Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933333AbXHHGs0 (ORCPT ); Wed, 8 Aug 2007 02:48:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755911AbXHHGsR (ORCPT ); Wed, 8 Aug 2007 02:48:17 -0400 Received: from mx1.redhat.com ([66.187.233.31]:36956 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755410AbXHHGsQ (ORCPT ); Wed, 8 Aug 2007 02:48:16 -0400 Message-ID: <46B96719.3050006@redhat.com> Date: Wed, 08 Aug 2007 02:47:53 -0400 From: Chris Snook User-Agent: Thunderbird 1.5.0.12 (Macintosh/20070509) MIME-Version: 1.0 To: Chris Friesen CC: Zan Lynx , Jerry Jiang , "Robert P. J. Day" , Linux Kernel Mailing List Subject: Re: why are some atomic_t's not volatile, while most are? References: <20070806123551.a6c3c154.wjiang@resilience.com> <46B72C58.5030502@redhat.com> <46B894E4.4010501@nortel.com> <46B8D6D7.2020206@redhat.com> <46B8DDF3.7050008@nortel.com> <46B8E1D3.8050501@redhat.com> <46B8E64E.7010708@nortel.com> <1186525977.232321.43.camel@localhost> <46B91D0C.5050704@redhat.com> <46B94B94.6000600@nortel.com> In-Reply-To: <46B94B94.6000600@nortel.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 964 Lines: 23 Chris Friesen wrote: > Chris Snook wrote: > >> This is not a problem, since indirect references will cause the CPU to >> fetch the data from memory/cache anyway. > > Isn't Zan's sample code (that shows the problem) already using indirect > references? Yeah, I misinterpreted his conclusion. I thought about this for a while, and realized that it's perfectly legal for the compiler to re-use a value obtained from atomic_read. All that matters is that the read itself was atomic. The use (or non-use) of the volatile keyword is really more relevant to the other atomic operations. If you want to guarantee a re-read from memory, use barrier(). This, incidentally, uses volatile under the hood. -- Chris - 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/