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


On Sat, 18 Aug 2007, Segher Boessenkool wrote:

> > > > 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.
>
> I'm not sure what in that mail you mean, but anyway...
>
> Yes, of course, the fact that "volatile" creates a side effect
> prevents certain things from being reordered wrt the atomic_dec();
> but the atomic_dec() has a side effect *already* so the volatile
> doesn't change anything.

That's precisely what that sub-thread (read down to the last mail
there, and not the first mail only) shows. So yes, "volatile" does
have something to do with re-ordering (as guaranteed by the C
standard).


> > > 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.
>
> The "asm volatile" implementation does have exactly the same
> reordering guarantees as the "volatile cast" thing,

I don't think so.

> if that is
> implemented by GCC in the "obvious" way. Even a "plain" asm()
> will do the same.

Read the relevant GCC documentation.

[ of course, if the (latest) GCC documentation is *yet again*
wrong, then alright, not much I can do about it, is there. ]
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