make atomic_read() behave consistently across all architectures

As recent discussions[1], and bugs[2] have shown, there is a great deal of confusion about the expected behavior of atomic_read(), compounded by the fact that it is not the same on all architectures. Since users expect calls to atomic_read() to actually perform a read, it is not desirable to allow the compiler to optimize this away. Requiring the use of barrier() in this case is inefficient, since we only want to re-load the atomic_t variable, not everything else in scope.

This patchset makes the behavior of atomic_read uniform by removing the volatile keyword from all atomic_t and atomic64_t definitions that currently have it, and instead explicitly casts the variable as volatile in atomic_read(). This leaves little room for creative optimization by the compiler, and is in keeping with the principles behind "volatile considered harmful".

Busy-waiters should still use cpu_relax(), but fast paths may be able to reduce their use of barrier() between some atomic_read() calls.

-- Chris

1) http://lkml.org/lkml/2007/7/1/52
2) http://lkml.org/lkml/2007/8/8/122