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Date:	Sat, 18 Aug 2007 01:17:43 +0200
From:	Segher Boessenkool <>
To:	Satyam Sharma <>
Cc:	Christoph Lameter <>,
	Paul Mackerras <>,,, Stefan Richter <>,
	Linux Kernel Mailing List <>,
	David Miller <>,
	"Paul E. McKenney" <>,
	Ilpo Järvinen <>,,,,
	Netdev <>,,,,
	Andrew Morton <>,, Chris Snook <>,
	Herbert Xu <>,
	Linus Torvalds <>,,
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

>>> 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
> (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.


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