Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752784Ab3JAQsS (ORCPT ); Tue, 1 Oct 2013 12:48:18 -0400 Received: from mga02.intel.com ([134.134.136.20]:41005 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751403Ab3JAQsO (ORCPT ); Tue, 1 Oct 2013 12:48:14 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.90,1014,1371106800"; d="scan'208";a="412506715" Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Tim Chen To: Waiman Long Cc: Jason Low , Paul McKenney , 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 In-Reply-To: <5249A8A4.9060400@hp.com> 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> Content-Type: text/plain; charset="UTF-8" Date: Tue, 01 Oct 2013 09:48:12 -0700 Message-ID: <1380646092.11046.6.camel@schen9-DESK> Mime-Version: 1.0 X-Mailer: Evolution 2.32.3 (2.32.3-1.fc14) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1837 Lines: 47 On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote: > On 09/30/2013 12:10 PM, Jason Low wrote: > > On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote: > >> On 09/28/2013 12:34 AM, Jason Low wrote: > >>>> Also, below is what the mcs_spin_lock() and mcs_spin_unlock() > >>>> functions would look like after applying the proposed changes. > >>>> > >>>> static noinline > >>>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) > >>>> { > >>>> struct mcs_spin_node *prev; > >>>> > >>>> /* Init node */ > >>>> node->locked = 0; > >>>> node->next = NULL; > >>>> > >>>> prev = xchg(lock, node); > >>>> if (likely(prev == NULL)) { > >>>> /* Lock acquired. No need to set node->locked since it > >>>> won't be used */ > >>>> return; > >>>> } > >>>> ACCESS_ONCE(prev->next) = node; > >>>> /* Wait until the lock holder passes the lock down */ > >>>> while (!ACCESS_ONCE(node->locked)) > >>>> arch_mutex_cpu_relax(); > >>>> smp_mb(); > >> I wonder if a memory barrier is really needed here. > > If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check > > so that the check occurs after an instruction in the critical section, > > then the barrier may be necessary. > > > > In that case, just a barrier() call should be enough. The cpu could still be executing out of order load instruction from the critical section before checking node->locked? Probably smp_mb() is still needed. Tim -- 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/