From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751797Ab1HXWCe (ORCPT ); Wed, 24 Aug 2011 18:02:34 -0400 Received: from terminus.zytor.com ([198.137.202.10]:38746 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750924Ab1HXWCd (ORCPT ); Wed, 24 Aug 2011 18:02:33 -0400 Message-ID: <4E5574C3.6080104@zytor.com> Date: Wed, 24 Aug 2011 15:01:39 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:6.0) Gecko/20110816 Thunderbird/6.0 MIME-Version: 1.0 To: Linus Torvalds CC: Jeremy Fitzhardinge , Peter Zijlstra , Ingo Molnar , the arch/x86 maintainers , Linux Kernel Mailing List , Nick Piggin , Jeremy Fitzhardinge Subject: Re: [PATCH 07/12] x86: use cmpxchg_flag() where applicable References: <3d7aea62fac48dd6e2da236d86234113fcd78178.1314221624.git.jeremy.fitzhardinge@citrix.com> In-Reply-To: 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 On 08/24/2011 02:56 PM, Linus Torvalds wrote: > Ok, I see nothing horrible in this series. > > The one reaction I have is that the cmpxchg_flag() thing returns an > 8-bit value, but then a lot of the users end up having to extend it to > a full "int" purely for calling convention reasons (eg I think > 'down_write_trylock()' will have 'sete + movzl' - not a new problem, > but since the whole point was to remove extraneous instructions and we > no longer have the silly 'testl', it now annoys me more). > > So it seems a bit sad. But I guess it doesn't really matter. > I think it is a net lose. The most common case is probably going to be to use it immediately, in which case we have: cmpxchg -> sete -> compare -> conditional versus cmpxchg -> compare -> conditional For doubleword cmpxchg it's another matter entirely, because doubleword comparisons are significantly more expensive that sete + test. So unless there is actual data showing this is better, I would like to see this dropped for now. -hpa