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


On Fri, 17 Aug 2007, Segher Boessenkool wrote:

> > > atomic_dec() already has volatile behavior everywhere, so this is
> > > semantically
> > > okay, but this code (and any like it) should be calling cpu_relax() each
> > > iteration through the loop, unless there's a compelling reason not to.
> > > I'll
> > > allow that for some hardware drivers (possibly this one) such a compelling
> > > reason may exist, but hardware-independent core subsystems probably have
> > > no
> > > excuse.
> >
> > No it does not have any volatile semantics. atomic_dec() can be reordered
> > at will by the compiler within the current basic unit if you do not add a
> > barrier.
>
> "volatile" has nothing to do with reordering.

If you're talking of "volatile" the type-qualifier keyword, then
http://lkml.org/lkml/2007/8/16/231 (and sub-thread below it) shows
otherwise.

> atomic_dec() writes
> to memory, so it _does_ have "volatile semantics", implicitly, as
> long as the compiler cannot optimise the atomic variable away
> completely -- any store counts as a side effect.

I don't think an atomic_dec() implemented as an inline "asm volatile"
or one that uses a "forget" macro would have the same re-ordering
guarantees as an atomic_dec() that uses a volatile access cast.
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