Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754155AbXHQDGN (ORCPT ); Thu, 16 Aug 2007 23:06:13 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751009AbXHQDF7 (ORCPT ); Thu, 16 Aug 2007 23:05:59 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:59802 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750800AbXHQDF5 (ORCPT ); Thu, 16 Aug 2007 23:05:57 -0400 Date: Thu, 16 Aug 2007 20:03:55 -0700 (PDT) From: Linus Torvalds To: Paul Mackerras cc: Christoph Lameter , Chris Snook , Ilpo J?rvinen , Herbert Xu , Satyam Sharma , "Paul E. McKenney" , Stefan Richter , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Netdev , Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, David Miller , schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: <18117.1287.779351.836552@cargo.ozlabs.ibm.com> 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> <18117.1287.779351.836552@cargo.ozlabs.ibm.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: 1247 Lines: 31 On Fri, 17 Aug 2007, Paul Mackerras wrote: > > Volatile doesn't mean it can't be reordered; volatile means the > accesses can't be eliminated. It also does limit re-ordering. Of course, since *normal* accesses aren't necessarily limited wrt re-ordering, the question then becomes one of "with regard to *what* does it limit re-ordering?". A C compiler that re-orders two different volatile accesses that have a sequence point in between them is pretty clearly a buggy compiler. So at a minimum, it limits re-ordering wrt other volatiles (assuming sequence points exists). It also means that the compiler cannot move it speculatively across conditionals, but other than that it's starting to get fuzzy. In general, I'd *much* rather we used barriers. Anything that "depends" on volatile is pretty much set up to be buggy. But I'm certainly also willing to have that volatile inside "atomic_read/atomic_set()" if it avoids code that would otherwise break - ie if it hides a bug. Linus - 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/