Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757397AbXHQJZ5 (ORCPT ); Fri, 17 Aug 2007 05:25:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750925AbXHQJZq (ORCPT ); Fri, 17 Aug 2007 05:25:46 -0400 Received: from webmail.icp-qv1-irony3.iinet.net.au ([203.59.1.108]:9612 "EHLO webmail.icp-qv1-irony3.iinet.net.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751009AbXHQJZo (ORCPT ); Fri, 17 Aug 2007 05:25:44 -0400 X-Greylist: delayed 619 seconds by postgrey-1.27 at vger.kernel.org; Fri, 17 Aug 2007 05:25:43 EDT X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Ao8CAPsDxUbLrQKa/2dsb2JhbAA X-IronPort-AV: i="4.19,274,1183305600"; d="scan'208"; a="183003105:sNHT9611358" Message-ID: <46C5672E.4060003@cyberone.com.au> Date: Fri, 17 Aug 2007 19:15:26 +1000 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20051007 Debian/1.7.12-1 X-Accept-Language: en MIME-Version: 1.0 To: Satyam Sharma CC: Stefan Richter , paulmck@linux.vnet.ibm.com, Herbert Xu , Paul Mackerras , Christoph Lameter , 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 References: <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> <46C40587.7050708@s5r6.in-berlin.de> <20070816081049.GA1431@gondor.apana.org.au> <46C41EE4.9090806@s5r6.in-berlin.de> <46C42767.4070104@s5r6.in-berlin.de> <20070816104250.GB2927@gondor.apana.org.au> <20070816163441.GB16957@linux.vnet.ibm.com> <46C512EB.7020603@yahoo.com.au> <46C54D74.60101@s5r6.in-berlin.de> <46C556F1.8000407@yahoo.com.au> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1625 Lines: 35 Satyam Sharma wrote: > > On Fri, 17 Aug 2007, Nick Piggin wrote: >>>Sure, now >>>that I learned of these properties I can start to audit code and insert >>>barriers where I believe they are needed, but this simply means that >>>almost all occurrences of atomic_read will get barriers (unless there >>>already are implicit but more or less obvious barriers like msleep). >> >>You might find that these places that appear to need barriers are >>buggy for other reasons anyway. Can you point to some in-tree code >>we can have a look at? > > > Such code was mentioned elsewhere (query nodemgr_host_thread in cscope) > that managed to escape the requirement for a barrier only because of > some completely un-obvious compilation-unit-scope thing. But I find such > an non-explicit barrier quite bad taste. Stefan, do consider plunking an > explicit call to barrier() there. It is very obvious. msleep calls schedule() (ie. sleeps), which is always a barrier. The "unobvious" thing is that you wanted to know how the compiler knows a function is a barrier -- answer is that if it does not *know* it is not a barrier, it must assume it is a barrier. If the whole msleep call chain including the scheduler were defined static in the current compilation unit, then it would still be a barrier because it would actually be able to see the barriers in schedule(void), if nothing else. - 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/