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

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Date:	Thu, 11 Sep 2014 16:53:38 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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.

> 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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