From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S939557AbXHIMxw (ORCPT ); Thu, 9 Aug 2007 08:53:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S939138AbXHIMxO (ORCPT ); Thu, 9 Aug 2007 08:53:14 -0400 Received: from mx1.redhat.com ([66.187.233.31]:54886 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932812AbXHIMxN (ORCPT ); Thu, 9 Aug 2007 08:53:13 -0400 Message-ID: <46BB0E1F.2030003@redhat.com> Date: Thu, 09 Aug 2007 08:52:47 -0400 From: Chris Snook User-Agent: Thunderbird 2.0.0.0 (X11/20070419) MIME-Version: 1.0 To: "Robert P. J. Day" CC: Jerry Jiang , Chris Friesen , Zan Lynx , 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> <46B96719.3050006@redhat.com> <20070808162755.61f50fbf.wjiang@resilience.com> <46BA2D72.9060202@redhat.com> In-Reply-To: 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 Robert P. J. Day wrote: > On Wed, 8 Aug 2007, Chris Snook wrote: > >> Jerry Jiang wrote: >>> 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 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. >>>> >>> >>> So for example, without volatile >>> >>> int a = read_atomic(v); >>> int b = read_atomic(v); >>> >>> the compiler will optimize it as b = a, But with volatile, it will be forced >>> to fetch v's value from memory >>> again. >>> >>> So, come back our initial question, >>> include/asm-v850/atomic.h:typedef struct { int counter; } atomic_t; >>> >>> Why is it right without volatile? >> Because atomic_t doesn't promise a memory fetch every time. It merely >> promises that any atomic_* operations will, in fact, be atomic. For example, >> posted today: >> >> http://lkml.org/lkml/2007/8/8/122 > > i'm sure that, when this is all done, i'll finally have an answer to > my original question, "why are some atomic_t's not volatile, while > most are?" > > i'm almost scared to ask any more questions. :-) > > rday Momentarily I'll be posting a patchset that makes all atomic_t and atomic64_t declarations non-volatile, and casts them to volatile inside of atomic[64]_read. This will ensure consistent behavior across all architectures, and is in keeping with the philosophy that memory reads should be enforced in running code, not declarations. I hope you don't mind that we're mooting the question by making the code more sensible. -- Chris