Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757629AbXHRAIp (ORCPT ); Fri, 17 Aug 2007 20:08:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752796AbXHRAIe (ORCPT ); Fri, 17 Aug 2007 20:08:34 -0400 Received: from gate.crashing.org ([63.228.1.57]:57610 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752348AbXHRAId (ORCPT ); Fri, 17 Aug 2007 20:08:33 -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 02:04:35 +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: 1508 Lines: 41 >>>> 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. "asm volatile" creates a side effect. Side effects aren't allowed to be reordered wrt sequence points. This is exactly the same reason as why "volatile accesses" cannot be reordered. >> if that is >> implemented by GCC in the "obvious" way. Even a "plain" asm() >> will do the same. > > Read the relevant GCC documentation. I did, yes. > [ of course, if the (latest) GCC documentation is *yet again* > wrong, then alright, not much I can do about it, is there. ] There was (and is) nothing wrong about the "+m" documentation, if that is what you are talking about. It could be extended now, to allow "+m" -- but that takes more than just "fixing" the documentation. 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/