linux-kernel - Re: [PATCH 1/2] signal: simplify deadlock-avoidance in lock_task

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Message-ID: <20140923103053.7da6b7f8@gandalf.local.home>
Date:	Tue, 23 Sep 2014 10:30:53 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rik van Riel <riel@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] signal: simplify deadlock-avoidance in
 lock_task_sighand()

On Tue, 23 Sep 2014 16:20:37 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Sep 23, 2014 at 07:45:22AM -0400, Rik van Riel wrote:
> 
> > > I really thing the preempt_disable/enable is not needed.
> > > 
> > > Paul, Thomas, care to comment?
> > 
> > I suspect you are right.  On normal kernels, rcu_read_lock() will
> > ensure preemption is disabled.
> > 
> > On -rt, the locks within are all sleepable mutexes.
> > 
> > Either way, things should be ok.
> 
> But with CONFIG_PREEMPT we get preemptible RCU but not the
> spinlock->rt_mutex conversion.

Right, I was about to say that. That is, rcu_read_lock() in mainline
does not always disable preemption (See CONFIG_PREEMPT_RCU).

But I still think this should be safe. I don't see the siglock ever
being taking in the scheduler or by rt_mutex_unlock().

-- Steve
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