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

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: