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Date: Sat, 18 Aug 2007 01:17:43 +0200 From: Segher Boessenkool <segher@...nel.crashing.org> To: Satyam Sharma <satyam@...radead.org> Cc: Christoph Lameter <clameter@....com>, Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com, horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, David Miller <davem@...emloft.net>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>, ak@...e.de, cfriesen@...tel.com, rpjday@...dspring.com, Netdev <netdev@...r.kernel.org>, jesper.juhl@...il.com, linux-arch@...r.kernel.org, zlynx@....org, Andrew Morton <akpm@...ux-foundation.org>, schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>, Herbert Xu <herbert@...dor.apana.org.au>, Linus Torvalds <torvalds@...ux-foundation.org>, wensong@...ux-vs.org, wjiang@...ilience.com 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 > 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 - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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