Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763001AbXHQMmD (ORCPT ); Fri, 17 Aug 2007 08:42:03 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758132AbXHQMlu (ORCPT ); Fri, 17 Aug 2007 08:41:50 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:46092 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756991AbXHQMlt (ORCPT ); Fri, 17 Aug 2007 08:41:49 -0400 Date: Fri, 17 Aug 2007 18:20:25 +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: <46C58B93.5000408@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> <46C58B93.5000408@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: 2663 Lines: 74 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". > 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. > > > 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". 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. > 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. Satyam - 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/