From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754128AbaIVTOn (ORCPT ); Mon, 22 Sep 2014 15:14:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61785 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751563AbaIVTOm (ORCPT ); Mon, 22 Sep 2014 15:14:42 -0400 Date: Mon, 22 Sep 2014 21:11:30 +0200 From: Oleg Nesterov To: Steven Rostedt Cc: Andrew Morton , "Paul E. McKenney" , Peter Zijlstra , Rik van Riel , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] signal: simplify deadlock-avoidance in lock_task_sighand() Message-ID: <20140922191130.GA4527@redhat.com> References: <20140922164404.GA28910@redhat.com> <20140922164437.GA28939@redhat.com> <20140922145828.4d06108a@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140922145828.4d06108a@gandalf.local.home> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/22, Steven Rostedt wrote: > > On Mon, 22 Sep 2014 18:44:37 +0200 > Oleg Nesterov wrote: > > > __lock_task_sighand() does local_irq_save() to prevent the potential > > deadlock, we can use preempt_disable() with the same effect. And in > > this case we can do preempt_disable/enable + rcu_read_lock/unlock only > > once outside of the main loop and simplify the code. This also shaves > > 112 bytes from signal.o. > > > > With this patch the main loop runs with preemption disabled, but this > > should be fine because restart is very unlikely: it can only happen if > > we race with de_thread() and ->sighand is shared. And the latter is only > > possible if CLONE_SIGHAND was used without CLONE_THREAD, most probably > > nobody does this nowadays. > > > > Signed-off-by: Oleg Nesterov > > --- > > kernel/signal.c | 31 +++++++++++++------------------ > > 1 files changed, 13 insertions(+), 18 deletions(-) > > > > diff --git a/kernel/signal.c b/kernel/signal.c > > index 8f0876f..61a1f55 100644 > > --- a/kernel/signal.c > > +++ b/kernel/signal.c > > @@ -1261,30 +1261,25 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, > > unsigned long *flags) > > { > > struct sighand_struct *sighand; > > - > > + /* > > + * We are going to do rcu_read_unlock() under spin_lock_irqsave(). > > + * Make sure we can not be preempted after rcu_read_lock(), see > > + * rcu_read_unlock() comment header for details. > > + */ > > + preempt_disable(); > > The sad part is, this is going to break -rt. Hmm, why?? > That > is, is -rt susceptible to this deadlock as well? In fact this deadlock is not really possible in any case, scheduler locks should be fine under ->siglock (for example, signal_wake_up() is called under this lock). But, the comment above rcu_read_unlock() says: Given that the set of locks acquired by rt_mutex_unlock() might change at any time, a somewhat more future-proofed approach is to make sure that that preemption never happens ... so this patch doesn't try to change the rules. But perhaps we can simply remove this preempt_disable/enable? Or. We can shift rcu_read_unlock() from lock_task_sighand() to unlock_task_sighand(). This way we can avoid preempt_disable too, but I'd prefer to not do this. Oleg.