Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761688AbXHQRxW (ORCPT ); Fri, 17 Aug 2007 13:53:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756825AbXHQRxJ (ORCPT ); Fri, 17 Aug 2007 13:53:09 -0400 Received: from gate.crashing.org ([63.228.1.57]:34537 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756207AbXHQRxH (ORCPT ); Fri, 17 Aug 2007 13:53:07 -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: Paul Mackerras , heiko.carstens@de.ibm.com, horms@verge.net.au, Stefan Richter , Satyam Sharma , 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, Andrew Morton , zlynx@acm.org, 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: Fri, 17 Aug 2007 19:41:13 +0200 To: Christoph Lameter X-Mailer: Apple Mail (2.623) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1096 Lines: 31 >> atomic_dec() already has volatile behavior everywhere, so this is >> semantically >> okay, but this code (and any like it) should be calling cpu_relax() >> each >> iteration through the loop, unless there's a compelling reason not >> to. I'll >> allow that for some hardware drivers (possibly this one) such a >> compelling >> reason may exist, but hardware-independent core subsystems probably >> have no >> excuse. > > 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. 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. 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/