Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965034Ab0HFVKR (ORCPT ); Fri, 6 Aug 2010 17:10:17 -0400 Received: from terminus.zytor.com ([198.137.202.10]:43272 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935106Ab0HFVKN (ORCPT ); Fri, 6 Aug 2010 17:10:13 -0400 Message-ID: <4C5C7A0A.9080107@zytor.com> Date: Fri, 06 Aug 2010 14:09:30 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.7) Gecko/20100720 Fedora/3.1.1-1.fc13 Thunderbird/3.1.1 MIME-Version: 1.0 To: Jeremy Fitzhardinge CC: Jan Beulich , Peter Zijlstra , Xen-devel , Avi Kivity , Nick Piggin , Linux Kernel Mailing List Subject: Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C References: <1280761639.1923.213.camel@laptop> <4C56E1A1.6020005@goop.org> <4C5C1F80020000780000EA6D@vpn.id2.novell.com> <4C5C21E3.1020004@goop.org> <4C5C6DCE.4020206@zytor.com> <4C5C71A5.3050500@goop.org> In-Reply-To: <4C5C71A5.3050500@goop.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1401 Lines: 43 On 08/06/2010 01:33 PM, Jeremy Fitzhardinge wrote: > On 08/06/2010 01:17 PM, H. Peter Anvin wrote: >> On 08/06/2010 07:53 AM, Jeremy Fitzhardinge wrote: >>> On 08/06/2010 05:43 AM, Jan Beulich wrote: >>>> You certainly mean "the compiler currently treats this as being:" - I >>>> don't think there's a guarantee it'll always be doing so. >>>> >>>>> for (;;) { >>>>> if (inc.tickets.head == inc.tickets.tail) >>>>> goto out; >>>>> ... >>>>> } >>>>> out: barrier(); >>>>> } >>>>> >>>>> (Which would probably be a reasonable way to clarify the code.) >>>> I therefore think it needs to be written this way. >>> >>> Agreed. >>> >> >> A call/return to an actual out-of-line function is a barrier (and will >> always be a barrier, as it is the fundamental ABI sequence points), >> but to an inline function it is not. > > Yes. So the goto explicitly puts the barrier into the control flow which > should stop the compiler from doing anything unexpected. > In this particular case, though, I would somewhat expect the more conventional: while (inc.tickets.head != inc.tickets.tail) { cpu_relax(); inc.tickets.head = ACCESS_ONCE(lock->tickets_head); } -hpa -- 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/