linux-kernel - Re: futex_wait_setup sleeping while atomic bug.

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Date:	Fri, 12 Sep 2014 10:12:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Darren Hart <darren@...art.com>
Subject: Re: futex_wait_setup sleeping while atomic bug.

On Thu, 11 Sep 2014, Davidlohr Bueso wrote:

> On Thu, 2014-09-11 at 23:52 +0200, Thomas Gleixner wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> > Date: Thu, 11 Sep 2014 23:44:35 +0200
> > Subject: futex: Unlock hb->lock in futex_wait_requeue_pi() error path
> 
> That's the second time we are bitten by bugs in when requeing, now pi.
> We need to reconsider some of our testing tools to stress these paths
> better, imo.

Testing tools are nice. What we really need is more competent eyes
looking at that code ...
 
> > futex_wait_requeue_pi() calls futex_wait_setup(). If
> > futex_wait_setup() succeeds it returns with hb->lock held and
> > preemption disabled. Now the sanity check after this does:
> > 
> >         if (match_futex(&q.key, &key2)) {
> > 	   	ret = -EINVAL;
> > 		goto out_put_keys;
> > 	}
> > 
> > which releases the keys but does not release hb->lock. So we happily
> > return to user space with hb->lock held and therefor preemption
> > disabled.
> > 
> > Unlock hb->lock before taking the exit route.
> > 
> > Reported-by: Dave "Trinity" Jones <davej@...hat.com>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
> 
> 
> 
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