Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932285AbaFKShk (ORCPT ); Wed, 11 Jun 2014 14:37:40 -0400 Received: from g4t3427.houston.hp.com ([15.201.208.55]:55273 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932111AbaFKShh (ORCPT ); Wed, 11 Jun 2014 14:37:37 -0400 From: Jason Low To: mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, tim.c.chen@linux.intel.com, paulmck@linux.vnet.ibm.com, rostedt@goodmis.org, davidlohr@hp.com, Waiman.Long@hp.com, scott.norton@hp.com, aswin@hp.com, jason.low2@hp.com Subject: [PATCH v2 3/4] mutex: Try to acquire mutex only if it is unlocked Date: Wed, 11 Jun 2014 11:37:22 -0700 Message-Id: <1402511843-4721-4-git-send-email-jason.low2@hp.com> X-Mailer: git-send-email 1.7.9.5 In-Reply-To: <1402511843-4721-1-git-send-email-jason.low2@hp.com> References: <1402511843-4721-1-git-send-email-jason.low2@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- kernel/locking/mutex.c | 7 ++++--- 1 files 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); -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/