From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754360AbaIQARD (ORCPT ); Tue, 16 Sep 2014 20:17:03 -0400 Received: from g6t1525.atlanta.hp.com ([15.193.200.68]:41745 "EHLO g6t1525.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752975AbaIQARB (ORCPT ); Tue, 16 Sep 2014 20:17:01 -0400 Message-ID: <1410913017.2447.22.camel@j-VirtualBox> Subject: [PATCH v3] locking/rwsem: Avoid double checking before try acquiring write lock From: Jason Low To: Peter Zijlstra , Ingo Molnar , Tim Chen , Peter Hurley , Davidlohr Bueso Cc: linux-kernel@vger.kernel.org, Aswin Chandramouleeswaran , Chegu Vinod , Jason Low Date: Tue, 16 Sep 2014 17:16:57 -0700 Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit 9b0fc9c09f1b checks for if there are known active lockers in order to avoid write trylocking using expensive cmpxchg() when it likely wouldn't get the lock. However, a subsequent patch was added such that we directly check for sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg(). Thus, commit 9b0fc9c09f1b now just adds overhead. This patch modifies it so that we only do a check for if count == RWSEM_WAITING_BIAS. Also, add a comment on why we do an "extra check" of count before the cmpxchg(). Cc: Peter Hurley Cc: Tim Chen Signed-off-by: Jason Low --- kernel/locking/rwsem-xadd.c | 20 +++++++++++--------- 1 files changed, 11 insertions(+), 9 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 12166ec..7628c3f 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -250,16 +250,18 @@ EXPORT_SYMBOL(rwsem_down_read_failed); static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) { - if (!(count & RWSEM_ACTIVE_MASK)) { - /* try acquiring the write lock */ - if (sem->count == RWSEM_WAITING_BIAS && - cmpxchg(&sem->count, RWSEM_WAITING_BIAS, - RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { - if (!list_is_singular(&sem->wait_list)) - rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); - return true; - } + /* + * Try acquiring the write lock. Check count first in order + * to reduce unnecessary expensive cmpxchg() operations. + */ + if (count == RWSEM_WAITING_BIAS && + cmpxchg(&sem->count, RWSEM_WAITING_BIAS, + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { + if (!list_is_singular(&sem->wait_list)) + rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); + return true; } + return false; } -- 1.7.1