From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752762AbaFKT4V (ORCPT ); Wed, 11 Jun 2014 15:56:21 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:50889 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750963AbaFKT4T (ORCPT ); Wed, 11 Jun 2014 15:56:19 -0400 Date: Wed, 11 Jun 2014 12:56:13 -0700 From: "Paul E. McKenney" To: Oleg Nesterov Cc: Thomas Gleixner , Steven Rostedt , Linus Torvalds , LKML , Peter Zijlstra , Andrew Morton , Ingo Molnar , Clark Williams Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc) Message-ID: <20140611195613.GM4581@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140610165704.GA3110@redhat.com> <20140610141306.04a4bcf3@gandalf.local.home> <20140611155251.GA4581@linux.vnet.ibm.com> <20140611170705.GA26816@redhat.com> <20140611171734.GA27457@redhat.com> <20140611172958.GF4581@linux.vnet.ibm.com> <20140611175934.GA28912@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140611175934.GA28912@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061119-7164-0000-0000-000002626A52 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 11, 2014 at 07:59:34PM +0200, Oleg Nesterov wrote: > On 06/11, Paul E. McKenney wrote: > > > > On Wed, Jun 11, 2014 at 07:17:34PM +0200, Oleg Nesterov wrote: > > > On 06/11, Oleg Nesterov wrote: > > > > > > > > On 06/11, Paul E. McKenney wrote: > > > > > > > > > raw_spin_unlock_irqrestore(&rnp->lock, flags); > > > > > rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */ > > > > > rt_mutex_unlock(&mtx); /* Keep lockdep happy. */ > > > > > > > > > > + /* Wait until boostee is done accessing mtx before reinitializing. */ > > > > > + wait_for_completion(&rnp->boost_completion); > > > > > + > > > > > > > > I must have missed something, I dont understand why we need ->boost_completion. > > > > > > > > What if you simply move that rt_mutex into rcu_node ? > > > > > > > > Or. Given that rcu_boost_kthread() never exits, it can declare this mutex > > > > on stack and pass the pointer to rcu_boost() ? > > > > > > Ah, please ignore, I forgot about init_proxy_locked(). Although perhaps this > > > can be solved easily. > > > > You beat me to it. ;-) > > > > I was thinking of ->boost_completion as the way to solve it easily, but > > what did you have in mind? > > I meant, rcu_boost() could probably just do "mtx->owner = t", we know that > it was unlocked by us and nobody else can use it until we set > t->rcu_boost_mutex. My concern with this is that rcu_read_unlock_special() could hypothetically get preempted (either by kernel or hypervisor), so that it might be a long time until it makes its reference. But maybe that reference would be harmless in this case. > And if we move it into rcu_node, then we can probably kill ->rcu_boost_mutex, > rcu_read_unlock_special() could check rnp->boost_mutex->owner == current. If this was anywhere near a hot code path, I would be sorely tempted. > But you know, I also think that the dentist removed the rest of my brains > along my tooth, so I am not sure if I actually have something in mind. Ouch!!! When that happens to me, most of my brains return some time after the anesthetic wears off, but I know the feeling! Thanx, Paul