Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932138Ab3CFXWM (ORCPT ); Wed, 6 Mar 2013 18:22:12 -0500 Received: from mail-pb0-f53.google.com ([209.85.160.53]:52222 "EHLO mail-pb0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757206Ab3CFXWJ (ORCPT ); Wed, 6 Mar 2013 18:22:09 -0500 From: Michel Lespinasse To: Alex Shi , Ingo Molnar , David Howells , Peter Zijlstra , Thomas Gleixner , Yuanhan Liu , Rik van Riel Cc: Andrew Morton , linux-kernel@vger.kernel.org Subject: [PATCH 07/12] rwsem: use cmpxchg for trying to steal write lock Date: Wed, 6 Mar 2013 15:21:46 -0800 Message-Id: <1362612111-28673-8-git-send-email-walken@google.com> X-Mailer: git-send-email 1.8.1.3 In-Reply-To: <1362612111-28673-1-git-send-email-walken@google.com> References: <1362612111-28673-1-git-send-email-walken@google.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1886 Lines: 61 Using rwsem_atomic_update to try stealing the write lock forced us to undo the adjustment in the failure path. We can have simpler and faster code by using cmpxchg instead. Signed-off-by: Michel Lespinasse --- lib/rwsem.c | 26 ++++++-------------------- 1 file changed, 6 insertions(+), 20 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index 2360bf204098..64c2dc007be2 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) return sem; } -/* Try to get write sem, caller holds sem->wait_lock: */ -static int try_get_writer_sem(struct rw_semaphore *sem) -{ - long oldcount, adjustment; - - adjustment = RWSEM_ACTIVE_WRITE_BIAS; - if (list_is_singular(&sem->wait_list)) - adjustment -= RWSEM_WAITING_BIAS; - -try_again_write: - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; - if (!(oldcount & RWSEM_ACTIVE_MASK)) - return 1; - /* some one grabbed the sem already */ - if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) - return 0; - goto try_again_write; -} - /* * wait for the read lock to be granted */ @@ -236,7 +217,12 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) while (true) { set_task_state(tsk, TASK_UNINTERRUPTIBLE); - if (try_get_writer_sem(sem)) + /* Try acquiring the write lock. */ + count = RWSEM_ACTIVE_WRITE_BIAS; + if (!list_is_singular(&sem->wait_list)) + count += RWSEM_WAITING_BIAS; + if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == + RWSEM_WAITING_BIAS) break; raw_spin_unlock_irq(&sem->wait_lock); -- 1.8.1.3 -- 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/