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From: Thomas Gleixner <tglx@linutronix.de>
To: Dave Jones <davej@redhat.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Darren Hart <darren@dvhart.com>
Subject: Re: futex_wait_setup sleeping while atomic bug.
Date: Thu, 11 Sep 2014 23:52:02 +0200 (CEST)
Message-ID: <alpine.DEB.2.10.1409112318500.4178@nanos> (raw)
In-Reply-To: <20140911151040.GB3008@redhat.com>

On Thu, 11 Sep 2014, Dave Jones wrote:

> Hit this overnight on Linus tree from yesterday.
> 
> BUG: sleeping function called from invalid context at include/linux/freezer.h:56
> in_atomic(): 1, irqs_disabled(): 0, pid: 31948, name: trinity-c121
> Preemption disabled at:[<ffffffffb90f2512>] futex_wait_setup+0xb2/0x140

Huch? So we are in a preempt disabled region in futex_wait_setup and
we get interrupted and end up in the signal delivery path?

I really love that futex stuff....

But fortunately the preemption disabled hint made it into mainline so
looking at the callsites of futex_wait_setup() makes it pretty clear
where the shit hits the fan. Patch below.

Thanks,

	tglx

------------------->
From: Thomas Gleixner <tglx@linutronix.de>
Date: Thu, 11 Sep 2014 23:44:35 +0200
Subject: futex: Unlock hb->lock in futex_wait_requeue_pi() error path

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@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
diff --git a/kernel/futex.c b/kernel/futex.c
index d3a9d946d0b7..815d7af2ffe8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2592,6 +2592,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 	 * shared futexes. We need to compare the keys:
 	 */
 	if (match_futex(&q.key, &key2)) {
+		queue_unlock(hb);
 		ret = -EINVAL;
 		goto out_put_keys;
 	}

  reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-11 15:10 Dave Jones
2014-09-11 21:52 ` Thomas Gleixner [this message]
2014-09-11 22:16   ` Darren Hart
2014-09-11 23:53   ` Davidlohr Bueso
2014-09-12  0:07     ` Darren Hart
2014-09-12  8:12     ` Thomas Gleixner
2014-09-12 20:10   ` [tip:locking/urgent] futex: Unlock hb-> lock in futex_wait_requeue_pi() error path tip-bot for Thomas Gleixner

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