From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751477AbaFDTBq (ORCPT ); Wed, 4 Jun 2014 15:01:46 -0400 Received: from g9t1613g.houston.hp.com ([15.240.0.71]:55911 "EHLO g9t1613g.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750777AbaFDTBp (ORCPT ); Wed, 4 Jun 2014 15:01:45 -0400 From: Jason Low To: mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, tim.c.chen@linux.intel.com, peter@hurleysoftware.com, riel@redhat.com, hpa@zytor.com, walken@google.com, davidlohr@hp.com, Waiman.Long@hp.com, aswin@hp.com, scott.norton@hp.com, chegu_vinod@hp.com, Jason Low Subject: [RFC PATCH] rwsem: Avoid double checking before trylock for rwsem_try_write_lock() Date: Wed, 4 Jun 2014 12:01:11 -0700 Message-Id: <1401908471-8765-1-git-send-email-jason.low2@hp.com> X-Mailer: git-send-email 1.7.9.5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch applies on top of linux-next. Commit 9b0fc9c09f1b added an extra check in rwsem_down_write_failed() for if there are known active lockers in order to avoid doing the cmpxchg() that would likely be unnecessary. However, a subsequent change was made such that we check for sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg(). Thus, the first check added in commit 9b0fc9c09f1b now adds extra overhead. This patch deletes it. Note: we are now changing the rwsem_try_write_lock() as the relevant code was recently moved into that function. Signed-off-by: Jason Low --- kernel/locking/rwsem-xadd.c | 16 +++++++--------- 1 files changed, 7 insertions(+), 9 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 1f99664..6d01f2f 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -247,15 +247,13 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) 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 */ + 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; } return false; } -- 1.7.1