From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f42.google.com (mail-pa0-f42.google.com [209.85.220.42]) by kanga.kvack.org (Postfix) with ESMTP id 582306B0133 for ; Wed, 6 Nov 2013 20:26:57 -0500 (EST) Received: by mail-pa0-f42.google.com with SMTP id kp14so505306pab.15 for ; Wed, 06 Nov 2013 17:26:56 -0800 (PST) Received: from psmtp.com ([74.125.245.176]) by mx.google.com with SMTP id it5si617819pbc.335.2013.11.06.17.26.55 for ; Wed, 06 Nov 2013 17:26:55 -0800 (PST) Subject: [PATCH v4 2/5] MCS Lock: optimizations and extra comments From: Tim Chen In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Wed, 06 Nov 2013 17:26:49 -0800 Message-ID: <1383787609.11046.365.camel@schen9-DESK> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: 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 , "Figo.zhang" 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: 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 -- 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