From 449f7413c876a229fd95362cc12bc7ade18d0661 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 2 Jan 2014 15:03:50 -0800 Subject: [PATCH] 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 Reviewed-by: Josh Triplett --- Documentation/memory-barriers.txt | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 102dc19c4119..f9ff060d8320 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: -- 2.39.5