Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures | Date | Sat, 18 Aug 2007 01:17:43 +0200 |
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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.
>> 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, if that is implemented by GCC in the "obvious" way. Even a "plain" asm() will do the same.
Segher
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