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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 117 |
1 files changed, 80 insertions, 37 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 3729cbe60e41..147ae8ec836f 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -4,8 +4,40 @@ By: David Howells <dhowells@redhat.com> Paul E. McKenney <paulmck@linux.vnet.ibm.com> + Will Deacon <will.deacon@arm.com> + Peter Zijlstra <peterz@infradead.org> -Contents: +========== +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. + +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 architecure 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. @@ -31,15 +63,15 @@ Contents: (*) Implicit kernel memory barriers. - - Locking functions. + - Lock acquisition functions. - Interrupt disabling functions. - Sleep and wake-up functions. - Miscellaneous functions. - (*) Inter-CPU locking barrier effects. + (*) Inter-CPU acquiring barrier effects. - - Locks vs memory accesses. - - Locks vs I/O accesses. + - Acquires vs memory accesses. + - Acquires vs I/O accesses. (*) Where are memory barriers needed? @@ -61,6 +93,7 @@ Contents: (*) The things CPUs get up to. - And then there's the Alpha. + - Virtual Machine Guests. (*) Example uses. @@ -148,7 +181,7 @@ As a further example, consider this sequence of events: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; Q = P; P = &B D = *Q; @@ -430,8 +463,9 @@ And a couple of implicit varieties: 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 smp_load_acquire() - operations. + ACQUIRE operations include LOCK operations and both smp_load_acquire() + and smp_cond_acquire() operations. The later builds the necessary ACQUIRE + semantics from relying on a control dependency and smp_rmb(). Memory operations that occur before an ACQUIRE operation may appear to happen after it completes. @@ -464,6 +498,11 @@ And a couple of implicit varieties: 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_ops.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 @@ -517,7 +556,7 @@ following sequence of events: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; <write barrier> WRITE_ONCE(P, &B) @@ -544,7 +583,7 @@ between the address load and the data load: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; <write barrier> WRITE_ONCE(P, &B); @@ -813,9 +852,10 @@ In summary: 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, - as optimizing compilers do not necessarily respect barrier() - in this case. + 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 @@ -1731,15 +1771,15 @@ The Linux kernel has eight basic CPU memory barriers: All memory barriers except the data dependency barriers imply a compiler -barrier. Data dependencies do not impose any additional compiler ordering. +barrier. Data dependencies do not impose any additional compiler ordering. Aside: In the case of data 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 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 +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. @@ -1794,6 +1834,7 @@ There are some more advanced barrier functions: (*) lockless_dereference(); + This can be thought of as a pointer-fetch wrapper around the smp_read_barrier_depends() data-dependency barrier. @@ -1858,7 +1899,7 @@ This is a variation on the mandatory write barrier that causes writes to weakly ordered I/O regions to be partially ordered. Its effects may go beyond the CPU->Hardware interface and actually affect the hardware at some level. -See the subsection "Locks vs I/O accesses" for more information. +See the subsection "Acquires vs I/O accesses" for more information. =============================== @@ -1873,8 +1914,8 @@ provide more substantial guarantees, but these may not be relied upon outside of arch specific code. -ACQUIRING FUNCTIONS -------------------- +LOCK ACQUISITION FUNCTIONS +-------------------------- The Linux kernel has a number of locking constructs: @@ -1895,7 +1936,7 @@ for each construct. These operations all imply certain barriers: Memory operations issued before the ACQUIRE may be completed after the ACQUIRE operation has completed. An smp_mb__before_spinlock(), combined with a following ACQUIRE, orders prior stores against - subsequent loads and stores. Note that this is weaker than smp_mb()! + subsequent loads and stores. Note that this is weaker than smp_mb()! The smp_mb__before_spinlock() primitive is free on many architectures. (2) RELEASE operation implication: @@ -2090,9 +2131,9 @@ or: event_indicated = 1; wake_up_process(event_daemon); -A write memory barrier is implied by wake_up() and co. if and only if they wake -something up. The barrier occurs before the task state is cleared, and so sits -between the STORE to indicate the event and the STORE to set TASK_RUNNING: +A write memory barrier is implied by wake_up() and co. if and only if they +wake something up. The barrier occurs before the task state is cleared, and so +sits between the STORE to indicate the event and the STORE to set TASK_RUNNING: CPU 1 CPU 2 =============================== =============================== @@ -2206,7 +2247,7 @@ three CPUs; then should the following sequence of events occur: 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: +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 @@ -2486,9 +2527,9 @@ The following operations are special locking primitives: clear_bit_unlock(); __clear_bit_unlock(); -These implement ACQUIRE-class and RELEASE-class operations. These should be used in -preference to other operations when implementing locking primitives, because -their implementations can be optimised on many architectures. +These implement ACQUIRE-class and RELEASE-class operations. These should be +used in preference to other operations when implementing locking primitives, +because their implementations can be optimised on many architectures. [!] Note that special memory barrier primitives are available for these situations because on some CPUs the atomic instructions used imply full memory @@ -2568,12 +2609,12 @@ 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. If this isn't sufficient then an +registers that form implicit I/O barriers. If this isn't sufficient then an mmiowb() may need to be used explicitly. 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 +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. @@ -2587,8 +2628,8 @@ functions: (*) inX(), outX(): These are intended to talk to I/O space rather than memory space, but - that's primarily a CPU-specific concept. The i386 and x86_64 processors do - indeed have special I/O space access cycles and instructions, but many + that's primarily a CPU-specific concept. The i386 and x86_64 processors + do indeed have special I/O space access cycles and instructions, but many CPUs don't have such a concept. The PCI bus, amongst others, defines an I/O space concept which - on such @@ -2610,7 +2651,7 @@ functions: Whether these are guaranteed to be fully ordered and uncombined with respect to each other on the issuing CPU depends on the characteristics - defined for the memory window through which they're accessing. On later + defined for the memory window through which they're accessing. On later i386 architecture machines, for example, this is controlled by way of the MTRR registers. @@ -2635,10 +2676,10 @@ functions: (*) readX_relaxed(), writeX_relaxed() These are similar to readX() and writeX(), but provide weaker memory - ordering guarantees. Specifically, they do not guarantee ordering with + ordering guarantees. Specifically, they do not guarantee ordering with respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee - ordering with respect to LOCK or UNLOCK operations. If the latter is - required, an mmiowb() barrier can be used. Note that relaxed accesses to + ordering with respect to LOCK or UNLOCK operations. If the latter is + required, an mmiowb() barrier can be used. Note that relaxed accesses to the same peripheral are guaranteed to be ordered with respect to each other. @@ -3040,8 +3081,9 @@ The Alpha defines the Linux kernel's memory barrier model. See the subsection on "Cache Coherency" above. + 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 @@ -3050,7 +3092,7 @@ 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 +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. @@ -3058,6 +3100,7 @@ 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 ============ |