From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760991AbaGEKsr (ORCPT ); Sat, 5 Jul 2014 06:48:47 -0400 Received: from terminus.zytor.com ([198.137.202.10]:58199 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755559AbaGEKso (ORCPT ); Sat, 5 Jul 2014 06:48:44 -0400 Date: Sat, 5 Jul 2014 03:47:50 -0700 From: tip-bot for Jason Low Message-ID: Cc: linux-kernel@vger.kernel.org, hpa@zytor.com, mingo@kernel.org, torvalds@linux-foundation.org, peterz@infradead.org, jason.low2@hp.com, Waiman.Long@hp.com, tglx@linutronix.de Reply-To: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, peterz@infradead.org, jason.low2@hp.com, Waiman.Long@hp.com, tglx@linutronix.de In-Reply-To: <1402511843-4721-4-git-send-email-jason.low2@hp.com> References: <1402511843-4721-4-git-send-email-jason.low2@hp.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] locking/mutexes: Try to acquire mutex only if it is unlocked Git-Commit-ID: 0d968dd8c6aced585b86fa7ba8ce4573bf19e848 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Gitweb: http://git.kernel.org/tip/0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Author: Jason Low AuthorDate: Wed, 11 Jun 2014 11:37:22 -0700 Committer: Ingo Molnar CommitDate: Sat, 5 Jul 2014 11:25:42 +0200 locking/mutexes: Try to acquire mutex only if it is unlocked Upon entering the slowpath in __mutex_lock_common(), we try once more to acquire the mutex. We only try to acquire if (lock->count >= 0). However, what we actually want here is to try to acquire if the mutex is unlocked (lock->count == 1). This patch changes it so that we only try-acquire the mutex upon entering the slowpath if it is unlocked, rather than if the lock count is non-negative. This helps further reduce unnecessary atomic xchg() operations. Furthermore, this patch uses !mutex_is_locked(lock) to do the initial checks for if the lock is free rather than directly calling atomic_read() on the lock->count, in order to improve readability. Signed-off-by: Jason Low Acked-by: Waiman Long Signed-off-by: Peter Zijlstra Cc: akpm@linux-foundation.org Cc: tim.c.chen@linux.intel.com Cc: paulmck@linux.vnet.ibm.com Cc: rostedt@goodmis.org Cc: davidlohr@hp.com Cc: scott.norton@hp.com Cc: aswin@hp.com Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1402511843-4721-4-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/mutex.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4bd9546..e4d997b 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -432,7 +432,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, if (owner && !mutex_spin_on_owner(lock, owner)) break; - if ((atomic_read(&lock->count) == 1) && + /* Try to acquire the mutex if it is unlocked. */ + if (!mutex_is_locked(lock) && (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { lock_acquired(&lock->dep_map, ip); if (use_ww_ctx) { @@ -479,9 +480,9 @@ slowpath: /* * Once more, try to acquire the lock. Only try-lock the mutex if - * lock->count >= 0 to reduce unnecessary xchg operations. + * it is unlocked to reduce unnecessary xchg() operations. */ - if (atomic_read(&lock->count) >= 0 && (atomic_xchg(&lock->count, 0) == 1)) + if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1)) goto skip_wait; debug_mutex_lock_common(lock, &waiter);