Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965720AbXIKC3u (ORCPT ); Mon, 10 Sep 2007 22:29:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761911AbXIKC3n (ORCPT ); Mon, 10 Sep 2007 22:29:43 -0400 Received: from gate.crashing.org ([63.228.1.57]:53106 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761239AbXIKC3l (ORCPT ); Mon, 10 Sep 2007 22:29:41 -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: <8239c744e7ef64a6f5a15449d337ac68@kernel.crashing.org> 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: Tue, 11 Sep 2007 04:27:22 +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: 1013 Lines: 27 >> "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. > > Stores can be reordered. Only x86 has (mostly) implicit write ordering. > So no atomic_dec has no volatile semantics Read again: I said the C "volatile" construct has nothing to do with CPU memory access reordering. > and may be reordered on a variety > of processors. Writes to memory may not follow code order on several > processors. The _compiler_ isn't allowed to reorder things here. Yes, of course you do need stronger barriers for many purposes, volatile isn't all that useful you know. 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/