Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935079AbXHGUcz (ORCPT ); Tue, 7 Aug 2007 16:32:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760575AbXHGUcp (ORCPT ); Tue, 7 Aug 2007 16:32:45 -0400 Received: from mx1.redhat.com ([66.187.233.31]:47718 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760806AbXHGUcp (ORCPT ); Tue, 7 Aug 2007 16:32:45 -0400 Message-ID: <46B8D6D7.2020206@redhat.com> Date: Tue, 07 Aug 2007 16:32:23 -0400 From: Chris Snook User-Agent: Thunderbird 2.0.0.0 (X11/20070419) MIME-Version: 1.0 To: Chris Friesen CC: 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> In-Reply-To: <46B894E4.4010501@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: 1241 Lines: 29 Chris Friesen wrote: > Chris Snook wrote: > >> If your architecture doesn't support SMP, the volatile keyword doesn't >> do anything except add a useless memory fetch. > > I was under the impression that there were other cases as well > (interrupt handlers, for instance) where the value could be modified > "behind the back" of the current code. When you're accessing data that could be modified by an interrupt handler, you generally use a function that calls arch-specific inline assembler to explicitly fetch it from memory. > It seems like this would fall more into the case of the arch providing > guarantees when using locked/atomic access rather than anything > SMP-related, no?. But if you're not using SMP, the only way you get a race condition is if your compiler is reordering instructions that have side effects which are invisible to the compiler. This can happen with MMIO registers, but it's not an issue with an atomic_t we're declaring in real memory. -- 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/