Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762336AbXHHIWU (ORCPT ); Wed, 8 Aug 2007 04:22:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753457AbXHHIWJ (ORCPT ); Wed, 8 Aug 2007 04:22:09 -0400 Received: from 207.47.19.6.static.nextweb.net ([207.47.19.6]:19328 "EHLO ex1.resilience.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751004AbXHHIWG (ORCPT ); Wed, 8 Aug 2007 04:22:06 -0400 Date: Wed, 8 Aug 2007 16:16:34 +0800 From: Jerry Jiang To: Chris Snook Cc: Chris Friesen , Zan Lynx , "Robert P. J. Day" , Linux Kernel Mailing List Subject: Re: why are some atomic_t's not volatile, while most are? Message-Id: <20070808161634.3a49377a.wjiang@resilience.com> In-Reply-To: <46B96719.3050006@redhat.com> 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> <46B96719.3050006@redhat.com> Organization: Resilience X-Mailer: Sylpheed version 2.3.0beta5 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 08 Aug 2007 08:21:54.0437 (UTC) FILETIME=[2DC1AB50:01C7D995] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1151 Lines: 32 On Wed, 08 Aug 2007 02:47:53 -0400 Chris Snook wrote: > 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 Sorry, I can't understand it a bit .., Could you do in detail? -- Jerry > 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/