From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753305AbbCPJRF (ORCPT ); Mon, 16 Mar 2015 05:17:05 -0400 Received: from mail-wg0-f51.google.com ([74.125.82.51]:34676 "EHLO mail-wg0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751799AbbCPJRB (ORCPT ); Mon, 16 Mar 2015 05:17:01 -0400 Date: Mon, 16 Mar 2015 10:16:56 +0100 From: Ingo Molnar To: Jason Low Cc: Peter Zijlstra , Linus Torvalds , Davidlohr Bueso , LKML Subject: Re: [PATCH] locking/mutex: Refactor mutex_spin_on_owner() Message-ID: <20150316091656.GA29357@gmail.com> References: <1425932094.2475.400.camel@j-VirtualBox> <20150310081148.GA20417@gmail.com> <1426005428.2460.4.camel@j-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1426005428.2460.4.camel@j-VirtualBox> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Jason Low wrote: > On Tue, 2015-03-10 at 09:11 +0100, Ingo Molnar wrote: > > * Jason Low wrote: > > > > > This patch applies on top of tip. > > > > > > ------------------------------------------------------------------- > > > Similar to what Linus suggested for rwsem_spin_on_owner(), in > > > mutex_spin_on_owner(), instead of having while (true) and breaking > > > out of the spin loop on lock->owner != owner, we can have the loop > > > directly check for while (lock->owner == owner). This improves the > > > readability of the code. > > > > > > Signed-off-by: Jason Low > > > --- > > > kernel/locking/mutex.c | 17 +++++------------ > > > 1 files changed, 5 insertions(+), 12 deletions(-) > > > > > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > > > index 16b2d3c..1c3b7c5 100644 > > > --- a/kernel/locking/mutex.c > > > +++ b/kernel/locking/mutex.c > > > @@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock, > > > static noinline > > > bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) > > > { > > > - bool ret; > > > - > > > rcu_read_lock(); > > > - while (true) { > > > - /* Return success when the lock owner changed */ > > > - if (lock->owner != owner) { > > > - ret = true; > > > - break; > > > - } > > > - > > > + while (lock->owner == owner) { > > > /* > > > * Ensure we emit the owner->on_cpu, dereference _after_ > > > * checking lock->owner still matches owner, if that fails, > > > @@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) > > > */ > > > barrier(); > > > > > > + /* Stop spinning when need_resched or owner is not running. */ > > > if (!owner->on_cpu || need_resched()) { > > > - ret = false; > > > - break; > > > + rcu_read_unlock(); > > > + return false; > > > } > > > > > > cpu_relax_lowlatency(); > > > } > > > rcu_read_unlock(); > > > > > > - return ret; > > > + return true; > > > > A nit: having multiple return statements in a function is not the > > cleanest approach, especially when we are holding locks. > > > > It's better to add an 'out_unlock' label to before the > > rcu_read_unlock() and use that plus 'ret'. > > Okay, I can update this patch. Should we make another similar update > for the rwsem then? Yeah, I suppose so. Thanks, Ingo