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

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Message-ID: <20140922172405.71c4a110@gandalf.local.home>
Date:	Mon, 22 Sep 2014 17:24:05 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.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 Mon, 22 Sep 2014 21:11:30 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> On 09/22, Steven Rostedt wrote:
> >
> > On Mon, 22 Sep 2014 18:44:37 +0200
> > Oleg Nesterov <oleg@...hat.com> 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 <oleg@...hat.com>
> > > ---
> > >  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??

Because in -rt, siglock is a mutex.

> 
> > That
> > is, is -rt susceptible to this deadlock as well?

As siglock is a mutex, this shouldn't be a problem.

> 
> 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.

> 
> 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.

I really thing the preempt_disable/enable is not needed.

Paul, Thomas, care to comment?

-- Steve
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