Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756738AbXHRA2L (ORCPT ); Fri, 17 Aug 2007 20:28:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751434AbXHRA17 (ORCPT ); Fri, 17 Aug 2007 20:27:59 -0400 Received: from gate.crashing.org ([63.228.1.57]:47672 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750900AbXHRA16 (ORCPT ); Fri, 17 Aug 2007 20:27:58 -0400 In-Reply-To: References: <20070816003948.GY9645@linux.vnet.ibm.com> <18115.44462.622801.683446@cargo.ozlabs.ibm.com> <20070816020042.GA30650@gondor.apana.org.au> <18115.45316.702491.681906@cargo.ozlabs.ibm.com> <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <20070816053945.GB32442@gondor.apana.org.au> <18115.62741.807704.969977@cargo.ozlabs.ibm.com> <20070816070907.GA964@gondor.apana.org.au> <46C4ABA5.9010804@redhat.com> Mime-Version: 1.0 (Apple Message framework v623) Content-Type: text/plain; charset=US-ASCII; format=flowed Message-Id: Content-Transfer-Encoding: 7bit Cc: Christoph Lameter , Paul Mackerras , heiko.carstens@de.ibm.com, horms@verge.net.au, Stefan Richter , Linux Kernel Mailing List , David Miller , "Paul E. McKenney" , =?ISO-8859-1?Q?Ilpo_J=E4rvinen?= , ak@suse.de, cfriesen@nortel.com, rpjday@mindspring.com, Netdev , jesper.juhl@gmail.com, linux-arch@vger.kernel.org, zlynx@acm.org, Andrew Morton , schwidefsky@de.ibm.com, Chris Snook , Herbert Xu , Linus Torvalds , wensong@linux-vs.org, wjiang@resilience.com 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 To: Satyam Sharma X-Mailer: Apple Mail (2.623) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1537 Lines: 41 >>> 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 linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/