From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753730AbaJCFad (ORCPT ); Fri, 3 Oct 2014 01:30:33 -0400 Received: from terminus.zytor.com ([198.137.202.10]:60953 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752595AbaJCFa3 (ORCPT ); Fri, 3 Oct 2014 01:30:29 -0400 Date: Thu, 2 Oct 2014 22:29:20 -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, tim.c.chen@linux.intel.com, peter@hurleysoftware.com, jason.low2@hp.com, dbueso@suse.de, chegu_vinod@hp.com, tglx@linutronix.de, aswin@hp.com Reply-To: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, peterz@infradead.org, tim.c.chen@linux.intel.com, jason.low2@hp.com, peter@hurleysoftware.com, dbueso@suse.de, chegu_vinod@hp.com, tglx@linutronix.de, aswin@hp.com In-Reply-To: <1410913017.2447.22.camel@j-VirtualBox> References: <1410913017.2447.22.camel@j-VirtualBox> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] locking/rwsem: Avoid double checking before try acquiring write lock Git-Commit-ID: debfab74e453f079cd8b12b0604387a8c510ef3a 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: debfab74e453f079cd8b12b0604387a8c510ef3a Gitweb: http://git.kernel.org/tip/debfab74e453f079cd8b12b0604387a8c510ef3a Author: Jason Low AuthorDate: Tue, 16 Sep 2014 17:16:57 -0700 Committer: Ingo Molnar CommitDate: Fri, 3 Oct 2014 06:09:29 +0200 locking/rwsem: Avoid double checking before try acquiring write lock Commit 9b0fc9c09f1b ("rwsem: skip initial trylock in rwsem_down_write_failed") 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(). Signed-off-by: Jason Low Acked-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Aswin Chandramouleeswaran Cc: Chegu Vinod Cc: Peter Hurley Cc: Tim Chen Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox Signed-off-by: Ingo Molnar --- kernel/locking/rwsem-xadd.c | 20 +++++++++++--------- 1 file 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; }