Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937437AbXHPAld (ORCPT ); Wed, 15 Aug 2007 20:41:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S937410AbXHPAki (ORCPT ); Wed, 15 Aug 2007 20:40:38 -0400 Received: from netops-testserver-4-out.sgi.com ([192.48.171.29]:55902 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S937402AbXHPAkf (ORCPT ); Wed, 15 Aug 2007 20:40:35 -0400 Date: Wed, 15 Aug 2007 17:40:33 -0700 (PDT) From: Christoph Lameter X-X-Sender: clameter@schroedinger.engr.sgi.com To: Paul Mackerras cc: Satyam Sharma , Stefan Richter , 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, Herbert Xu , "Paul E. McKenney" Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: <18115.39815.730873.346526@cargo.ozlabs.ibm.com> Message-ID: References: <20070809131423.GA9927@shell.boston.redhat.com> <46C2D6F3.3070707@s5r6.in-berlin.de> <18115.35524.56393.347841@cargo.ozlabs.ibm.com> <18115.39815.730873.346526@cargo.ozlabs.ibm.com> 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: 1201 Lines: 24 On Thu, 16 Aug 2007, Paul Mackerras wrote: > Those barriers are for when we need ordering between atomic variables > and other memory locations. An atomic variable by itself doesn't and > shouldn't need any barriers for other CPUs to be able to see what's > happening to it. It does not need any barriers. As soon as one cpu acquires the cacheline for write it will be invalidated in the caches of the others. So the other cpu will have to refetch. No need for volatile. The issue here may be that the compiler has fetched the atomic variable earlier and put it into a register. However, that prefetching is limited because it cannot cross functions calls etc. The only problem could be loops where the compiler does not refetch the variable since it assumes that it does not change and there are no function calls in the body of the loop. But AFAIK these loops need cpu_relax and other measures anyways to avoid bad effects from busy waiting. - 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/