On Sat, 18 Aug 2007, Segher Boessenkool wrote:

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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).

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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. ]