Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764092AbXHQM5A (ORCPT ); Fri, 17 Aug 2007 08:57:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759049AbXHQM4t (ORCPT ); Fri, 17 Aug 2007 08:56:49 -0400 Received: from webmail.icp-qv1-irony3.iinet.net.au ([203.59.1.108]:32270 "EHLO webmail.icp-qv1-irony3.iinet.net.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758923AbXHQM4s (ORCPT ); Fri, 17 Aug 2007 08:56:48 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Ao8CAIY3xUbLrQKa/2dsb2JhbAA X-IronPort-AV: i="4.19,275,1183305600"; d="scan'208"; a="183085072:sNHT10326522" Message-ID: <46C59B09.8040004@cyberone.com.au> Date: Fri, 17 Aug 2007 22:56:41 +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> <46C5672E.4060003@cyberone.com.au> <46C58B93.5000408@cyberone.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: 3168 Lines: 102 Satyam Sharma wrote: > >On Fri, 17 Aug 2007, Nick Piggin wrote: > > >>Satyam Sharma wrote: >> >>>On Fri, 17 Aug 2007, Nick Piggin wrote: >>> >>>>Satyam Sharma wrote: >>>> >>>>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. >>> >>Where did I say it is a barrier because it sleeps? >> > >Just below. What you wrote: > > >>It is always a barrier because, at the lowest level, schedule() (and thus >>anything that sleeps) is defined to always be a barrier. >> > >"It is always a barrier because, at the lowest level, anything that sleeps >is defined to always be a barrier". > ... because it must call schedule and schedule is a barrier. >>Regardless of >>whatever obscure means the compiler might need to infer the barrier. >> >>In other words, you can ignore those obscure details because schedule() is >>always going to have an explicit barrier in it. >> > >I didn't quite understand what you said here, so I'll tell what I think: > >* foo() is a compiler barrier if the definition of foo() is invisible to > the compiler at a callsite. > >* foo() is also a compiler barrier if the definition of foo() includes > a barrier, and it is inlined at the callsite. > >If the above is wrong, or if there's something else at play as well, >do let me know. > Right. >>>>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? >>> >>No. If you accept that barrier() is implemented correctly, and you know >>that sleeping is defined to be a barrier, >> > >Curiously, that's the second time you've said "sleeping is defined to >be a (compiler) barrier". > _In Linux,_ sleeping is defined to be a compiler barrier. >How does the compiler even know if foo() is >a function that "sleeps"? Do compilers have some notion of "sleeping" >to ensure they automatically assume a compiler barrier whenever such >a function is called? Or are you saying that the compiler can see the >barrier() inside said function ... nopes, you're saying quite the >opposite below. > You're getting too worried about the compiler implementation. Start by assuming that it does work ;) >>then its perfectly clear. You >>don't have to know how the compiler "knows" that some function contains >>a barrier. >> > >I think I do, why not? Would appreciate if you could elaborate on this. > If a function is not completely visible to the compiler (so it can't determine whether a barrier could be in it or not), then it must always assume it will contain a barrier so it always does the right thing. - 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/