From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753688Ab3ITC71 (ORCPT ); Thu, 19 Sep 2013 22:59:27 -0400 Received: from g5t0009.atlanta.hp.com ([15.192.0.46]:7861 "EHLO g5t0009.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752067Ab3ITC7Z (ORCPT ); Thu, 19 Sep 2013 22:59:25 -0400 Message-ID: <523BB9FE.2030003@hp.com> Date: Thu, 19 Sep 2013 22:59:10 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Linus Torvalds CC: Will Deacon , Linux Kernel Mailing List Subject: Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates References: <1379614006-3844-1-git-send-email-will.deacon@arm.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/19/2013 02:11 PM, Linus Torvalds wrote: > On Thu, Sep 19, 2013 at 1:06 PM, Will Deacon wrote: >> The cmpxchg() function tends not to support 64-bit arguments on 32-bit >> architectures. This could be either due to use of unsigned long arguments >> (like on ARM) or lack of instruction support (cmpxchgq on x86). However, >> these architectures may implement a specific cmpxchg64() function to >> provide 64-bit cmpxchg support instead > I'm certainly ok with this, but I wonder how much point there is to > use the cmpxchg alternatives for 32-bit architectures at all... > > From a performance standpoint, lockref really is expected to mainly > help with big machines. Only insane people would do big machines with > 32-bit kernels these days. > Of course, it may be that cmpxchg is actually faster on some > architectures, but at least on x86-32, cmpxchg8b is traditionally > quite slow. > > In other words, I'd actually like to see some numbers if there are > loads where this actually helps and matters... > > Linus I agreed that 32-bit machines are not likely to be big enough to get benefit from this feature. However, I do see a minor problem with the current code. If a user tries to turn on CMPXCHG_LOCKREF on a 32-bit build, he/she will get a compilation error because a 64-bit data type is not supported in 32-bit mode's cmpxchg(). Because of this, my original patch also uses cmpxchg64() which is equivalent to cmpxchg() in 64-bit machine for 64-bit data type. I would suggest either change to use cmpxchg64() or add the dependence line "depends on 64BIT" to CMPXCHG_LOCKREF. -Longman