On Wed, 3 Apr 2013, Johannes Weiner wrote: > > As stated, it doesn't. I made the comment "for what it's worth" that > > ACCESS_ONCE() doesn't do anything to "prevent the compiler from > > re-fetching" as the changelog insists it does. > > That's exactly what it does: > > /* > * Prevent the compiler from merging or refetching accesses. > > This is the guarantee ACCESS_ONCE() gives, users should absolutely be > allowed to rely on this literal definition. The underlying gcc > implementation does not matter one bit. That's the whole point of > abstraction! > The C99 and earlier C standards do not provide any way of "preventing the compiler from refetching accesses," and in fact C99 leaves an access to a volatile qualified object as implementation defined. (If you disagree, then specify what exactly about ACCESS_ONCE() prevents the compiler from doing so.) I agree that comment is confusing unless you specify that gcc's implementation provides that guarantee and I would tend to agree with Paul's assessment that the wide majority (all?) of compilers do the same. I would hesitate to say positively that gcc will continue to implement anything in the future other than what the standard specifies, though. But I do agree that the comment is confusing. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>