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Diffstat (limited to 'Documentation/memory-barriers.txt')
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1 files changed, 30 insertions, 28 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index b6882680247e..7deee1441640 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1343,13 +1343,13 @@ 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 -is visible at the same time to all CPUs, or, alternatively, that all -CPUs agree on the order in which all stores took place. However, use of -full multicopy atomicity would rule out valuable hardware optimizations, -so a weaker form called ``other multicopy atomicity'' instead guarantees -that a given store is observed at the same time by all -other- CPUs. The -remainder of this document discusses this weaker form, but for brevity -will call it simply ``multicopy atomicity''. +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: @@ -1360,24 +1360,26 @@ The following example demonstrates multicopy atomicity: <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 -that CPU 3's load from Y returns 1. This indicates that CPU 2's load -from X in some sense follows CPU 1's store to X and that CPU 2's store -to Y in some sense preceded CPU 3's load from Y. The question is then -"Can CPU 3's load from X return 0?" +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 came after CPU 2's load, it +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 is an example of multicopy atomicity: if a load executing -on CPU A follows a load from the same variable executing on CPU B, then -an understandable but incorrect expectation is that CPU A's load must -either return the same value that CPU B's load did, or must return some -later value. - -In the Linux kernel, the above use of a general memory barrier compensates -for any lack of multicopy atomicity. Therefore, in the above example, -if CPU 2's load from X returns 1 and its load from Y returns 0, and CPU 3's -load from Y returns 1, then CPU 3's load from X must also 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 @@ -1396,11 +1398,11 @@ 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. Therefore, -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. A general barrier is therefore required to ensure -that all CPUs agree on the combined order of CPU 1's and CPU 2's accesses. +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- |