Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754175Ab3JBTch (ORCPT ); Wed, 2 Oct 2013 15:32:37 -0400 Received: from g1t0027.austin.hp.com ([15.216.28.34]:6601 "EHLO g1t0027.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753983Ab3JBTcg (ORCPT ); Wed, 2 Oct 2013 15:32:36 -0400 Message-ID: <524C74C3.4060908@hp.com> Date: Wed, 02 Oct 2013 15:32:19 -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: Tim Chen CC: Paul McKenney , Jason Low , Ingo Molnar , Andrew Morton , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , linux-kernel@vger.kernel.org, linux-mm Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file References: <1380147049.3467.67.camel@schen9-DESK> <20130927152953.GA4464@linux.vnet.ibm.com> <1380310733.3467.118.camel@schen9-DESK> <20130927203858.GB9093@linux.vnet.ibm.com> <1380322005.3467.186.camel@schen9-DESK> <20130927230137.GE9093@linux.vnet.ibm.com> <20130928021947.GF9093@linux.vnet.ibm.com> <52499E13.8050109@hp.com> <1380557440.14213.6.camel@j-VirtualBox> <5249A8A4.9060400@hp.com> <1380646092.11046.6.camel@schen9-DESK> <524B2A01.4080403@hp.com> <1380662188.11046.37.camel@schen9-DESK> <524B75F0.2070005@hp.com> <1380739391.11046.73.camel@schen9-DESK> In-Reply-To: <1380739391.11046.73.camel@schen9-DESK> 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 Content-Length: 1947 Lines: 42 On 10/02/2013 02:43 PM, Tim Chen wrote: > On Tue, 2013-10-01 at 21:25 -0400, Waiman Long wrote: >> >> If the lock and unlock functions are done right, there should be no >> overlap of critical section. So it is job of the lock/unlock functions >> to make sure that critical section code won't leak out. There should be >> some kind of memory barrier at the beginning of the lock function and >> the end of the unlock function. >> >> The critical section also likely to have branches. The CPU may >> speculatively execute code on the 2 branches, but one of them will be >> discarded once the branch condition is known. Also >> arch_mutex_cpu_relax() is a compiler barrier by itself. So we may not >> need a barrier() after all. The while statement is a branch instruction, >> any code after that can only be speculatively executed and cannot be >> committed until the branch is done. > But the condition code may be checked after speculative execution? > The condition may not be true during speculative execution and only > turns true when we check the condition, and take that branch? > > The thing that bothers me is without memory barrier after the while > statement, we could speculatively execute before affirming the lock is > in acquired state. Then when we check the lock, the lock is set > to acquired state in the mean time. > We could be loading some memory entry *before* > the node->locked has been set true. I think a smp_rmb (if not a > smp_mb) should be set after the while statement. Yes, I think a smp_rmb() make sense here to correspond to the smp_wmb() in the unlock path. BTW, you need to move the node->locked = 0; statement before xchg() if you haven't done so. -Longman -- 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/