From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754269Ab1HTRJu (ORCPT ); Sat, 20 Aug 2011 13:09:50 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:45229 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754135Ab1HTRJq (ORCPT ); Sat, 20 Aug 2011 13:09:46 -0400 Date: Sat, 20 Aug 2011 10:09:32 -0700 From: "Paul E. McKenney" To: Arnaud Lacombe Cc: Thomas Gleixner , Peter Zijlstra , laijs@cn.fujitsu.com, darren@dvhart.com, rostedt@goodmis.org, mingo@elte.hu, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked with irqs disabled Message-ID: <20110820170932.GA4261@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110719201410.GA10553@linux.vnet.ibm.com> <1311247968.29152.122.camel@twins> <20110723220348.GD2415@linux.vnet.ibm.com> <20110724051744.GF2415@linux.vnet.ibm.com> <20110724155634.GG2415@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 19, 2011 at 09:31:05PM -0400, Arnaud Lacombe wrote: > Hi, > > On Sun, Jul 24, 2011 at 11:56 AM, Paul E. McKenney > wrote: > > On Sun, Jul 24, 2011 at 11:00:41AM +0200, Thomas Gleixner wrote: > >> On Sat, 23 Jul 2011, Paul E. McKenney wrote: > >> > On Sun, Jul 24, 2011 at 02:05:13AM +0200, Thomas Gleixner wrote: > >> > > On Sun, 24 Jul 2011, Thomas Gleixner wrote: > >> > > > > > Thomas, I'm inclined to merge this, any objections? > >> > > > > > >> > > > > FWIW, it has been passing tests here. > >> > > > > >> > > > If it's only the unlock path, I'm fine with that change. > >> > > > > >> > > > Acked-by-me > >> > > > >> > > Hrmpft. That's requiring all places to take the lock irq safe. Not > >> > > really amused. For -RT that's a hotpath and we can really do without > >> > > the irq fiddling there. That needs a bit more thought. > >> > > >> > Indeed...  If I make only some of the lock acquisitions irq safe, lockdep > >> > will yell at me.  And rightfully so, as that could result in deadlock. > >> > > >> > So, what did you have in mind? > >> > >> Have no real good idea yet for this. Could you grab rt and check > >> whether you can observe any impact when the patch is applied? > > > > Hmmm, wait a minute...  There might be a way to do this with zero > > impact on the fastpath, given that I am allocating an rt_mutex on > > the stack that is used only by RCU priority boosting, and that only > > rt_mutex_init_proxy_locked(), rt_mutex_lock(), and rt_mutex_unlock() > > are used. > > > > So I could do the following: > > > > o       Use lockdep_set_class_and_name() to make the ->wait_lock() > >        field of my rt_mutex have a separate lockdep class.  I guess > >        I should allocate a global variable for lock_class_key > >        rather than allocating it on the stack.  ;-) > > > > o       Make all calls from RCU priority boosting to rt_mutex_lock() > >        and rt_mutex_unlock() have irqs disabled. > > > > o       Make __rt_mutex_slowlock() do the following when sleeping: > > > >        raw_spin_unlock(&lock->wait_lock); > > > >        debug_rt_mutex_print_deadlock(waiter); > > > >        { > >                int was_disabled = irqs_disabled(); > > > >                if (was_disabled) > >                        local_irq_enable(); > > > FWIW, the final construct you opted for in -next: > > if (was_disabled = irqs_disabled()) > local_irq_enable(); > > triggers: > > /linux/linux/kernel/rtmutex.c: In function '__rt_mutex_slowlock': > /linux/linux/kernel/rtmutex.c:605:3: warning: suggest parentheses > around assignment used as truth value But I -do- have parentheses around that assignment!!! Sigh, gcc strikes again. Does the following patch help? If so, I will fold it into commit 83841f02. Thanx, Paul ------------------------------------------------------------------------ Signed-off-by: Paul E. McKenney diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c index 0222e34..2548f44 100644 --- a/kernel/rtmutex.c +++ b/kernel/rtmutex.c @@ -602,7 +602,8 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state, raw_spin_unlock(&lock->wait_lock); - if (was_disabled = irqs_disabled()) + was_disabled = irqs_disabled(); + if (was_disabled) local_irq_enable(); debug_rt_mutex_print_deadlock(waiter);