From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754152Ab3JBWjD (ORCPT ); Wed, 2 Oct 2013 18:39:03 -0400 Received: from mga02.intel.com ([134.134.136.20]:47427 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754471Ab3JBWiq (ORCPT ); Wed, 2 Oct 2013 18:38:46 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.90,1022,1371106800"; d="scan'208";a="413212371" Subject: [PATCH v8 6/9] MCS Lock: optimizations and extra comments From: Tim Chen To: Ingo Molnar , Andrew Morton Cc: Linus Torvalds , 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 , Jason Low , Waiman Long , linux-kernel@vger.kernel.org, linux-mm In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Wed, 02 Oct 2013 15:38:35 -0700 Message-ID: <1380753515.11046.88.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. 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f49.google.com (mail-pb0-f49.google.com [209.85.160.49]) by kanga.kvack.org (Postfix) with ESMTP id B62AE6B003C for ; Wed, 2 Oct 2013 18:38:48 -0400 (EDT) Received: by mail-pb0-f49.google.com with SMTP id xb4so1534429pbc.8 for ; Wed, 02 Oct 2013 15:38:48 -0700 (PDT) Subject: [PATCH v8 6/9] MCS Lock: optimizations and extra comments From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Wed, 02 Oct 2013 15:38:35 -0700 Message-ID: <1380753515.11046.88.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Ingo Molnar , Andrew Morton Cc: Linus Torvalds , 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 , Jason Low , Waiman Long , linux-kernel@vger.kernel.org, linux-mm 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. 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org