Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762287AbXHQSZ6 (ORCPT ); Fri, 17 Aug 2007 14:25:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755114AbXHQSZq (ORCPT ); Fri, 17 Aug 2007 14:25:46 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:41377 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750779AbXHQSZo (ORCPT ); Fri, 17 Aug 2007 14:25:44 -0400 Date: Sat, 18 Aug 2007 00:08:01 +0530 (IST) From: Satyam Sharma X-X-Sender: satyam@enigma.security.iitk.ac.in To: Segher Boessenkool 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-8859-15?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 Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: Message-ID: 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 Content-Type: TEXT/PLAIN; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1511 Lines: 37 On Fri, 17 Aug 2007, Segher Boessenkool wrote: > > > 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. 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. > 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. - 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/