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Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence

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The ACCESS_ONCE() primitive provides cache coherence, but the
documentation does not clearly state this.  This commit therefore upgrades
the documentation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2014-01-02 15:03:50 -08:00
parent 6e67669678
commit 449f7413c8
1 changed files with 17 additions and 0 deletions
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Documentation/memory-barriers.txt
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@ -1249,6 +1249,23 @@ The ACCESS_ONCE() function 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] = ACCESS_ONCE(x);
a[1] = ACCESS_ONCE(x);
In short, ACCESS_ONCE() provides 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: