Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence
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>
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 102dc19..f9ff060 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1249,6 +1249,23 @@
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: