From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755363Ab3KERnX (ORCPT ); Tue, 5 Nov 2013 12:43:23 -0500 Received: from mga02.intel.com ([134.134.136.20]:10977 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754870Ab3KERmg (ORCPT ); Tue, 5 Nov 2013 12:42:36 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.93,535,1378882800"; d="scan'208";a="404017426" Subject: [PATCH v2 2/4] MCS Lock: optimizations and extra comments 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 , Will Deacon In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Tue, 05 Nov 2013 09:42:33 -0800 Message-ID: <1383673353.11046.278.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 Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node check in mcs_spin_unlock() likely() as it is likely that a race did not occur most of the time. Also add in more comments describing how the local node is used in MCS locks. 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 b5de3b0..96f14299 100644 --- a/include/linux/mcs_spinlock.h +++ b/include/linux/mcs_spinlock.h @@ -18,6 +18,12 @@ struct mcs_spinlock { }; /* + * In order to acquire the lock, the caller should declare a local node and + * pass a reference of the node to this function in addition to the lock. + * If the lock has already been acquired, then this will proceed to spin + * on this node->locked until the previous lock holder sets the node->locked + * in mcs_spin_unlock(). + * * We don't inline mcs_spin_lock() so that perf can correctly account for the * time spent in this lock function. */ @@ -33,7 +39,6 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired */ - node->locked = 1; return; } ACCESS_ONCE(prev->next) = node; @@ -43,6 +48,10 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) arch_mutex_cpu_relax(); } +/* + * Releases the lock. The caller should pass in the corresponding node that + * was used to acquire the lock. + */ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) { struct mcs_spinlock *next = ACCESS_ONCE(node->next); @@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod /* * Release the lock by setting it to NULL */ - if (cmpxchg(lock, node, NULL) == node) + if (likely(cmpxchg(lock, node, NULL) == node)) return; /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) -- 1.7.4.4