Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758911AbXHRVy2 (ORCPT ); Sat, 18 Aug 2007 17:54:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757583AbXHRVyS (ORCPT ); Sat, 18 Aug 2007 17:54:18 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:41446 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757544AbXHRVyR (ORCPT ); Sat, 18 Aug 2007 17:54:17 -0400 Date: Sat, 18 Aug 2007 14:54:09 -0700 From: "Paul E. McKenney" To: Linus Torvalds Cc: Satyam Sharma , Christoph Lameter , 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 Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Message-ID: <20070818215409.GC7628@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <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 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1487 Lines: 47 On Fri, Aug 17, 2007 at 09:13:35PM -0700, 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. > > 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. But when talking about "volatile", > that's exactly what ytou always get (and always have gotten - this is > not a regression, and I doubt gcc is alone in this). One of the gcc guys claimed that he thought that the two-instruction sequence would be faster on some x86 machines. I pointed out that there might be a concern about code size. I chose not to point out that people might also care about the other x86 machines. ;-) Thanx, Paul - 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/