Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754805Ab0GMSVI (ORCPT ); Tue, 13 Jul 2010 14:21:08 -0400 Received: from claw.goop.org ([74.207.240.146]:33861 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752725Ab0GMSVG (ORCPT ); Tue, 13 Jul 2010 14:21:06 -0400 Message-ID: <4C3CAE8F.10900@goop.org> Date: Tue, 13 Jul 2010 11:21:03 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Lightning/1.0b2pre Thunderbird/3.0.5 MIME-Version: 1.0 To: Linus Torvalds CC: Peter Palfrader , Avi Kivity , Greg KH , linux-kernel@vger.kernel.org, stable@kernel.org, stable-review@kernel.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Glauber Costa , Zachary Amsden , Marcelo Tosatti Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock References: <20100707124731.GJ15122@anguilla.noreply.org> <4C359D5A.1050906@redhat.com> <20100713102350.GW15122@anguilla.noreply.org> <4C3C68C8.4060409@redhat.com> <20100713141902.GB15122@anguilla.noreply.org> <4C3C8CE5.1080705@redhat.com> <20100713162207.GC15122@anguilla.noreply.org> <4C3C9589.4090602@redhat.com> <4C3C96EC.8060901@redhat.com> <4C3C9839.4090404@redhat.com> <20100713172526.GE15122@anguilla.noreply.org> In-Reply-To: X-Enigmail-Version: 1.0.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1516 Lines: 33 On 07/13/2010 10:59 AM, Linus Torvalds wrote: > Btw, this second patch was a bit more aggressive than the first one, > and actually removes the "memory" clobber entirely, and the fake cast > of the target type. > > That shouldn't matter _except_ if people actually use cmpxchg to > implement their own locking, since now the compiler could potentially > move unrelated memory references around the lock. Of course, if you > use cmpxchg to implement your own locking, you're probably doing > something wrong anyway (ie you'll get the wrong memory barriers on > various architectures), so it should all be fine. > There are some places which rely on xchg/cmpxchg being a barrier in arch-specific code. For example, the Xen code uses as part of the shared memory protocol with the hypervisor. > But I thought I'd mention it. And I don't really know how much gcc > moves memory accesses around a "asm volatile" - the gcc docs are > historically very vague ("volatile asms aren't moved around > 'significantly'", whatever 'significant' means) > "asm volatile"'s only real meaning is that it will not get elided if it appears its output is unused (assuming it is reachable at all). I don't think you can consider it having any meaningful effects on ordering. J -- 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/