Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763640AbXHQJvp (ORCPT ); Fri, 17 Aug 2007 05:51:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754687AbXHQJvf (ORCPT ); Fri, 17 Aug 2007 05:51:35 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:41953 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753066AbXHQJvd (ORCPT ); Fri, 17 Aug 2007 05:51:33 -0400 Date: Fri, 17 Aug 2007 15:33:33 +0530 (IST) From: Satyam Sharma X-X-Sender: satyam@enigma.security.iitk.ac.in To: Nick Piggin 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 In-Reply-To: <46C5672E.4060003@cyberone.com.au> Message-ID: 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> <46C5672E.4060003@cyberone.com.au> 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: 1897 Lines: 45 On Fri, 17 Aug 2007, Nick Piggin wrote: > 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. Probably you didn't mean that, but no, schedule() is not barrier because it sleeps. It's a barrier because it's invisible. > 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. True, that's clearly what happens here. But are you're definitely joking that this is "obvious" in terms of code-clarity, right? Just 5 minutes back you mentioned elsewhere you like seeing lots of explicit calls to barrier() (with comments, no less, hmm? :-) - 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/