From mboxrd@z Thu Jan 1 00:00:00 1970 From: Satyam Sharma Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Sat, 18 Aug 2007 19:06:33 +0530 (IST) Message-ID: References: <20070815234021.GA28775@gondor.apana.org.au> <3694fb2e4ed1e4d9bf873c0d050c911e@kernel.crashing.org> <46C3B50E.7010702@yahoo.com.au> <194369f4c96ea0e24decf8f9197d5bad@kernel.crashing.org> <46C505B2.6030704@yahoo.com.au> <18117.4848.695269.72976@cargo.ozlabs.ibm.com> <46C516BA.60700@yahoo.com.au> <20070817235912.GA24314@linux.vnet.ibm.com> <20070818000913.GA25585@gondor.apana.org.au> <20070818010818.GQ8464@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Cc: Christoph Lameter , "Paul E. McKenney" , Herbert Xu , Nick Piggin , Paul Mackerras , Segher Boessenkool , heiko.carstens@de.ibm.com, horms@verge.net.au, linux-kernel@vger.kernel.org, rpjday@mindspring.com, ak@suse.de, netdev@vger.kernel.org, cfriesen@nortel.com, akpm@linux-foundation.org, jesper.juhl@gmail.com, linux-arch@vger.kernel.org, zlynx@acm.org, schwidefsky@de.ibm.com, Chris Snook , davem@davemloft.net, wensong@linux-vs.org, wjiang@resilience.com To: Linus Torvalds Return-path: Received: from pentafluge.infradead.org ([213.146.154.40]:45661 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750987AbXHRNYF (ORCPT ); Sat, 18 Aug 2007 09:24:05 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, 17 Aug 2007, Linus Torvalds wrote: > On Sat, 18 Aug 2007, Satyam Sharma wrote: > > > > No code does (or would do, or should do): > > > > x.counter++; > > > > on an "atomic_t x;" anyway. > > That's just an example of a general problem. > > No, you don't use "x.counter++". But you *do* use > > if (atomic_read(&x) <= 1) > > and loading into a register is stupid and pointless, when you could just > do it as a regular memory-operand to the cmp instruction. True, but that makes this a bad/poor code generation issue with the compiler, not something that affects the _correctness_ of atomic ops if "volatile" is used for that counter object (as was suggested), because we'd always use the atomic_inc() etc primitives to do increments, which are always (should be!) implemented to be atomic. > And as far as the compiler is concerned, the problem is the 100% same: > combining operations with the volatile memop. > > The fact is, a compiler that thinks that > > movl mem,reg > cmpl $val,reg > > is any better than > > cmpl $val,mem > > is just not a very good compiler. Absolutely, this is definitely a bug report worth opening with gcc. And what you've said to explain this previously sounds definitely correct -- seeing "volatile" for any access does appear to just scare the hell out of gcc and makes it generate such (poor) code. Satyam