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
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