From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754598Ab2HaST1 (ORCPT ); Fri, 31 Aug 2012 14:19:27 -0400 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:42601 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754434Ab2HaSTZ (ORCPT ); Fri, 31 Aug 2012 14:19:25 -0400 X-Originating-IP: 217.70.178.134 X-Originating-IP: 173.246.103.110 Date: Fri, 31 Aug 2012 11:19:17 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org Subject: Re: [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault Message-ID: <20120831181917.GG4259@jtriplet-mobl1> References: <20120830185607.GA32148@linux.vnet.ibm.com> <1346352988-32444-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346352988-32444-9-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1346352988-32444-9-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 30, 2012 at 11:56:22AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > The rcu_print_detail_task_stall_rnp() function invokes > rcu_preempt_blocked_readers_cgp() to verify that there are some preempted > RCU readers blocking the current grace period outside of the protection > of the rcu_node structure's ->lock. This means that the last blocked > reader might exit its RCU read-side critical section and remove itself > from the ->blkd_tasks list before the ->lock is acquired, resulting in > a segmentation fault when the subsequent code attempts to dereference > the now-NULL gp_tasks pointer. > > This commit therefore moves the test under the lock. This will not > have measurable effect on lock contention because this code is invoked > only when printing RCU CPU stall warnings, in other words, in the common > case, never. > > Signed-off-by: Paul E. McKenney > --- > kernel/rcutree_plugin.h | 6 ++++-- > 1 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index 139a803..c02dc1d 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -422,9 +422,11 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) > unsigned long flags; > struct task_struct *t; > > - if (!rcu_preempt_blocked_readers_cgp(rnp)) > - return; > raw_spin_lock_irqsave(&rnp->lock, flags); > + if (!rcu_preempt_blocked_readers_cgp(rnp)) { > + raw_spin_unlock_irqrestore(&rnp->lock, flags); > + return; > + } > t = list_entry(rnp->gp_tasks, > struct task_struct, rcu_node_entry); > list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) Given the small number of lines of code inside the critical section here, I think this would look clearer without the early return and duplicate lock release: raw_spin_lock_irqsave(&rnp->lock, flags); if (rcu_preempt_blocked_readers_cgp(rnp)) { ... } raw_spin_unlock_irqrestore(&rnp->lock, flags); - Josh Triplett