[PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxx>
---
 Documentation/memory-barriers.txt | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 102dc19c4119..ad6db1d48f1f 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -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:
-- 
1.8.1.5

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