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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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