Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761393AbXHPFkg (ORCPT ); Thu, 16 Aug 2007 01:40:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752366AbXHPFkL (ORCPT ); Thu, 16 Aug 2007 01:40:11 -0400 Received: from rhun.apana.org.au ([64.62.148.172]:4187 "EHLO arnor.apana.org.au" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752064AbXHPFkH (ORCPT ); Thu, 16 Aug 2007 01:40:07 -0400 Date: Thu, 16 Aug 2007 13:39:45 +0800 From: Herbert Xu To: Paul Mackerras Cc: Satyam Sharma , Christoph Lameter , "Paul E. McKenney" , Stefan Richter , Chris Snook , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Linus Torvalds , netdev@vger.kernel.org, Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, 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 Message-ID: <20070816053945.GB32442@gondor.apana.org.au> References: <18115.35524.56393.347841@cargo.ozlabs.ibm.com> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <18115.52863.638655.658466@cargo.ozlabs.ibm.com> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1552 Lines: 39 On Thu, Aug 16, 2007 at 02:11:43PM +1000, Paul Mackerras wrote: > > The uses of atomic_read where one might want it to allow caching of > the result seem to me to fall into 3 categories: > > 1. Places that are buggy because of a race arising from the way it's > used. > > 2. Places where there is a race but it doesn't matter because we're > doing some clever trick. > > 3. Places where there is some locking in place that eliminates any > potential race. Agreed. > In case 1, adding volatile won't solve the race, of course, but it's > hard to argue that we shouldn't do something because it will slow down > buggy code. Case 2 is hopefully pretty rare and accompanied by large > comment blocks, and in those cases caching the result of atomic_read > explicitly in a local variable would probably make the code clearer. > And in case 3 there is no reason to use atomic_t at all; we might as > well just use an int. Since adding volatile doesn't help any of the 3 cases, and takes away optimisations from both 2 and 3, I wonder what is the point of the addition after all? Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt - 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/