Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754351Ab3I0XyK (ORCPT ); Fri, 27 Sep 2013 19:54:10 -0400 Received: from mail-bk0-f49.google.com ([209.85.214.49]:60060 "EHLO mail-bk0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753696Ab3I0XyI (ORCPT ); Fri, 27 Sep 2013 19:54:08 -0400 MIME-Version: 1.0 In-Reply-To: <20130927230137.GE9093@linux.vnet.ibm.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> Date: Fri, 27 Sep 2013 16:54:06 -0700 X-Google-Sender-Auth: X7z1ZSFA25RHtrJh0rYXIhFBirE Message-ID: Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Jason Low To: Paul McKenney Cc: Tim Chen , Jason Low , Waiman Long , 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 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2601 Lines: 72 On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney wrote: > Yep. The previous lock holder's smp_wmb() won't keep either the compiler > or the CPU from reordering things for the new lock holder. They could for > example reorder the critical section to precede the node->locked check, > which would be very bad. Paul, Tim, Longman, How would you like the proposed changes below? --- Subject: [PATCH] MCS: optimizations and barrier corrections Delete the node->locked = 1 assignment if the lock is free as it won't be used. Delete the smp_wmb() in mcs_spin_lock() and add a full memory barrier at the end of the mcs_spin_lock() function. As Paul McKenney suggested, "you do need a full memory barrier here in order to ensure that you see the effects of the previous lock holder's critical section." And in the mcs_spin_unlock(), move the memory barrier so that it is before the "ACCESS_ONCE(next->locked) = 1;". Signed-off-by: Jason Low Signed-off-by: Paul E. McKenney Signed-off-by: Tim Chen --- include/linux/mcslock.h | 7 +++---- 1 files changed, 3 insertions(+), 4 deletions(-) diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h index 20fd3f0..edd57d2 100644 --- a/include/linux/mcslock.h +++ b/include/linux/mcslock.h @@ -26,15 +26,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) prev = xchg(lock, node); if (likely(prev == NULL)) { - /* Lock acquired */ - node->locked = 1; + /* Lock acquired. No need to set node->locked since it won't be used */ return; } ACCESS_ONCE(prev->next) = node; - smp_wmb(); /* Wait until the lock holder passes the lock down */ while (!ACCESS_ONCE(node->locked)) arch_mutex_cpu_relax(); + smp_mb(); } static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node) @@ -51,8 +50,8 @@ static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *n while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); } - ACCESS_ONCE(next->locked) = 1; smp_wmb(); + ACCESS_ONCE(next->locked) = 1; } #endif -- 1.7.1 -- 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/