Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760991AbXHQGOP (ORCPT ); Fri, 17 Aug 2007 02:14:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758188AbXHQGOD (ORCPT ); Fri, 17 Aug 2007 02:14:03 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:56668 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758080AbXHQGOA (ORCPT ); Fri, 17 Aug 2007 02:14:00 -0400 Date: Fri, 17 Aug 2007 11:56:22 +0530 (IST) From: Satyam Sharma X-X-Sender: satyam@enigma.security.iitk.ac.in To: Herbert Xu cc: Paul Mackerras , Linus Torvalds , Christoph Lameter , Chris Snook , Ilpo Jarvinen , "Paul E. McKenney" , Stefan Richter , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Netdev , Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, David Miller , 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: <20070817035342.GA14744@gondor.apana.org.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> <46C4ABA5.9010804@redhat.com> <18117.1287.779351.836552@cargo.ozlabs.ibm.com> <18117.6495.397597.582736@cargo.ozlabs.ibm.com> <20070817035342.GA14744@gondor.apana.org.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: 2114 Lines: 53 On Fri, 17 Aug 2007, Herbert Xu wrote: > On Fri, Aug 17, 2007 at 01:43:27PM +1000, Paul Mackerras wrote: > > > > The cost of doing so seems to me to be well down in the noise - 44 > > bytes of extra kernel text on a ppc64 G5 config, and I don't believe > > the extra few cycles for the occasional extra load would be measurable > > (they should all hit in the L1 dcache). I don't mind if x86[-64] have > > atomic_read/set be nonvolatile and find all the missing barriers, but > > for now on powerpc, I think that not having to find those missing > > barriers is worth the 0.00076% increase in kernel text size. > > BTW, the sort of missing barriers that triggered this thread > aren't that subtle. It'll result in a simple lock-up if the > loop condition holds upon entry. At which point it's fairly > straightforward to find the culprit. Not necessarily. A barrier-less buggy code such as below: atomic_set(&v, 0); ... /* some initial code */ while (atomic_read(&v)) ; ... /* code that MUST NOT be executed unless v becomes non-zero */ (where v->counter is has no volatile access semantics) could be generated by the compiler to simply *elid* or *do away* with the loop itself, thereby making the: "/* code that MUST NOT be executed unless v becomes non-zero */" to be executed even when v is zero! That is subtle indeed, and causes no hard lockups. Granted, the above IS buggy code. But, the stated objective is to avoid heisenbugs. And we have driver / subsystem maintainers such as Stefan coming up and admitting that often a lot of code that's written to use atomic_read() does assume the read will not be elided by the compiler. See, I agree, "volatility" semantics != what we often want. However, if what we want is compiler barrier, for only the object under consideration, "volatility" semantics aren't really "nonsensical" or anything. - 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/