Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756788Ab3C1QoV (ORCPT ); Thu, 28 Mar 2013 12:44:21 -0400 Received: from mailout02.c08.mtsvc.net ([205.186.168.190]:51568 "EHLO mailout02.c08.mtsvc.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755844Ab3C1QoU (ORCPT ); Thu, 28 Mar 2013 12:44:20 -0400 Message-ID: <1364489046.3559.14.camel@thor.lan> Subject: Re: [PATCH v2 07/13] rwsem: use cmpxchg for trying to steal write lock From: Peter Hurley To: Michel Lespinasse Cc: Alex Shi , Ingo Molnar , David Howells , Peter Zijlstra , Thomas Gleixner , Yuanhan Liu , Rik van Riel , Dave Chinner , Andrew Morton , linux-kernel@vger.kernel.org Date: Thu, 28 Mar 2013 12:44:06 -0400 In-Reply-To: <1363344869-15732-8-git-send-email-walken@google.com> References: <1363344869-15732-1-git-send-email-walken@google.com> <1363344869-15732-8-git-send-email-walken@google.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.3-0pjh1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Authenticated-User: 125194 peter@hurleysoftware.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2248 Lines: 67 On Fri, 2013-03-15 at 03:54 -0700, Michel Lespinasse wrote: > 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) This is just my opinion but I think this is way more readable if this stays in try_get_writer_sem() and you change that to static inline rather than inlining here. Especially because 9/13 adds a scope. > break; > > raw_spin_unlock_irq(&sem->wait_lock); -- 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/