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    SubjectRe: [PATCH 0/24] make atomic_read() behave consistently across all architectures


    On Fri, 17 Aug 2007, Paul Mackerras wrote:
    >
    > Volatile doesn't mean it can't be reordered; volatile means the
    > accesses can't be eliminated.

    It also does limit re-ordering.

    Of course, since *normal* accesses aren't necessarily limited wrt
    re-ordering, the question then becomes one of "with regard to *what* does
    it limit re-ordering?".

    A C compiler that re-orders two different volatile accesses that have a
    sequence point in between them is pretty clearly a buggy compiler. So at a
    minimum, it limits re-ordering wrt other volatiles (assuming sequence
    points exists). It also means that the compiler cannot move it
    speculatively across conditionals, but other than that it's starting to
    get fuzzy.

    In general, I'd *much* rather we used barriers. Anything that "depends" on
    volatile is pretty much set up to be buggy. But I'm certainly also willing
    to have that volatile inside "atomic_read/atomic_set()" if it avoids code
    that would otherwise break - ie if it hides a bug.

    Linus
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