From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754060Ab3KDXhT (ORCPT ); Mon, 4 Nov 2013 18:37:19 -0500 Received: from mga09.intel.com ([134.134.136.24]:7907 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752839Ab3KDXhO (ORCPT ); Mon, 4 Nov 2013 18:37:14 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.93,535,1378882800"; d="scan'208";a="429779974" Subject: [PATCH 3/4] MCS Lock: Barrier corrections From: Tim Chen To: Ingo Molnar , Andrew Morton , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, linux-mm , linux-arch@vger.kernel.org, Linus Torvalds , Waiman Long , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , "Paul E.McKenney" , Tim Chen , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Mon, 04 Nov 2013 15:37:10 -0800 Message-ID: <1383608230.11046.262.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 This patch corrects the way memory barriers are used in the MCS lock and removes ones that are not needed. Also add comments on all barriers. Reviewed-by: Paul E. McKenney Reviewed-by: Tim Chen Signed-off-by: Jason Low --- include/linux/mcs_spinlock.h | 13 +++++++++++-- 1 files changed, 11 insertions(+), 2 deletions(-) diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h index 96f14299..93d445d 100644 --- a/include/linux/mcs_spinlock.h +++ b/include/linux/mcs_spinlock.h @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) node->locked = 0; node->next = NULL; + /* xchg() provides a memory barrier */ prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired */ 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(); + + /* Make sure subsequent operations happen after the lock is acquired */ + smp_rmb(); } /* @@ -58,6 +61,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod if (likely(!next)) { /* + * cmpxchg() provides a memory barrier. * Release the lock by setting it to NULL */ if (likely(cmpxchg(lock, node, NULL) == node)) @@ -65,9 +69,14 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); + } else { + /* + * Make sure all operations within the critical section + * happen before the lock is released. + */ + smp_wmb(); } ACCESS_ONCE(next->locked) = 1; - smp_wmb(); } #endif /* __LINUX_MCS_SPINLOCK_H */ -- 1.7.4.4