From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756670AbaIWTHE (ORCPT ); Tue, 23 Sep 2014 15:07:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:65506 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754196AbaIWTHB (ORCPT ); Tue, 23 Sep 2014 15:07:01 -0400 Date: Tue, 23 Sep 2014 21:03:48 +0200 From: Oleg Nesterov To: Steven Rostedt , "Paul E. McKenney" Cc: Andrew Morton , 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: <20140923190348.GA13976@redhat.com> References: <20140922164404.GA28910@redhat.com> <20140922164437.GA28939@redhat.com> <20140922145828.4d06108a@gandalf.local.home> <20140922191130.GA4527@redhat.com> <20140922172405.71c4a110@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140922172405.71c4a110@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 21:11:30 +0200 > Oleg Nesterov wrote: > > > > > @@ -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?? > > Because in -rt, siglock is a mutex. Yes, thanks... I thougt that -rt should handle this somehow, we have more examples of preempt_disable() + spin_lock(). OK, let's forger this patch. It was supposed to be a cleanup, it should not disturb -rt. > > 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 ... > > Hmm, I'm not sure we need to worry about this. As in -rt siglock is a > mutex, which is rt_mutex() itself, I highly doubt we will have > rt_mutex_unlock() grab siglock, otherwise that would cause havoc in -rt. Yes. And, the changelog in a841796f "signal: align __lock_task_sighand() irq disabling and RCU" says: It is therefore possible that this RCU read-side critical section will be preempted and later RCU priority boosted, which means that rcu_read_unlock() will call rt_mutex_unlock() in order to deboost itself, but with interrupts disabled. This results in lockdep splats ... It is quite possible that a better long-term fix is to make rt_mutex_unlock() disable irqs when acquiring the rt_mutex structure's ->wait_lock. but this doesn't look right, raw_spin_lock(&lock->wait_lock) should be fine with irqs disabled or I am totally confused. rt_mutex_adjust_prio() does _irqsave/irqrestore, so this can't enable interrupts. Paul, will you agree if we turn it into struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, unsigned long *flags) { struct sighand_struct *sighand; rcu_read_lock(); for (;;) { sighand = rcu_dereference(tsk->sighand); if (unlikely(sighand == NULL)) break; spin_lock_irqsave(&sighand->siglock, *flags); if (likely(sighand == tsk->sighand)) break; spin_unlock_irqrestore(&sighand->siglock, *flags); } /* * On the succesfull return we hold ->siglock. According to comment * above rcu_read_unlock() this is against the rules, but scheduler * locks are fine under this lock, signal_wake_up() takes them too. */ rcu_read_unlock(); return sighand; } ? Or I can leave this code alone, this is the minor cleanup. Just to me this sequence local_irq_save(); rcu_read_lock(); spin_lock(); looks a bit confusing/annoying even with the comment. Oleg.