit-field member + declaration. It is not safe to concurrently update two + bit-fields in the same structure if all members declared + between them are also bit-fields, no matter what the + sizes of those intervening bit-fields happen to be. + + +========================= +WHAT ARE MEMORY BARRIERS? +========================= + +As can be seen above, independent memory operations are effectively performed +in random order, but this can be a problem for CPU-CPU interaction and for I/O. +What is required is some way of intervening to instruct the compiler and the +CPU to restrict the order. + +Memory barriers are such interventions. They impose a perceived partial +ordering over the memory operations on either side of the barrier. + +Such enforcement is important because the CPUs and other devices in a system +can use a variety of tricks to improve performance, including reordering, +deferral and combination of memory operations; speculative loads; speculative +branch prediction and various types of caching. Memory barriers are used to +override or suppress these tricks, allowing the code to sanely control the +interaction of multiple CPUs and/or devices. + + +VARIETIES OF MEMORY BARRIER +--------------------------- + +Memory barriers come in four basic varieties: + + (1) Write (or store) memory barriers. + + A write memory barrier gives a guarantee that all the STORE operations + specified before the barrier will appear to happen before all the STORE + operations specified after the barrier with respect to the other + components of the system. + + A write barrier is a partial ordering on stores only; it is not required + to have any effect on loads. + + A CPU can be viewed as committing a sequence of store operations to the + memory system as time progresses. All stores _before_ a write barrier + will occur _before_ all the stores after the write barrier. + + [!] Note that write barriers should normally be paired with read or + address-dependency barriers; see the "SMP barrier pairing" subsection. + + + (2) Address-dependency barriers (historical). + + An address-dependency barrier is a weaker form of read barrier. In the + case where two loads are performed such that the second depends on the + result of the first (eg: the first load retrieves the address to which + the second load will be directed), an address-dependency barrier would + be required to make sure that the target of the second load is updated + after the address obtained by the first load is accessed. + + An address-dependency barrier is a partial ordering on interdependent + loads only; it is not required to have any effect on stores, independent + loads or overlapping loads. + + As mentioned in (1), the other CPUs in the system can be viewed as + committing sequences of stores to the memory system that the CPU being + considered can then perceive. An address-dependency barrier issued by + the CPU under consideration guarantees that for any load preceding it, + if that load touches one of a sequence of stores from another CPU, then + by the time the barrier completes, the effects of all the stores prior to + that touched by the load will be perceptible to any loads issued after + the address-dependency barrier. + + See the "Examples of memory barrier sequences" subsection for diagrams + showing the ordering constraints. + + [!] Note that the first load really has to have an _address_ dependency and + not a control dependency. If the address for the second load is dependent + on the first load, but the dependency is through a conditional rather than + actually loading the address itself, then it's a _control_ dependency and + a full read barrier or better is required. See the "Control dependencies" + subsection for more information. + + [!] Note that address-dependency barriers should normally be paired with + write barriers; see the "SMP barrier pairing" subsection. + + [!] Kernel release v5.9 removed kernel APIs for explicit address- + dependency barriers. Nowadays, APIs for marking loads from shared + variables such as READ_ONCE() and rcu_dereference() provide implicit + address-dependency barriers. + + (3) Read (or load) memory barriers. + + A read barrier is an address-dependency barrier plus a guarantee that all + the LOAD operations specified before the barrier will appear to happen + before all the LOAD operations specified after the barrier with respect to + the other components of the system. + + A read barrier is a partial ordering on loads only; it is not required to + have any effect on stores. + + Read memory barriers imply address-dependency barriers, and so can + substitute for them. + + [!] Note that read barriers should normally be paired with write barriers; + see the "SMP barrier pairing" subsection. + + + (4) General memory barriers. + + A general memory barrier gives a guarantee that all the LOAD and STORE + operations specified before the barrier will appear to happen before all + the LOAD and STORE operations specified after the barrier with respect to + the other components of the system. + + A general memory barrier is a partial ordering over both loads and stores. + + General memory barriers imply both read and write memory barriers, and so + can substitute for either. + + +And a couple of implicit varieties: + + (5) ACQUIRE operations. + + This acts as a one-way permeable barrier. It guarantees that all memory + operations after the ACQUIRE operation will appear to happen after the + ACQUIRE operation with respect to the other components of the system. + ACQUIRE operations include LOCK operations and both smp_load_acquire() + and smp_cond_load_acquire() operations. + + Memory operations that occur before an ACQUIRE operation may appear to + happen after it completes. + + An ACQUIRE operation should almost always be paired with a RELEASE + operation. + + + (6) RELEASE operations. + + This also acts as a one-way permeable barrier. It guarantees that all + memory operations before the RELEASE operation will appear to happen + before the RELEASE operation with respect to the other components of the + system. RELEASE operations include UNLOCK operations and + smp_store_release() operations. + + Memory operations that occur after a RELEASE operation may appear to + happen before it completes. + + The use of ACQUIRE and RELEASE operations generally precludes the need + for other sorts of memory barrier. In addition, a RELEASE+ACQUIRE pair is + -not- guaranteed to act as a full memory barrier. However, after an + ACQUIRE on a given variable, all memory accesses preceding any prior + RELEASE on that same variable are guaranteed to be visible. In other + words, within a given variable's critical section, all accesses of all + previous critical sections for that variable are guaranteed to have + completed. + + This means that ACQUIRE acts as a minimal "acquire" operation and + RELEASE acts as a minimal "release" operation. + +A subset of the atomic operations described in atomic_t.txt have ACQUIRE and +RELEASE variants in addition to fully-ordered and relaxed (no barrier +semantics) definitions. For compound atomics performing both a load and a +store, ACQUIRE semantics apply only to the load and RELEASE semantics apply +only to the store portion of the operation. + +Memory barriers are only required where there's a possibility of interaction +between two CPUs or between a CPU and a device. If it can be guaranteed that +there won't be any such interaction in any particular piece of code, then +memory barriers are unnecessary in that piece of code. + + +Note that these are the _minimum_ guarantees. Different architectures may give +more substantial guarantees, but they may _not_ be relied upon outside of arch +specific code. + + +WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS? +---------------------------------------------- + +There are certain things that the Linux kernel memory barriers do not guarantee: + + (*) There is no guarantee that any of the memory accesses specified before a + memory barrier will be _complete_ by the completion of a memory barrier + instruction; the barrier can be considered to draw a line in that CPU's + access queue that accesses of the appropriate type may not cross. + + (*) There is no guarantee that issuing a memory barrier on one CPU will have + any direct effect on another CPU or any other hardware in the system. The + indirect effect will be the order in which the second CPU sees the effects + of the first CPU's accesses occur, but see the next point: + + (*) There is no guarantee that a CPU will see the correct order of effects + from a second CPU's accesses, even _if_ the second CPU uses a memory + barrier, unless the first CPU _also_ uses a matching memory barrier (see + the subsection on "SMP Barrier Pairing"). + + (*) There is no guarantee that some intervening piece of off-the-CPU + hardware[*] will not reorder the memory accesses. CPU cache coherency + mechanisms should propagate the indirect effects of a memory barrier + between CPUs, but might not do so in order. + + [*] For information on bus mastering DMA and coherency please read: + + Documentation/driver-api/pci/pci.rst + Documentation/core-api/dma-api-howto.rst + Documentation/core-api/dma-api.rst + + +ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) +---------------------------------------- + +As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for +DEC Alpha, which means that about the only people who need to pay attention +to this section are those working on DEC Alpha architecture-specific code +and those working on READ_ONCE() itself. For those who need it, and for +those who are interested in the history, here is the story of +address-dependency barriers. + +[!] While address dependencies are observed in both load-to-load and +load-to-store relations, address-dependency barriers are not necessary +for load-to-store situations. + +The requirement of address-dependency barriers is a little subtle, and +it's not always obvious that they're needed. To illustrate, consider the +following sequence of events: + + CPU 1 CPU 2 + =============== =============== + { A == 1, B == 2, C == 3, P == &A, Q == &C } + B = 4; + <write barrier> + WRITE_ONCE(P, &B); + Q = READ_ONCE_OLD(P); + D = *Q; + +[!] READ_ONCE_OLD() corresponds to READ_ONCE() of pre-4.15 kernel, which +doesn't imply an address-dependency barrier. + +There's a clear address dependency here, and it would seem that by the end of +the sequence, Q must be either &A or &B, and that: + + (Q == &A) implies (D == 1) + (Q == &B) implies (D == 4) + +But! CPU 2's perception of P may be updated _before_ its perception of B, thus +leading to the following situation: + + (Q == &B) and (D == 2) ???? + +While this may seem like a failure of coherency or causality maintenance, it +isn't, and this behaviour can be observed on certain real CPUs (such as the DEC +Alpha). + +To deal with this, READ_ONCE() provides an implicit address-dependency barrier +since kernel release v4.15: + + CPU 1 CPU 2 + =============== =============== + { A == 1, B == 2, C == 3, P == &A, Q == &C } + B = 4; + <write barrier> + WRITE_ONCE(P, &B); + Q = READ_ONCE(P); + <implicit address-dependency barrier> + D = *Q; + +This enforces the occurrence of one of the two implications, and prevents the +third possibility from arising. + + +[!] Note that this extremely counterintuitive situation arises most easily on +machines with split caches, so that, for example, one cache bank processes +even-numbered cache lines and the other bank processes odd-numbered cache +lines. The pointer P might be stored in an odd-numbered cache line, and the +variable B might be stored in an even-numbered cache line. Then, if the +even-numbered bank of the reading CPU's cache is extremely busy while the +odd-numbered bank is idle, one can see the new value of the pointer P (&B), +but the old value of the variable B (2). + + +An address-dependency barrier is not required to order dependent writes +because the CPUs that the Linux kernel supports don't do writes until they +are certain (1) that the write will actually happen, (2) of the location of +the write, and (3) of the value to be written. +But please carefully read the "CONTROL DEPENDENCIES" section and the +Documentation/RCU/rcu_dereference.rst file: The compiler can and does break +dependencies in a great many highly creative ways. + + CPU 1 CPU 2 + =============== =============== + { A == 1, B == 2, C = 3, P == &A, Q == &C } + B = 4; + <write barrier> + WRITE_ONCE(P, &B); + Q = READ_ONCE_OLD(P); + WRITE_ONCE(*Q, 5); + +Therefore, no address-dependency barrier is required to order the read into +Q with the store into *Q. In other words, this outcome is prohibited, +even without an implicit address-dependency barrier of modern READ_ONCE(): + + (Q == &B) && (B == 4) + +Please note that this pattern should be rare. After all, the whole point +of dependency ordering is to -prevent- writes to the data structure, along +with the expensive cache misses associated with those writes. This pattern +can be used to record rare error conditions and the like, and the CPUs' +naturally occurring ordering prevents such records from being lost. + + +Note well that the ordering provided by an address dependency is local to +the CPU containing it. See the section on "Multicopy atomicity" for +more information. + + +The address-dependency barrier is very important to the RCU system, +for example. See rcu_assign_pointer() and rcu_dereference() in +include/linux/rcupdate.h. This permits the current target of an RCU'd +pointer to be replaced with a new modified target, without the replacement +target appearing to be incompletely initialised. + +See also the subsection on "Cache Coherency" for a more thorough example. + + +CONTROL DEPENDENCIES +-------------------- + +Control dependencies can be a bit tricky because current compilers do +not understand them. The purpose of this section is to help you prevent +the compiler's ignorance from breaking your code. + +A load-load control dependency requires a full read memory barrier, not +simply an (implicit) address-dependency barrier to make it work correctly. +Consider the following bit of code: + + q = READ_ONCE(a); + <implicit address-dependency barrier> + if (q) { + /* BUG: No address dependency!!! */ + p = READ_ONCE(b); + } + +This will not have the desired effect because there is no actual address +dependency, but rather a control dependency that the CPU may short-circuit +by attempting to predict the outcome in advance, so that other CPUs see +the load from b as having happened before the load from a. In such a case +what's actually required is: + + q = READ_ONCE(a); + if (q) { + <read barrier> + p = READ_ONCE(b); + } + +However, stores are not speculated. This means that ordering -is- provided +for load-store control dependencies, as in the following example: + + q = READ_ONCE(a); + if (q) { + WRITE_ONCE(b, 1); + } + +Control dependencies pair normally with other types of barriers. +That said, please note that neither READ_ONCE() nor WRITE_ONCE() +are optional! Without the READ_ONCE(), the compiler might combine the +load from 'a' with other loads from 'a'. Without the WRITE_ONCE(), +the compiler might combine the store to 'b' with other stores to 'b'. +Either can result in highly counterintuitive effects on ordering. + +Worse yet, if the compiler is able to prove (say) that the value of +variable 'a' is always non-zero, it would be well within its rights +to optimize the original example by eliminating the "if" statement +as follows: + + q = a; + b = 1; /* BUG: Compiler and CPU can both reorder!!! */ + +So don't leave out the READ_ONCE(). + +It is tempting to try to enforce ordering on identical stores on both +branches of the "if" statement as follows: + + q = READ_ONCE(a); + if (q) { + barrier(); + WRITE_ONCE(b, 1); + do_something(); + } else { + barrier(); + WRITE_ONCE(b, 1); + do_something_else(); + } + +Unfortunately, current compilers will transform this as follows at high +optimization levels: + + q = READ_ONCE(a); + barrier(); + WRITE_ONCE(b, 1); /* BUG: No ordering vs. load from a!!! */ + if (q) { + /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */ + do_something(); + } else { + /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */ + do_something_else(); + } + +Now there is no conditional between the load from 'a' and the store to +'b', which means that the CPU is within its rights to reorder them: +The conditional is absolutely required, and must be present in the +assembly code even after all compiler optimizations have been applied. +Therefore, if you need ordering in this example, you need explicit +memory barriers, for example, smp_store_release(): + + q = READ_ONCE(a); + if (q) { + smp_store_release(&b, 1); + do_something(); + } else { + smp_store_release(&b, 1); + do_something_else(); + } + +In contrast, without explicit memory barriers, two-legged-if control +ordering is guaranteed only when the stores differ, for example: + + q = READ_ONCE(a); + if (q) { + WRITE_ONCE(b, 1); + do_something(); + } else { + WRITE_ONCE(b, 2); + do_something_else(); + } + +The initial READ_ONCE() is still required to prevent the compiler from +proving the value of 'a'. + +In addition, you need to be careful what you do with the local variable 'q', +otherwise the compiler might be able to guess the value and again remove +the needed conditional. For example: + + q = READ_ONCE(a); + if (q % MAX) { + WRITE_ONCE(b, 1); + do_something(); + } else { + WRITE_ONCE(b, 2); + do_something_else(); + } + +If MAX is defined to be 1, then the compiler knows that (q % MAX) is +equal to zero, in which case the compiler is within its rights to +transform the above code into the following: + + q = READ_ONCE(a); + WRITE_ONCE(b, 2); + do_something_else(); + +Given this transformation, the CPU is not required to respect the ordering +between the load from variable 'a' and the store to variable 'b'. It is +tempting to add a barrier(), but this does not help. The conditional +is gone, and the barrier won't bring it back. Therefore, if you are +relying on this ordering, you should make sure that MAX is greater than +one, perhaps as follows: + + q = READ_ONCE(a); + BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */ + if (q % MAX) { + WRITE_ONCE(b, 1); + do_something(); + } else { + WRITE_ONCE(b, 2); + do_something_else(); + } + +Please note once again that the stores to 'b' differ. If they were +identical, as noted earlier, the compiler could pull this store outside +of the 'if' statement. + +You must also be careful not to rely too much on boolean short-circuit +evaluation. Consider this example: + + q = READ_ONCE(a); + if (q || 1 > 0) + WRITE_ONCE(b, 1); + +Because the first condition cannot fault and the second condition is +always true, the compiler can transform this example as following, +defeating control dependency: + + q = READ_ONCE(a); + WRITE_ONCE(b, 1); + +This example underscores the need to ensure that the compiler cannot +out-guess your code. More generally, although READ_ONCE() does force +the compiler to actually emit code for a given load, it does not force +the compiler to use the results. + +In addition, control dependencies apply only to the then-clause and +else-clause of the if-statement in question. In particular, it does +not necessarily apply to code following the if-statement: + + q = READ_ONCE(a); + if (q) { + WRITE_ONCE(b, 1); + } else { + WRITE_ONCE(b, 2); + } + WRITE_ONCE(c, 1); /* BUG: No ordering against the read from 'a'. */ + +It is tempting to argue that there in fact is ordering because the +compiler cannot reorder volatile accesses and also cannot reorder +the writes to 'b' with the condition. Unfortunately for this line +of reasoning, the compiler might compile the two writes to 'b' as +conditional-move instructions, as in this fanciful pseudo-assembly +language: + + ld r1,a + cmp r1,$0 + cmov,ne r4,$1 + cmov,eq r4,$2 + st r4,b + st $1,c + +A weakly ordered CPU would have no dependency of any sort between the load +from 'a' and the store to 'c'. The control dependencies would extend +only to the pair of cmov instructions and the store depending on them. +In short, control dependencies apply only to the stores in the then-clause +and else-clause of the if-statement in question (including functions +invoked by those two clauses), not to code following that if-statement. + + +Note well that the ordering provided by a control dependency is local +to the CPU containing it. See the section on "Multicopy atomicity" +for more information. + + +In summary: + + (*) Control dependencies can order prior loads against later stores. + However, they do -not- guarantee any other sort of ordering: + Not prior loads against later loads, nor prior stores against + later anything. If you need these other forms of ordering, + use smp_rmb(), smp_wmb(), or, in the case of prior stores and + later loads, smp_mb(). + + (*) If both legs of the "if" statement begin with identical stores to + the same variable, then those stores must be ordered, either by + preceding both of them with smp_mb() or by using smp_store_release() + to carry out the stores. Please note that it is -not- sufficient + to use barrier() at beginning of each leg of the "if" statement + because, as shown by the example above, optimizing compilers can + destroy the control dependency while respecting the letter of the + barrier() law. + + (*) Control dependencies require at least one run-time conditional + between the prior load and the subsequent store, and this + conditional must involve the prior load. If the compiler is able + to optimize the conditional away, it will have also optimized + away the ordering. Careful use of READ_ONCE() and WRITE_ONCE() + can help to preserve the needed conditional. + + (*) Control dependencies require that the compiler avoid reordering the + dependency into nonexistence. Careful use of READ_ONCE() or + atomic{,64}_read() can help to preserve your control dependency. + Please see the COMPILER BARRIER section for more information. + + (*) Control dependencies apply only to the then-clause and else-clause + of the if-statement containing the control dependency, including + any functions that these two clauses call. Control dependencies + do -not- apply to code following the if-statement containing the + control dependency. + + (*) Control dependencies pair normally with other types of barriers. + + (*) Control dependencies do -not- provide multicopy atomicity. If you + need all the CPUs to see a given store at the same time, use smp_mb(). + + (*) Compilers do not understand control dependencies. It is therefore + your job to ensure that they do not break your code. + + +SMP BARRIER PAIRING +------------------- + +When dealing with CPU-CPU interactions, certain types of memory barrier should +always be paired. A lack of appropriate pairing is almost certainly an error. + +General barriers pair with each other, though they also pair with most +other types of barriers, albeit without multicopy atomicity. An acquire +barrier pairs with a release barrier, but both may also pair with other +barriers, including of course general barriers. A write barrier pairs +with an address-dependency barrier, a control dependency, an acquire barrier, +a release barrier, a read barrier, or a general barrier. Similarly a +read barrier, control dependency, or an address-dependency barrier pairs +with a write barrier, an acquire barrier, a release barrier, or a +general barrier: + + CPU 1 CPU 2 + =============== =============== + WRITE_ONCE(a, 1); + <write barrier> + WRITE_ONCE(b, 2); x = READ_ONCE(b); + <read barrier> + y = READ_ONCE(a); + +Or: + + CPU 1 CPU 2 + =============== =============================== + a = 1; + <write barrier> + WRITE_ONCE(b, &a); x = READ_ONCE(b); + <implicit address-dependency barrier> + y = *x; + +Or even: + + CPU 1 CPU 2 + =============== =============================== + r1 = READ_ONCE(y); + <general barrier> + WRITE_ONCE(x, 1); if (r2 = READ_ONCE(x)) { + <implicit control dependency> + WRITE_ONCE(y, 1); + } + + assert(r1 == 0 || r2 == 0); + +Basically, the read barrier always has to be there, even though it can be of +the "weaker" type. + +[!] Note that the stores before the write barrier would normally be expected to +match the loads after the read barrier or the address-dependency barrier, and +vice versa: + + CPU 1 CPU 2 + =================== =================== + WRITE_ONCE(a, 1); }---- --->{ v = READ_ONCE(c); + WRITE_ONCE(b, 2); } \ / { w = READ_ONCE(d); + <write barrier> \ <read barrier> + WRITE_ONCE(c, 3); } / \ { x = READ_ONCE(a); + WRITE_ONCE(d, 4); }---- --->{ y = READ_ONCE(b); + + +EXAMPLES OF MEMORY BARRIER SEQUENCES +------------------------------------ + +Firstly, write barriers act as partial orderings on store operations. +Consider the following sequence of events: + + CPU 1 + ======================= + STORE A = 1 + STORE B = 2 + STORE C = 3 + <write barrier> + STORE D = 4 + STORE E = 5 + +This sequence of events is committed to the memory coherence system in an order +that the rest of the system might perceive as the unordered set of { STORE A, +STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E +}: + + +-------+ : : + | | +------+ + | |------>| C=3 | } /\ + | | : +------+ }----- \ -----> Events perceptible to + | | : | A=1 | } \/ the rest of the system + | | : +------+ } + | CPU 1 | : | B=2 | } + | | +------+ } + | | wwwwwwwwwwwwwwww } <--- At this point the write barrier + | | +------+ } requires all stores prior to the + | | : | E=5 | } barrier to be committed before + | | : +------+ } further stores may take place + | |------>| D=4 | } + | | +------+ + +-------+ : : + | + | Sequence in which stores are committed to the + | memory system by CPU 1 + V + + +Secondly, address-dependency barriers act as partial orderings on address- +dependent loads. Consider the following sequence of events: + + CPU 1 CPU 2 + ======================= ======================= + { B = 7; X = 9; Y = 8; C = &Y } + STORE A = 1 + STORE B = 2 + <write barrier> + STORE C = &B LOAD X + STORE D = 4 LOAD C (gets &B) + LOAD *C (reads B) + +Without intervention, CPU 2 may perceive the events on CPU 1 in some +effectively random order, despite the write barrier issued by CPU 1: + + +-------+ : : : : + | | +------+ +-------+ | Sequence of update + | |------>| B=2 |----- --->| Y->8 | | of perception on + | | : +------+ \ +-------+ | CPU 2 + | CPU 1 | : | A=1 | \ --->| C->&Y | V + | | +------+ | +-------+ + | | wwwwwwwwwwwwwwww | : : + | | +------+ | : : + | | : | C=&B |--- | : : +-------+ + | | : +------+ \ | +-------+ | | + | |------>| D=4 | ----------->| C->&B |------>| | + | | +------+ | +-------+ | | + +-------+ : : | : : | | + | : : | | + | : : | CPU 2 | + | +-------+ | | + Apparently incorrect ---> | | B->7 |------>| | + perception of B (!) | +-------+ | | + | : : | | + | +-------+ | | + The load of X holds ---> \ | X->9 |------>| | + up the maintenance \ +-------+ | | + of coherence of B ----->| B->2 | +-------+ + +-------+ + : : + + +In the above example, CPU 2 perceives that B is 7, despite the load of *C +(which would be B) coming after the LOAD of C. + +If, however, an address-dependency barrier were to be placed between the load +of C and the load of *C (ie: B) on CPU 2: + + CPU 1 CPU 2 + ======================= ======================= + { B = 7; X = 9; Y = 8; C = &Y } + STORE A = 1 + STORE B = 2 + <write barrier> + STORE C = &B LOAD X + STORE D = 4 LOAD C (gets &B) + <address-dependency barrier> + LOAD *C (reads B) + +then the following will occur: + + +-------+ : : : : + | | +------+ +-------+ + | |------>| B=2 |----- --->| Y->8 | + | | : +------+ \ +-------+ + | CPU 1 | : | A=1 | \ --->| C->&Y | + | | +------+ | +-------+ + | | wwwwwwwwwwwwwwww | : : + | | +------+ | : : + | | : | C=&B |--- | : : +-------+ + | | : +------+ \ | +-------+ | | + | |------>| D=4 | ----------->| C->&B |------>| | + | | +------+ | +-------+ | | + +-------+ : : | : : | | + | : : | | + | : : | CPU 2 | + | +-------+ | | + | | X->9 |------>| | + | +-------+ | | + Makes sure all effects ---> \ aaaaaaaaaaaaaaaaa | | + prior to the store of C \ +-------+ | | + are perceptible to ----->| B->2 |------>| | + subsequent loads +-------+ | | + : : +-------+ + + +And thirdly, a read barrier acts as a partial order on loads. Consider the +following sequence of events: + + CPU 1 CPU 2 + ======================= ======================= + { A = 0, B = 9 } + STORE A=1 + <write barrier> + STORE B=2 + LOAD B + LOAD A + +Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in +some effectively random order, despite the write barrier issued by CPU 1: + + +-------+ : : : : + | | +------+ +-------+ + | |------>| A=1 |------ --->| A->0 | + | | +------+ \ +-------+ + | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | + | | +------+ | +-------+ + | |------>| B=2 |--- | : : + | | +------+ \ | : : +-------+ + +-------+ : : \ | +-------+ | | + ---------->| B->2 |------>| | + | +-------+ | CPU 2 | + | | A->0 |------>| | + | +-------+ | | + | : : +-------+ + \ : : + \ +-------+ + ---->| A->1 | + +-------+ + : : + + +If, however, a read barrier were to be placed between the load of B and the +load of A on CPU 2: + + CPU 1 CPU 2 + ======================= ======================= + { A = 0, B = 9 } + STORE A=1 + <write barrier> + STORE B=2 + LOAD B + <read barrier> + LOAD A + +then the partial ordering imposed by CPU 1 will be perceived correctly by CPU +2: + + +-------+ : : : : + | | +------+ +-------+ + | |------>| A=1 |------ --->| A->0 | + | | +------+ \ +-------+ + | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | + | | +------+ | +-------+ + | |------>| B=2 |--- | : : + | | +------+ \ | : : +-------+ + +-------+ : : \ | +-------+ | | + ---------->| B->2 |------>| | + | +-------+ | CPU 2 | + | : : | | + | : : | | + At this point the read ----> \ rrrrrrrrrrrrrrrrr | | + barrier causes all effects \ +-------+ | | + prior to the storage of B ---->| A->1 |------>| | + to be perceptible to CPU 2 +-------+ | | + : : +-------+ + + +To illustrate this more completely, consider what could happen if the code +contained a load of A either side of the read barrier: + + CPU 1 CPU 2 + ======================= ======================= + { A = 0, B = 9 } + STORE A=1 + <write barrier> + STORE B=2 + LOAD B + LOAD A [first load of A] + <read barrier> + LOAD A [second load of A] + +Even though the two loads of A both occur after the load of B, they may both +come up with different values: + + +-------+ : : : : + | | +------+ +-------+ + | |------>| A=1 |------ --->| A->0 | + | | +------+ \ +-------+ + | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | + | | +------+ | +-------+ + | |------>| B=2 |--- | : : + | | +------+ \ | : : +-------+ + +-------+ : : \ | +-------+ | | + ---------->| B->2 |------>| | + | +-------+ | CPU 2 | + | : : | | + | : : | | + | +-------+ | | + | | A->0 |------>| 1st | + | +-------+ | | + At this point the read ----> \ rrrrrrrrrrrrrrrrr | | + barrier causes all effects \ +-------+ | | + prior to the storage of B ---->| A->1 |------>| 2nd | + to be perceptible to CPU 2 +-------+ | | + : : +-------+ + + +But it may be that the update to A from CPU 1 becomes perceptible to CPU 2 +before the read barrier completes anyway: + + +-------+ : : : : + | | +------+ +-------+ + | |------>| A=1 |------ --->| A->0 | + | | +------+ \ +-------+ + | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 | + | | +------+ | +-------+ + | |------>| B=2 |--- | : : + | | +------+ \ | : : +-------+ + +-------+ : : \ | +-------+ | | + ---------->| B->2 |------>| | + | +-------+ | CPU 2 | + | : : | | + \ : : | | + \ +-------+ | | + ---->| A->1 |------>| 1st | + +-------+ | | + rrrrrrrrrrrrrrrrr | | + +-------+ | | + | A->1 |------>| 2nd | + +-------+ | | + : : +-------+ + + +The guarantee is that the second load will always come up with A == 1 if the +load of B came up with B == 2. No such guarantee exists for the first load of +A; that may come up with either A == 0 or A == 1. + + +READ MEMORY BARRIERS VS LOAD SPECULATION +---------------------------------------- + +Many CPUs speculate with loads: that is they see that they will need to load an +item from memory, and they find a time where they're not using the bus for any +other loads, and so do the load in advance - even though they haven't actually +got to that point in the instruction execution flow yet. This permits the +actual load instruction to potentially complete immediately because the CPU +already has the value to hand. + +It may turn out that the CPU didn't actually need the value - perhaps because a +branch circumvented the load - in which case it can discard the value or just +cache it for later use. + +Consider: + + CPU 1 CPU 2 + ======================= ======================= + LOAD B + DIVIDE } Divide instructions generally + DIVIDE } take a long time to perform + LOAD A + +Which might appear as this: + + : : +-------+ + +-------+ | | + --->| B->2 |------>| | + +-------+ | CPU 2 | + : :DIVIDE | | + +-------+ | | + The CPU being busy doing a ---> --->| A->0 |~~~~ | | + division speculates on the +-------+ ~ | | + LOAD of A : : ~ | | + : :DIVIDE | | + : : ~ | | + Once the divisions are complete --> : : ~-->| | + the CPU can then perform the : : | | + LOAD with immediate effect : : +-------+ + + +Placing a read barrier or an address-dependency barrier just before the second +load: + + CPU 1 CPU 2 + ======================= ======================= + LOAD B + DIVIDE + DIVIDE + <read barrier> + LOAD A + +will force any value speculatively obtained to be reconsidered to an extent +dependent on the type of barrier used. If there was no change made to the +speculated memory location, then the speculated value will just be used: + + : : +-------+ + +-------+ | | + --->| B->2 |------>| | + +-------+ | CPU 2 | + : :DIVIDE | | + +-------+ | | + The CPU being busy doing a ---> --->| A->0 |~~~~ | | + division speculates on the +-------+ ~ | | + LOAD of A : : ~ | | + : :DIVIDE | | + : : ~ | | + : : ~ | | + rrrrrrrrrrrrrrrr~ | | + : : ~ | | + : : ~-->| | + : : | | + : : +-------+ + + +but if there was an update or an invalidation from another CPU pending, then +the speculation will be cancelled and the value reloaded: + + : : +-------+ + +-------+ | | + --->| B->2 |------>| | + +-------+ | CPU 2 | + : :DIVIDE | | + +-------+ | | + The CPU being busy doing a ---> --->| A->0 |~~~~ | | + division speculates on the +-------+ ~ | | + LOAD of A : : ~ | | + : :DIVIDE | | + : : ~ | | + : : ~ | | + rrrrrrrrrrrrrrrrr | | + +-------+ | | + The speculation is discarded ---> --->| A->1 |------>| | + and an updated value is +-------+ | | + retrieved : : +-------+ + + +MULTICOPY ATOMICITY +-------------------- + +Multicopy atomicity is a deeply intuitive notion about ordering that is +not always provided by real computer systems, namely that a given store +becomes visible at the same time to all CPUs, or, alternatively, that all +CPUs agree on the order in which all stores become visible. However, +support of full multicopy atomicity would rule out valuable hardware +optimizations, so a weaker form called ``other multicopy atomicity'' +instead guarantees only that a given store becomes visible at the same +time to all -other- CPUs. The remainder of this document discusses this +weaker form, but for brevity will call it simply ``multicopy atomicity''. + +The following example demonstrates multicopy atomicity: + + CPU 1 CPU 2 CPU 3 + ======================= ======================= ======================= + { X = 0, Y = 0 } + STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1) + <general barrier> <read barrier> + STORE Y=r1 LOAD X + +Suppose that CPU 2's load from X returns 1, which it then stores to Y, +and CPU 3's load from Y returns 1. This indicates that CPU 1's store +to X precedes CPU 2's load from X and that CPU 2's store to Y precedes +CPU 3's load from Y. In addition, the memory barriers guarantee that +CPU 2 executes its load before its store, and CPU 3 loads from Y before +it loads from X. The question is then "Can CPU 3's load from X return 0?" + +Because CPU 3's load from X in some sense comes after CPU 2's load, it +is natural to expect that CPU 3's load from X must therefore return 1. +This expectation follows from multicopy atomicity: if a load executing +on CPU B follows a load from the same variable executing on CPU A (and +CPU A did not originally store the value which it read), then on +multicopy-atomic systems, CPU B's load must return either the same value +that CPU A's load did or some later value. However, the Linux kernel +does not require systems to be multicopy atomic. + +The use of a general memory barrier in the example above compensates +for any lack of multicopy atomicity. In the example, if CPU 2's load +from X returns 1 and CPU 3's load from Y returns 1, then CPU 3's load +from X must indeed also return 1. + +However, dependencies, read barriers, and write barriers are not always +able to compensate for non-multicopy atomicity. For example, suppose +that CPU 2's general barrier is removed from the above example, leaving +only the data dependency shown below: + + CPU 1 CPU 2 CPU 3 + ======================= ======================= ======================= + { X = 0, Y = 0 } + STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1) + <data dependency> <read barrier> + STORE Y=r1 LOAD X (reads 0) + +This substitution allows non-multicopy atomicity to run rampant: in +this example, it is perfectly legal for CPU 2's load from X to return 1, +CPU 3's load from Y to return 1, and its load from X to return 0. + +The key point is that although CPU 2's data dependency orders its load +and store, it does not guarantee to order CPU 1's store. Thus, if this +example runs on a non-multicopy-atomic system where CPUs 1 and 2 share a +store buffer or a level of cache, CPU 2 might have early access to CPU 1's +writes. General barriers are therefore required to ensure that all CPUs +agree on the combined order of multiple accesses. + +General barriers can compensate not only for non-multicopy atomicity, +but can also generate additional ordering that can ensure that -all- +CPUs will perceive the same order of -all- operations. In contrast, a +chain of release-acquire pairs do not provide this additional ordering, +which means that only those CPUs on the chain are guaranteed to agree +on the combined order of the accesses. For example, switching to C code +in deference to the ghost of Herman Hollerith: + + int u, v, x, y, z; + + void cpu0(void) + { + r0 = smp_load_acquire(&x); + WRITE_ONCE(u, 1); + smp_store_release(&y, 1); + } + + void cpu1(void) + { + r1 = smp_load_acquire(&y); + r4 = READ_ONCE(v); + r5 = READ_ONCE(u); + smp_store_release(&z, 1); + } + + void cpu2(void) + { + r2 = smp_load_acquire(&z); + smp_store_release(&x, 1); + } + + void cpu3(void) + { + WRITE_ONCE(v, 1); + smp_mb(); + r3 = READ_ONCE(u); + } + +Because cpu0(), cpu1(), and cpu2() participate in a chain of +smp_store_release()/smp_load_acquire() pairs, the following outcome +is prohibited: + + r0 == 1 && r1 == 1 && r2 == 1 + +Furthermore, because of the release-acquire relationship between cpu0() +and cpu1(), cpu1() must see cpu0()'s writes, so that the following +outcome is prohibited: + + r1 == 1 && r5 == 0 + +However, the ordering provided by a release-acquire chain is local +to the CPUs participating in that chain and does not apply to cpu3(), +at least aside from stores. Therefore, the following outcome is possible: + + r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 + +As an aside, the following outcome is also possible: + + r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1 + +Although cpu0(), cpu1(), and cpu2() will see their respective reads and +writes in order, CPUs not involved in the release-acquire chain might +well disagree on the order. This disagreement stems from the fact that +the weak memory-barrier instructions used to implement smp_load_acquire() +and smp_store_release() are not required to order prior stores against +subsequent loads in all cases. This means that cpu3() can see cpu0()'s +store to u as happening -after- cpu1()'s load from v, even though +both cpu0() and cpu1() agree that these two operations occurred in the +intended order. + +However, please keep in mind that smp_load_acquire() is not magic. +In particular, it simply reads from its argument with ordering. It does +-not- ensure that any particular value will be read. Therefore, the +following outcome is possible: + + r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0 + +Note that this outcome can happen even on a mythical sequentially +consistent system where nothing is ever reordered. + +To reiterate, if your code requires full ordering of all operations, +use general barriers throughout. + + +======================== +EXPLICIT KERNEL BARRIERS +======================== + +The Linux kernel has a variety of different barriers that act at different +levels: + + (*) Compiler barrier. + + (*) CPU memory barriers. + + +COMPILER BARRIER +---------------- + +The Linux kernel has an explicit compiler barrier function that prevents the +compiler from moving the memory accesses either side of it to the other side: + + barrier(); + +This is a general barrier -- there are no read-read or write-write +variants of barrier(). However, READ_ONCE() and WRITE_ONCE() can be +thought of as weak forms of barrier() that affect only the specific +accesses flagged by the READ_ONCE() or WRITE_ONCE(). + +The barrier() function has the following effects: + + (*) Prevents the compiler from reordering accesses following the + barrier() to precede any accesses preceding the barrier(). + One example use for this property is to ease communication between + interrupt-handler code and the code that was interrupted. + + (*) Within a loop, forces the compiler to load the variables used + in that loop's conditional on each pass through that loop. + +The READ_ONCE() and WRITE_ONCE() functions can prevent any number of +optimizations that, while perfectly safe in single-threaded code, can +be fatal in concurrent code. Here are some examples of these sorts +of optimizations: + + (*) The compiler is within its rights to reorder loads and stores + to the same variable, and in some cases, the CPU is within its + rights to reorder loads to the same variable. This means that + the following code: + + a[0] = x; + a[1] = x; + + Might result in an older value of x stored in a[1] than in a[0]. + Prevent both the compiler and the CPU from doing this as follows: + + a[0] = READ_ONCE(x); + a[1] = READ_ONCE(x); + + In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for + accesses from multiple CPUs to a single variable. + + (*) The compiler is within its rights to merge successive loads from + the same variable. Such merging can cause the compiler to "optimize" + the following code: + + while (tmp = a) + do_something_with(tmp); + + into the following code, which, although in some sense legitimate + for single-threaded code, is almost certainly not what the developer + intended: + + if (tmp = a) + for (;;) + do_something_with(tmp); + + Use READ_ONCE() to prevent the compiler from doing this to you: + + while (tmp = READ_ONCE(a)) + do_something_with(tmp); + + (*) The compiler is within its rights to reload a variable, for example, + in cases where high register pressure prevents the compiler from + keeping all data of interest in registers. The compiler might + therefore optimize the variable 'tmp' out of our previous example: + + while (tmp = a) + do_something_with(tmp); + + This could result in the following code, which is perfectly safe in + single-threaded code, but can be fatal in concurrent code: + + while (a) + do_something_with(a); + + For example, the optimized version of this code could result in + passing a zero to do_something_with() in the case where the variable + a was modified by some other CPU between the "while" statement and + the call to do_something_with(). + + Again, use READ_ONCE() to prevent the compiler from doing this: + + while (tmp = READ_ONCE(a)) + do_something_with(tmp); + + Note that if the compiler runs short of registers, it might save + tmp onto the stack. The overhead of this saving and later restoring + is why compilers reload variables. Doing so is perfectly safe for + single-threaded code, so you need to tell the compiler about cases + where it is not safe. + + (*) The compiler is within its rights to omit a load entirely if it knows + what the value will be. For example, if the compiler can prove that + the value of variable 'a' is always zero, it can optimize this code: + + while (tmp = a) + do_something_with(tmp); + + Into this: + + do { } while (0); + + This transformation is a win for single-threaded code because it + gets rid of a load and a branch. The problem is that the compiler + will carry out its proof assuming that the current CPU is the only + one updating variable 'a'. If variable 'a' is shared, then the + compiler's proof will be erroneous. Use READ_ONCE() to tell the + compiler that it doesn't know as much as it thinks it does: + + while (tmp = READ_ONCE(a)) + do_something_with(tmp); + + But please note that the compiler is also closely watching what you + do with the value after the READ_ONCE(). For example, suppose you + do the following and MAX is a preprocessor macro with the value 1: + + while ((tmp = READ_ONCE(a)) % MAX) + do_something_with(tmp); + + Then the compiler knows that the result of the "%" operator applied + to MAX will always be zero, again allowing the compiler to optimize + the code into near-nonexistence. (It will still load from the + variable 'a'.) + + (*) Similarly, the compiler is within its rights to omit a store entirely + if it knows that the variable already has the value being stored. + Again, the compiler assumes that the current CPU is the only one + storing into the variable, which can cause the compiler to do the + wrong thing for shared variables. For example, suppose you have + the following: + + a = 0; + ... Code that does not store to variable a ... + a = 0; + + The compiler sees that the value of variable 'a' is already zero, so + it might well omit the second store. This would come as a fatal + surprise if some other CPU might have stored to variable 'a' in the + meantime. + + Use WRITE_ONCE() to prevent the compiler from making this sort of + wrong guess: + + WRITE_ONCE(a, 0); + ... Code that does not store to variable a ... + WRITE_ONCE(a, 0); + + (*) The compiler is within its rights to reorder memory accesses unless + you tell it not to. For example, consider the following interaction + between process-level code and an interrupt handler: + + void process_level(void) + { + msg = get_message(); + flag = true; + } + + void interrupt_handler(void) + { + if (flag) + process_message(msg); + } + + There is nothing to prevent the compiler from transforming + process_level() to the following, in fact, this might well be a + win for single-threaded code: + + void process_level(void) + { + flag = true; + msg = get_message(); + } + + If the interrupt occurs between these two statement, then + interrupt_handler() might be passed a garbled msg. Use WRITE_ONCE() + to prevent this as follows: + + void process_level(void) + { + WRITE_ONCE(msg, get_message()); + WRITE_ONCE(flag, true); + } + + void interrupt_handler(void) + { + if (READ_ONCE(flag)) + process_message(READ_ONCE(msg)); + } + + Note that the READ_ONCE() and WRITE_ONCE() wrappers in + interrupt_handler() are needed if this interrupt handler can itself + be interrupted by something that also accesses 'flag' and 'msg', + for example, a nested interrupt or an NMI. Otherwise, READ_ONCE() + and WRITE_ONCE() are not needed in interrupt_handler() other than + for documentation purposes. (Note also that nested interrupts + do not typically occur in modern Linux kernels, in fact, if an + interrupt handler returns with interrupts enabled, you will get a + WARN_ONCE() splat.) + + You should assume that the compiler can move READ_ONCE() and + WRITE_ONCE() past code not containing READ_ONCE(), WRITE_ONCE(), + barrier(), or similar primitives. + + This effect could also be achieved using barrier(), but READ_ONCE() + and WRITE_ONCE() are more selective: With READ_ONCE() and + WRITE_ONCE(), the compiler need only forget the contents of the + indicated memory locations, while with barrier() the compiler must + discard the value of all memory locations that it has currently + cached in any machine registers. Of course, the compiler must also + respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur, + though the CPU of course need not do so. + + (*) The compiler is within its rights to invent stores to a variable, + as in the following example: + + if (a) + b = a; + else + b = 42; + + The compiler might save a branch by optimizing this as follows: + + b = 42; + if (a) + b = a; + + In single-threaded code, this is not only safe, but also saves + a branch. Unfortunately, in concurrent code, this optimization + could cause some other CPU to see a spurious value of 42 -- even + if variable 'a' was never zero -- when loading variable 'b'. + Use WRITE_ONCE() to prevent this as follows: + + if (a) + WRITE_ONCE(b, a); + else + WRITE_ONCE(b, 42); + + The compiler can also invent loads. These are usually less + damaging, but they can result in cache-line bouncing and thus in + poor performance and scalability. Use READ_ONCE() to prevent + invented loads. + + (*) For aligned memory locations whose size allows them to be accessed + with a single memory-reference instruction, prevents "load tearing" + and "store tearing," in which a single large access is replaced by + multiple smaller accesses. For example, given an architecture having + 16-bit store instructions with 7-bit immediate fields, the compiler + might be tempted to use two 16-bit store-immediate instructions to + implement the following 32-bit store: + + p = 0x00010002; + + Please note that GCC really does use this sort of optimization, + which is not surprising given that it would likely take more + than two instructions to build the constant and then store it. + This optimization can therefore be a win in single-threaded code. + In fact, a recent bug (since fixed) caused GCC to incorrectly use + this optimization in a volatile store. In the absence of such bugs, + use of WRITE_ONCE() prevents store tearing in the following example: + + WRITE_ONCE(p, 0x00010002); + + Use of packed structures can also result in load and store tearing, + as in this example: + + struct __attribute__((__packed__)) foo { + short a; + int b; + short c; + }; + struct foo foo1, foo2; + ... + + foo2.a = foo1.a; + foo2.b = foo1.b; + foo2.c = foo1.c; + + Because there are no READ_ONCE() or WRITE_ONCE() wrappers and no + volatile markings, the compiler would be well within its rights to + implement these three assignment statements as a pair of 32-bit + loads followed by a pair of 32-bit stores. This would result in + load tearing on 'foo1.b' and store tearing on 'foo2.b'. READ_ONCE() + and WRITE_ONCE() again prevent tearing in this example: + + foo2.a = foo1.a; + WRITE_ONCE(foo2.b, READ_ONCE(foo1.b)); + foo2.c = foo1.c; + +All that aside, it is never necessary to use READ_ONCE() and +WRITE_ONCE() on a variable that has been marked volatile. For example, +because 'jiffies' is marked volatile, it is never necessary to +say READ_ONCE(jiffies). The reason for this is that READ_ONCE() and +WRITE_ONCE() are implemented as volatile casts, which has no effect when +its argument is already marked volatile. + +Please note that these compiler barriers have no direct effect on the CPU, +which may then reorder things however it wishes. + + +CPU MEMORY BARRIERS +------------------- + +The Linux kernel has seven basic CPU memory barriers: + + TYPE MANDATORY SMP CONDITIONAL + ======================= =============== =============== + GENERAL mb() smp_mb() + WRITE wmb() smp_wmb() + READ rmb() smp_rmb() + ADDRESS DEPENDENCY READ_ONCE() + + +All memory barriers except the address-dependency barriers imply a compiler +barrier. Address dependencies do not impose any additional compiler ordering. + +Aside: In the case of address dependencies, the compiler would be expected +to issue the loads in the correct order (eg. `a[b]` would have to load +the value of b before loading a[b]), however there is no guarantee in +the C specification that the compiler may not speculate the value of b +(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1) +tmp = a[b]; ). There is also the problem of a compiler reloading b after +having loaded a[b], thus having a newer copy of b than a[b]. A consensus +has not yet been reached about these problems, however the READ_ONCE() +macro is a good place to start looking. + +SMP memory barriers are reduced to compiler barriers on uniprocessor compiled +systems because it is assumed that a CPU will appear to be self-consistent, +and will order overlapping accesses correctly with respect to itself. +However, see the subsection on "Virtual Machine Guests" below. + +[!] Note that SMP memory barriers _must_ be used to control the ordering of +references to shared memory on SMP systems, though the use of locking instead +is sufficient. + +Mandatory barriers should not be used to control SMP effects, since mandatory +barriers impose unnecessary overhead on both SMP and UP systems. They may, +however, be used to control MMIO effects on accesses through relaxed memory I/O +windows. These barriers are required even on non-SMP systems as they affect +the order in which memory operations appear to a device by prohibiting both the +compiler and the CPU from reordering them. + + +There are some more advanced barrier functions: + + (*) smp_store_mb(var, value) + + This assigns the value to the variable and then inserts a full memory + barrier after it. It isn't guaranteed to insert anything more than a + compiler barrier in a UP compilation. + + + (*) smp_mb__before_atomic(); + (*) smp_mb__after_atomic(); + + These are for use with atomic RMW functions that do not imply memory + barriers, but where the code needs a memory barrier. Examples for atomic + RMW functions that do not imply a memory barrier are e.g. add, + subtract, (failed) conditional operations, _relaxed functions, + but not atomic_read or atomic_set. A common example where a memory + barrier may be required is when atomic ops are used for reference + counting. + + These are also used for atomic RMW bitop functions that do not imply a + memory barrier (such as set_bit and clear_bit). + + As an example, consider a piece of code that marks an object as being dead + and then decrements the object's reference count: + + obj->dead = 1; + smp_mb__before_atomic(); + atomic_dec(&obj->ref_count); + + This makes sure that the death mark on the object is perceived to be set + *before* the reference counter is decremented. + + See Documentation/atomic_{t,bitops}.txt for more information. + + + (*) dma_wmb(); + (*) dma_rmb(); + (*) dma_mb(); + + These are for use with consistent memory to guarantee the ordering + of writes or reads of shared memory accessible to both the CPU and a + DMA capable device. See Documentation/core-api/dma-api.rst file for more + information about consistent memory. + + For example, consider a device driver that shares memory with a device + and uses a descriptor status value to indicate if the descriptor belongs + to the device or the CPU, and a doorbell to notify it when new + descriptors are available: + + if (desc->status != DEVICE_OWN) { + /* do not read data until we own descriptor */ + dma_rmb(); + + /* read/modify data */ + read_data = desc->data; + desc->data = write_data; + + /* flush modifications before status update */ + dma_wmb(); + + /* assign ownership */ + desc->status = DEVICE_OWN; + + /* Make descriptor status visible to the device followed by + * notify device of new descriptor + */ + writel(DESC_NOTIFY, doorbell); + } + + The dma_rmb() allows us to guarantee that the device has released ownership + before we read the data from the descriptor, and the dma_wmb() allows + us to guarantee the data is written to the descriptor before the device + can see it now has ownership. The dma_mb() implies both a dma_rmb() and + a dma_wmb(). + + Note that the dma_*() barriers do not provide any ordering guarantees for + accesses to MMIO regions. See the later "KERNEL I/O BARRIER EFFECTS" + subsection for more information about I/O accessors and MMIO ordering. + + (*) pmem_wmb(); + + This is for use with persistent memory to ensure that stores for which + modifications are written to persistent storage reached a platform + durability domain. + + For example, after a non-temporal write to pmem region, we use pmem_wmb() + to ensure that stores have reached a platform durability domain. This ensures + that stores have updated persistent storage before any data access or + data transfer caused by subsequent instructions is initiated. This is + in addition to the ordering done by wmb(). + + For load from persistent memory, existing read memory barriers are sufficient + to ensure read ordering. + + (*) io_stop_wc(); + + For memory accesses with write-combining attributes (e.g. those returned + by ioremap_wc()), the CPU may wait for prior accesses to be merged with + subsequent ones. io_stop_wc() can be used to prevent the merging of + write-combining memory accesses before this macro with those after it when + such wait has performance implications. + +=============================== +IMPLICIT KERNEL MEMORY BARRIERS +=============================== + +Some of the other functions in the linux kernel imply memory barriers, amongst +which are locking and scheduling functions. + +This specification is a _minimum_ guarantee; any particular architecture may +provide more substantial guarantees, but these may not be relied upon outside +of arch specific code. + + +LOCK ACQUISITION FUNCTIONS +-------------------------- + +The Linux kernel has a number of locking constructs: + + (*) spin locks + (*) R/W spin locks + (*) mutexes + (*) semaphores + (*) R/W semaphores + +In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations +for each construct. These operations all imply certain barriers: + + (1) ACQUIRE operation implication: + + Memory operations issued after the ACQUIRE will be completed after the + ACQUIRE operation has completed. + + Memory operations issued before the ACQUIRE may be completed after + the ACQUIRE operation has completed. + + (2) RELEASE operation implication: + + Memory operations issued before the RELEASE will be completed before the + RELEASE operation has completed. + + Memory operations issued after the RELEASE may be completed before the + RELEASE operation has completed. + + (3) ACQUIRE vs ACQUIRE implication: + + All ACQUIRE operations issued before another ACQUIRE operation will be + completed before that ACQUIRE operation. + + (4) ACQUIRE vs RELEASE implication: + + All ACQUIRE operations issued before a RELEASE operation will be + completed before the RELEASE operation. + + (5) Failed conditional ACQUIRE implication: + + Certain locking variants of the ACQUIRE operation may fail, either due to + being unable to get the lock immediately, or due to receiving an unblocked + signal while asleep waiting for the lock to become available. Failed + locks do not imply any sort of barrier. + +[!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only +one-way barriers is that the effects of instructions outside of a critical +section may seep into the inside of the critical section. + +An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier +because it is possible for an access preceding the ACQUIRE to happen after the +ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and +the two accesses can themselves then cross: + + *A = a; + ACQUIRE M + RELEASE M + *B = b; + +may occur as: + + ACQUIRE M, STORE *B, STORE *A, RELEASE M + +When the ACQUIRE and RELEASE are a lock acquisition and release, +respectively, this same reordering can occur if the lock's ACQUIRE and +RELEASE are to the same lock variable, but only from the perspective of +another CPU not holding that lock. In short, a ACQUIRE followed by an +RELEASE may -not- be assumed to be a full memory barrier. + +Similarly, the reverse case of a RELEASE followed by an ACQUIRE does +not imply a full memory barrier. Therefore, the CPU's execution of the +critical sections corresponding to the RELEASE and the ACQUIRE can cross, +so that: + + *A = a; + RELEASE M + ACQUIRE N + *B = b; + +could occur as: + + ACQUIRE N, STORE *B, STORE *A, RELEASE M + +It might appear that this reordering could introduce a deadlock. +However, this cannot happen because if such a deadlock threatened, +the RELEASE would simply complete, thereby avoiding the deadlock. + + Why does this work? + + One key point is that we are only talking about the CPU doing + the reordering, not the compiler. If the compiler (or, for + that matter, the developer) switched the operations, deadlock + -could- occur. + + But suppose the CPU reordered the operations. In this case, + the unlock precedes the lock in the assembly code. The CPU + simply elected to try executing the later lock operation first. + If there is a deadlock, this lock operation will simply spin (or + try to sleep, but more on that later). The CPU will eventually + execute the unlock operation (which preceded the lock operation + in the assembly code), which will unravel the potential deadlock, + allowing the lock operation to succeed. + + But what if the lock is a sleeplock? In that case, the code will + try to enter the scheduler, where it will eventually encounter + a memory barrier, which will force the earlier unlock operation + to complete, again unraveling the deadlock. There might be + a sleep-unlock race, but the locking primitive needs to resolve + such races properly in any case. + +Locks and semaphores may not provide any guarantee of ordering on UP compiled +systems, and so cannot be counted on in such a situation to actually achieve +anything at all - especially with respect to I/O accesses - unless combined +with interrupt disabling operations. + +See also the section on "Inter-CPU acquiring barrier effects". + + +As an example, consider the following: + + *A = a; + *B = b; + ACQUIRE + *C = c; + *D = d; + RELEASE + *E = e; + *F = f; + +The following sequence of events is acceptable: + + ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE + + [+] Note that {*F,*A} indicates a combined access. + +But none of the following are: + + {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E + *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F + *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F + *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E + + + +INTERRUPT DISABLING FUNCTIONS +----------------------------- + +Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts +(RELEASE equivalent) will act as compiler barriers only. So if memory or I/O +barriers are required in such a situation, they must be provided from some +other means. + + +SLEEP AND WAKE-UP FUNCTIONS +--------------------------- + +Sleeping and waking on an event flagged in global data can be viewed as an +interaction between two pieces of data: the task state of the task waiting for +the event and the global data used to indicate the event. To make sure that +these appear to happen in the right order, the primitives to begin the process +of going to sleep, and the primitives to initiate a wake up imply certain +barriers. + +Firstly, the sleeper normally follows something like this sequence of events: + + for (;;) { + set_current_state(TASK_UNINTERRUPTIBLE); + if (event_indicated) + break; + schedule(); + } + +A general memory barrier is interpolated automatically by set_current_state() +after it has altered the task state: + + CPU 1 + =============================== + set_current_state(); + smp_store_mb(); + STORE current->state + <general barrier> + LOAD event_indicated + +set_current_state() may be wrapped by: + + prepare_to_wait(); + prepare_to_wait_exclusive(); + +which therefore also imply a general memory barrier after setting the state. +The whole sequence above is available in various canned forms, all of which +interpolate the memory barrier in the right place: + + wait_event(); + wait_event_interruptible(); + wait_event_interruptible_exclusive(); + wait_event_interruptible_timeout(); + wait_event_killable(); + wait_event_timeout(); + wait_on_bit(); + wait_on_bit_lock(); + + +Secondly, code that performs a wake up normally follows something like this: + + event_indicated = 1; + wake_up(&event_wait_queue); + +or: + + event_indicated = 1; + wake_up_process(event_daemon); + +A general memory barrier is executed by wake_up() if it wakes something up. +If it doesn't wake anything up then a memory barrier may or may not be +executed; you must not rely on it. The barrier occurs before the task state +is accessed, in particular, it sits between the STORE to indicate the event +and the STORE to set TASK_RUNNING: + + CPU 1 (Sleeper) CPU 2 (Waker) + =============================== =============================== + set_current_state(); STORE event_indicated + smp_store_mb(); wake_up(); + STORE current->state ... + <general barrier> <general barrier> + LOAD event_indicated if ((LOAD task->state) & TASK_NORMAL) + STORE task->state + +where "task" is the thread being woken up and it equals CPU 1's "current". + +To repeat, a general memory barrier is guaranteed to be executed by wake_up() +if something is actually awakened, but otherwise there is no such guarantee. +To see this, consider the following sequence of events, where X and Y are both +initially zero: + + CPU 1 CPU 2 + =============================== =============================== + X = 1; Y = 1; + smp_mb(); wake_up(); + LOAD Y LOAD X + +If a wakeup does occur, one (at least) of the two loads must see 1. If, on +the other hand, a wakeup does not occur, both loads might see 0. + +wake_up_process() always executes a general memory barrier. The barrier again +occurs before the task state is accessed. In particular, if the wake_up() in +the previous snippet were replaced by a call to wake_up_process() then one of +the two loads would be guaranteed to see 1. + +The available waker functions include: + + complete(); + wake_up(); + wake_up_all(); + wake_up_bit(); + wake_up_interruptible(); + wake_up_interruptible_all(); + wake_up_interruptible_nr(); + wake_up_interruptible_poll(); + wake_up_interruptible_sync(); + wake_up_interruptible_sync_poll(); + wake_up_locked(); + wake_up_locked_poll(); + wake_up_nr(); + wake_up_poll(); + wake_up_process(); + +In terms of memory ordering, these functions all provide the same guarantees of +a wake_up() (or stronger). + +[!] Note that the memory barriers implied by the sleeper and the waker do _not_ +order multiple stores before the wake-up with respect to loads of those stored +values after the sleeper has called set_current_state(). For instance, if the +sleeper does: + + set_current_state(TASK_INTERRUPTIBLE); + if (event_indicated) + break; + __set_current_state(TASK_RUNNING); + do_something(my_data); + +and the waker does: + + my_data = value; + event_indicated = 1; + wake_up(&event_wait_queue); + +there's no guarantee that the change to event_indicated will be perceived by +the sleeper as coming after the change to my_data. In such a circumstance, the +code on both sides must interpolate its own memory barriers between the +separate data accesses. Thus the above sleeper ought to do: + + set_current_state(TASK_INTERRUPTIBLE); + if (event_indicated) { + smp_rmb(); + do_something(my_data); + } + +and the waker should do: + + my_data = value; + smp_wmb(); + event_indicated = 1; + wake_up(&event_wait_queue); + + +MISCELLANEOUS FUNCTIONS +----------------------- + +Other functions that imply barriers: + + (*) schedule() and similar imply full memory barriers. + + +=================================== +INTER-CPU ACQUIRING BARRIER EFFECTS +=================================== + +On SMP systems locking primitives give a more substantial form of barrier: one +that does affect memory access ordering on other CPUs, within the context of +conflict on any particular lock. + + +ACQUIRES VS MEMORY ACCESSES +--------------------------- + +Consider the following: the system has a pair of spinlocks (M) and (Q), and +three CPUs; then should the following sequence of events occur: + + CPU 1 CPU 2 + =============================== =============================== + WRITE_ONCE(*A, a); WRITE_ONCE(*E, e); + ACQUIRE M ACQUIRE Q + WRITE_ONCE(*B, b); WRITE_ONCE(*F, f); + WRITE_ONCE(*C, c); WRITE_ONCE(*G, g); + RELEASE M RELEASE Q + WRITE_ONCE(*D, d); WRITE_ONCE(*H, h); + +Then there is no guarantee as to what order CPU 3 will see the accesses to *A +through *H occur in, other than the constraints imposed by the separate locks +on the separate CPUs. It might, for example, see: + + *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M + +But it won't see any of: + + *B, *C or *D preceding ACQUIRE M + *A, *B or *C following RELEASE M + *F, *G or *H preceding ACQUIRE Q + *E, *F or *G following RELEASE Q + + +================================= +WHERE ARE MEMORY BARRIERS NEEDED? +================================= + +Under normal operation, memory operation reordering is generally not going to +be a problem as a single-threaded linear piece of code will still appear to +work correctly, even if it's in an SMP kernel. There are, however, four +circumstances in which reordering definitely _could_ be a problem: + + (*) Interprocessor interaction. + + (*) Atomic operations. + + (*) Accessing devices. + + (*) Interrupts. + + +INTERPROCESSOR INTERACTION +-------------------------- + +When there's a system with more than one processor, more than one CPU in the +system may be working on the same data set at the same time. This can cause +synchronisation problems, and the usual way of dealing with them is to use +locks. Locks, however, are quite expensive, and so it may be preferable to +operate without the use of a lock if at all possible. In such a case +operations that affect both CPUs may have to be carefully ordered to prevent +a malfunction. + +Consider, for example, the R/W semaphore slow path. Here a waiting process is +queued on the semaphore, by virtue of it having a piece of its stack linked to +the semaphore's list of waiting processes: + + struct rw_semaphore { + ... + spinlock_t lock; + struct list_head waiters; + }; + + struct rwsem_waiter { + struct list_head list; + struct task_struct *task; + }; + +To wake up a particular waiter, the up_read() or up_write() functions have to: + + (1) read the next pointer from this waiter's record to know as to where the + next waiter record is; + + (2) read the pointer to the waiter's task structure; + + (3) clear the task pointer to tell the waiter it has been given the semaphore; + + (4) call wake_up_process() on the task; and + + (5) release the reference held on the waiter's task struct. + +In other words, it has to perform this sequence of events: + + LOAD waiter->list.next; + LOAD waiter->task; + STORE waiter->task; + CALL wakeup + RELEASE task + +and if any of these steps occur out of order, then the whole thing may +malfunction. + +Once it has queued itself and dropped the semaphore lock, the waiter does not +get the lock again; it instead just waits for its task pointer to be cleared +before proceeding. Since the record is on the waiter's stack, this means that +if the task pointer is cleared _before_ the next pointer in the list is read, +another CPU might start processing the waiter and might clobber the waiter's +stack before the up*() function has a chance to read the next pointer. + +Consider then what might happen to the above sequence of events: + + CPU 1 CPU 2 + =============================== =============================== + down_xxx() + Queue waiter + Sleep + up_yyy() + LOAD waiter->task; + STORE waiter->task; + Woken up by other event + <preempt> + Resume processing + down_xxx() returns + call foo() + foo() clobbers *waiter + </preempt> + LOAD waiter->list.next; + --- OOPS --- + +This could be dealt with using the semaphore lock, but then the down_xxx() +function has to needlessly get the spinlock again after being woken up. + +The way to deal with this is to insert a general SMP memory barrier: + + LOAD waiter->list.next; + LOAD waiter->task; + smp_mb(); + STORE waiter->task; + CALL wakeup + RELEASE task + +In this case, the barrier makes a guarantee that all memory accesses before the +barrier will appear to happen before all the memory accesses after the barrier +with respect to the other CPUs on the system. It does _not_ guarantee that all +the memory accesses before the barrier will be complete by the time the barrier +instruction itself is complete. + +On a UP system - where this wouldn't be a problem - the smp_mb() is just a +compiler barrier, thus making sure the compiler emits the instructions in the +right order without actually intervening in the CPU. Since there's only one +CPU, that CPU's dependency ordering logic will take care of everything else. + + +ATOMIC OPERATIONS +----------------- + +While they are technically interprocessor interaction considerations, atomic +operations are noted specially as some of them imply full memory barriers and +some don't, but they're very heavily relied on as a group throughout the +kernel. + +See Documentation/atomic_t.txt for more information. + + +ACCESSING DEVICES +----------------- + +Many devices can be memory mapped, and so appear to the CPU as if they're just +a set of memory locations. To control such a device, the driver usually has to +make the right memory accesses in exactly the right order. + +However, having a clever CPU or a clever compiler creates a potential problem +in that the carefully sequenced accesses in the driver code won't reach the +device in the requisite order if the CPU or the compiler thinks it is more +efficient to reorder, combine or merge accesses - something that would cause +the device to malfunction. + +Inside of the Linux kernel, I/O should be done through the appropriate accessor +routines - such as inb() or writel() - which know how to make such accesses +appropriately sequential. While this, for the most part, renders the explicit +use of memory barriers unnecessary, if the accessor functions are used to refer +to an I/O memory window with relaxed memory access properties, then _mandatory_ +memory barriers are required to enforce ordering. + +See Documentation/driver-api/device-io.rst for more information. + + +INTERRUPTS +---------- + +A driver may be interrupted by its own interrupt service routine, and thus the +two parts of the driver may interfere with each other's attempts to control or +access the device. + +This may be alleviated - at least in part - by disabling local interrupts (a +form of locking), such that the critical operations are all contained within +the interrupt-disabled section in the driver. While the driver's interrupt +routine is executing, the driver's core may not run on the same CPU, and its +interrupt is not permitted to happen again until the current interrupt has been +handled, thus the interrupt handler does not need to lock against that. + +However, consider a driver that was talking to an ethernet card that sports an +address register and a data register. If that driver's core talks to the card +under interrupt-disablement and then the driver's interrupt handler is invoked: + + LOCAL IRQ DISABLE + writew(ADDR, 3); + writew(DATA, y); + LOCAL IRQ ENABLE + <interrupt> + writew(ADDR, 4); + q = readw(DATA); + </interrupt> + +The store to the data register might happen after the second store to the +address register if ordering rules are sufficiently relaxed: + + STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA + + +If ordering rules are relaxed, it must be assumed that accesses done inside an +interrupt disabled section may leak outside of it and may interleave with +accesses performed in an interrupt - and vice versa - unless implicit or +explicit barriers are used. + +Normally this won't be a problem because the I/O accesses done inside such +sections will include synchronous load operations on strictly ordered I/O +registers that form implicit I/O barriers. + + +A similar situation may occur between an interrupt routine and two routines +running on separate CPUs that communicate with each other. If such a case is +likely, then interrupt-disabling locks should be used to guarantee ordering. + + +========================== +KERNEL I/O BARRIER EFFECTS +========================== + +Interfacing with peripherals via I/O accesses is deeply architecture and device +specific. Therefore, drivers which are inherently non-portable may rely on +specific behaviours of their target systems in order to achieve synchronization +in the most lightweight manner possible. For drivers intending to be portable +between multiple architectures and bus implementations, the kernel offers a +series of accessor functions that provide various degrees of ordering +guarantees: + + (*) readX(), writeX(): + + The readX() and writeX() MMIO accessors take a pointer to the + peripheral being accessed as an __iomem * parameter. For pointers + mapped with the default I/O attributes (e.g. those returned by + ioremap()), the ordering guarantees are as follows: + + 1. All readX() and writeX() accesses to the same peripheral are ordered + with respect to each other. This ensures that MMIO register accesses + by the same CPU thread to a particular device will arrive in program + order. + + 2. A writeX() issued by a CPU thread holding a spinlock is ordered + before a writeX() to the same peripheral from another CPU thread + issued after a later acquisition of the same spinlock. This ensures + that MMIO register writes to a particular device issued while holding + a spinlock will arrive in an order consistent with acquisitions of + the lock. + + 3. A writeX() by a CPU thread to the peripheral will first wait for the + completion of all prior writes to memory either issued by, or + propagated to, the same thread. This ensures that writes by the CPU + to an outbound DMA buffer allocated by dma_alloc_coherent() will be + visible to a DMA engine when the CPU writes to its MMIO control + register to trigger the transfer. + + 4. A readX() by a CPU thread from the peripheral will complete before + any subsequent reads from memory by the same thread can begin. This + ensures that reads by the CPU from an incoming DMA buffer allocated + by dma_alloc_coherent() will not see stale data after reading from + the DMA engine's MMIO status register to establish that the DMA + transfer has completed. + + 5. A readX() by a CPU thread from the peripheral will complete before + any subsequent delay() loop can begin execution on the same thread. + This ensures that two MMIO register writes by the CPU to a peripheral + will arrive at least 1us apart if the first write is immediately read + back with readX() and udelay(1) is called prior to the second + writeX(): + + writel(42, DEVICE_REGISTER_0); // Arrives at the device... + readl(DEVICE_REGISTER_0); + udelay(1); + writel(42, DEVICE_REGISTER_1); // ...at least 1us before this. + + The ordering properties of __iomem pointers obtained with non-default + attributes (e.g. those returned by ioremap_wc()) are specific to the + underlying architecture and therefore the guarantees listed above cannot + generally be relied upon for accesses to these types of mappings. + + (*) readX_relaxed(), writeX_relaxed(): + + These are similar to readX() and writeX(), but provide weaker memory + ordering guarantees. Specifically, they do not guarantee ordering with + respect to locking, normal memory accesses or delay() loops (i.e. + bullets 2-5 above) but they are still guaranteed to be ordered with + respect to other accesses from the same CPU thread to the same + peripheral when operating on __iomem pointers mapped with the default + I/O attributes. + + (*) readsX(), writesX(): + + The readsX() and writesX() MMIO accessors are designed for accessing + register-based, memory-mapped FIFOs residing on peripherals that are not + capable of performing DMA. Consequently, they provide only the ordering + guarantees of readX_relaxed() and writeX_relaxed(), as documented above. + + (*) inX(), outX(): + + The inX() and outX() accessors are intended to access legacy port-mapped + I/O peripherals, which may require special instructions on some + architectures (notably x86). The port number of the peripheral being + accessed is passed as an argument. + + Since many CPU architectures ultimately access these peripherals via an + internal virtual memory mapping, the portable ordering guarantees + provided by inX() and outX() are the same as those provided by readX() + and writeX() respectively when accessing a mapping with the default I/O + attributes. + + Device drivers may expect outX() to emit a non-posted write transaction + that waits for a completion response from the I/O peripheral before + returning. This is not guaranteed by all architectures and is therefore + not part of the portable ordering semantics. + + (*) insX(), outsX(): + + As above, the insX() and outsX() accessors provide the same ordering + guarantees as readsX() and writesX() respectively when accessing a + mapping with the default I/O attributes. + + (*) ioreadX(), iowriteX(): + + These will perform appropriately for the type of access they're actually + doing, be it inX()/outX() or readX()/writeX(). + +With the exception of the string accessors (insX(), outsX(), readsX() and +writesX()), all of the above assume that the underlying peripheral is +little-endian and will therefore perform byte-swapping operations on big-endian +architectures. + + +======================================== +ASSUMED MINIMUM EXECUTION ORDERING MODEL +======================================== + +It has to be assumed that the conceptual CPU is weakly-ordered but that it will +maintain the appearance of program causality with respect to itself. Some CPUs +(such as i386 or x86_64) are more constrained than others (such as powerpc or +frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside +of arch-specific code. + +This means that it must be considered that the CPU will execute its instruction +stream in any order it feels like - or even in parallel - provided that if an +instruction in the stream depends on an earlier instruction, then that +earlier instruction must be sufficiently complete[*] before the later +instruction may proceed; in other words: provided that the appearance of +causality is maintained. + + [*] Some instructions have more than one effect - such as changing the + condition codes, changing registers or changing memory - and different + instructions may depend on different effects. + +A CPU may also discard any instruction sequence that winds up having no +ultimate effect. For example, if two adjacent instructions both load an +immediate value into the same register, the first may be discarded. + + +Similarly, it has to be assumed that compiler might reorder the instruction +stream in any way it sees fit, again provided the appearance of causality is +maintained. + + +============================ +THE EFFECTS OF THE CPU CACHE +============================ + +The way cached memory operations are perceived across the system is affected to +a certain extent by the caches that lie between CPUs and memory, and by the +memory coherence system that maintains the consistency of state in the system. + +As far as the way a CPU interacts with another part of the system through the +caches goes, the memory system has to include the CPU's caches, and memory +barriers for the most part act at the interface between the CPU and its cache +(memory barriers logically act on the dotted line in the following diagram): + + <--- CPU ---> : <----------- Memory -----------> + : + +--------+ +--------+ : +--------+ +-----------+ + | | | | : | | | | +--------+ + | CPU | | Memory | : | CPU | | | | | + | Core |--->| Access |----->| Cache |<-->| | | | + | | | Queue | : | | | |--->| Memory | + | | | | : | | | | | | + +--------+ +--------+ : +--------+ | | | | + : | Cache | +--------+ + : | Coherency | + : | Mechanism | +--------+ + +--------+ +--------+ : +--------+ | | | | + | | | | : | | | | | | + | CPU | | Memory | : | CPU | | |--->| Device | + | Core |--->| Access |----->| Cache |<-->| | | | + | | | Queue | : | | | | | | + | | | | : | | | | +--------+ + +--------+ +--------+ : +--------+ +-----------+ + : + : + +Although any particular load or store may not actually appear outside of the +CPU that issued it since it may have been satisfied within the CPU's own cache, +it will still appear as if the full memory access had taken place as far as the +other CPUs are concerned since the cache coherency mechanisms will migrate the +cacheline over to the accessing CPU and propagate the effects upon conflict. + +The CPU core may execute instructions in any order it deems fit, provided the +expected program causality appears to be maintained. Some of the instructions +generate load and store operations which then go into the queue of memory +accesses to be performed. The core may place these in the queue in any order +it wishes, and continue execution until it is forced to wait for an instruction +to complete. + +What memory barriers are concerned with is controlling the order in which +accesses cross from the CPU side of things to the memory side of things, and +the order in which the effects are perceived to happen by the other observers +in the system. + +[!] Memory barriers are _not_ needed within a given CPU, as CPUs always see +their own loads and stores as if they had happened in program order. + +[!] MMIO or other device accesses may bypass the cache system. This depends on +the properties of the memory window through which devices are accessed and/or +the use of any special device communication instructions the CPU may have. + + +CACHE COHERENCY VS DMA +---------------------- + +Not all systems maintain cache coherency with respect to devices doing DMA. In +such cases, a device attempting DMA may obtain stale data from RAM because +dirty cache lines may be resident in the caches of various CPUs, and may not +have been written back to RAM yet. To deal with this, the appropriate part of +the kernel must flush the overlapping bits of cache on each CPU (and maybe +invalidate them as well). + +In addition, the data DMA'd to RAM by a device may be overwritten by dirty +cache lines being written back to RAM from a CPU's cache after the device has +installed its own data, or cache lines present in the CPU's cache may simply +obscure the fact that RAM has been updated, until at such time as the cacheline +is discarded from the CPU's cache and reloaded. To deal with this, the +appropriate part of the kernel must invalidate the overlapping bits of the +cache on each CPU. + +See Documentation/core-api/cachetlb.rst for more information on cache +management. + + +CACHE COHERENCY VS MMIO +----------------------- + +Memory mapped I/O usually takes place through memory locations that are part of +a window in the CPU's memory space that has different properties assigned than +the usual RAM directed window. + +Amongst these properties is usually the fact that such accesses bypass the +caching entirely and go directly to the device buses. This means MMIO accesses +may, in effect, overtake accesses to cached memory that were emitted earlier. +A memory barrier isn't sufficient in such a case, but rather the cache must be +flushed between the cached memory write and the MMIO access if the two are in +any way dependent. + + +========================= +THE THINGS CPUS GET UP TO +========================= + +A programmer might take it for granted that the CPU will perform memory +operations in exactly the order specified, so that if the CPU is, for example, +given the following piece of code to execute: + + a = READ_ONCE(*A); + WRITE_ONCE(*B, b); + c = READ_ONCE(*C); + d = READ_ONCE(*D); + WRITE_ONCE(*E, e); + +they would then expect that the CPU will complete the memory operation for each +instruction before moving on to the next one, leading to a definite sequence of +operations as seen by external observers in the system: + + LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E. + + +Reality is, of course, much messier. With many CPUs and compilers, the above +assumption doesn't hold because: + + (*) loads are more likely to need to be completed immediately to permit + execution progress, whereas stores can often be deferred without a + problem; + + (*) loads may be done speculatively, and the result discarded should it prove + to have been unnecessary; + + (*) loads may be done speculatively, leading to the result having been fetched + at the wrong time in the expected sequence of events; + + (*) the order of the memory accesses may be rearranged to promote better use + of the CPU buses and caches; + + (*) loads and stores may be combined to improve performance when talking to + memory or I/O hardware that can do batched accesses of adjacent locations, + thus cutting down on transaction setup costs (memory and PCI devices may + both be able to do this); and + + (*) the CPU's data cache may affect the ordering, and while cache-coherency + mechanisms may alleviate this - once the store has actually hit the cache + - there's no guarantee that the coherency management will be propagated in + order to other CPUs. + +So what another CPU, say, might actually observe from the above piece of code +is: + + LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B + + (Where "LOAD {*C,*D}" is a combined load) + + +However, it is guaranteed that a CPU will be self-consistent: it will see its +_own_ accesses appear to be correctly ordered, without the need for a memory +barrier. For instance with the following code: + + U = READ_ONCE(*A); + WRITE_ONCE(*A, V); + WRITE_ONCE(*A, W); + X = READ_ONCE(*A); + WRITE_ONCE(*A, Y); + Z = READ_ONCE(*A); + +and assuming no intervention by an external influence, it can be assumed that +the final result will appear to be: + + U == the original value of *A + X == W + Z == Y + *A == Y + +The code above may cause the CPU to generate the full sequence of memory +accesses: + + U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A + +in that order, but, without intervention, the sequence may have almost any +combination of elements combined or discarded, provided the program's view +of the world remains consistent. Note that READ_ONCE() and WRITE_ONCE() +are -not- optional in the above example, as there are architectures +where a given CPU might reorder successive loads to the same location. +On such architectures, READ_ONCE() and WRITE_ONCE() do whatever is +necessary to prevent this, for example, on Itanium the volatile casts +used by READ_ONCE() and WRITE_ONCE() cause GCC to emit the special ld.acq +and st.rel instructions (respectively) that prevent such reordering. + +The compiler may also combine, discard or defer elements of the sequence before +the CPU even sees them. + +For instance: + + *A = V; + *A = W; + +may be reduced to: + + *A = W; + +since, without either a write barrier or an WRITE_ONCE(), it can be +assumed that the effect of the storage of V to *A is lost. Similarly: + + *A = Y; + Z = *A; + +may, without a memory barrier or an READ_ONCE() and WRITE_ONCE(), be +reduced to: + + *A = Y; + Z = Y; + +and the LOAD operation never appear outside of the CPU. + + +AND THEN THERE'S THE ALPHA +-------------------------- + +The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that, +some versions of the Alpha CPU have a split data cache, permitting them to have +two semantically-related cache lines updated at separate times. This is where +the address-dependency barrier really becomes necessary as this synchronises +both caches with the memory coherence system, thus making it seem like pointer +changes vs new data occur in the right order. + +The Alpha defines the Linux kernel's memory model, although as of v4.15 +the Linux kernel's addition of smp_mb() to READ_ONCE() on Alpha greatly +reduced its impact on the memory model. + + +VIRTUAL MACHINE GUESTS +---------------------- + +Guests running within virtual machines might be affected by SMP effects even if +the guest itself is compiled without SMP support. This is an artifact of +interfacing with an SMP host while running an UP kernel. Using mandatory +barriers for this use-case would be possible but is often suboptimal. + +To handle this case optimally, low-level virt_mb() etc macros are available. +These have the same effect as smp_mb() etc when SMP is enabled, but generate +identical code for SMP and non-SMP systems. For example, virtual machine guests +should use virt_mb() rather than smp_mb() when synchronizing against a +(possibly SMP) host. + +These are equivalent to smp_mb() etc counterparts in all other respects, +in particular, they do not control MMIO effects: to control +MMIO effects, use mandatory barriers. + + +============ +EXAMPLE USES +============ + +CIRCULAR BUFFERS +---------------- + +Memory barriers can be used to implement circular buffering without the need +of a lock to serialise the producer with the consumer. See: + + Documentation/core-api/circular-buffers.rst + +for details. + + +========== +REFERENCES +========== + +Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek, +Digital Press) + Chapter 5.2: Physical Address Space Characteristics + Chapter 5.4: Caches and Write Buffers + Chapter 5.5: Data Sharing + Chapter 5.6: Read/Write Ordering + +AMD64 Architecture Programmer's Manual Volume 2: System Programming + Chapter 7.1: Memory-Access Ordering + Chapter 7.4: Buffering and Combining Memory Writes + +ARM Architecture Reference Manual (ARMv8, for ARMv8-A architecture profile) + Chapter B2: The AArch64 Application Level Memory Model + +IA-32 Intel Architecture Software Developer's Manual, Volume 3: +System Programming Guide + Chapter 7.1: Locked Atomic Operations + Chapter 7.2: Memory Ordering + Chapter 7.4: Serializing Instructions + +The SPARC Architecture Manual, Version 9 + Chapter 8: Memory Models + Appendix D: Formal Specification of the Memory Models + Appendix J: Programming with the Memory Models + +Storage in the PowerPC (Stone and Fitzgerald) + +UltraSPARC Programmer Reference Manual + Chapter 5: Memory Accesses and Cacheability + Chapter 15: Sparc-V9 Memory Models + +UltraSPARC III Cu User's Manual + Chapter 9: Memory Models + +UltraSPARC IIIi Processor User's Manual + Chapter 8: Memory Models + +UltraSPARC Architecture 2005 + Chapter 9: Memory + Appendix D: Formal Specifications of the Memory Models + +UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005 + Chapter 8: Memory Models + Appendix F: Caches and Cache Coherency + +Solaris Internals, Core Kernel Architecture, p63-68: + Chapter 3.3: Hardware Considerations for Locks and + Synchronization + +Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching +for Kernel Programmers: + Chapter 13: Other Memory Models + +Intel Itanium Architecture Software Developer's Manual: Volume 1: + Section 2.6: Speculation + Section 4.4: Memory Access |