linux-kernel - Re: 3.5-rc6 futex_wait_requeue

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Message-ID: <5008969F.5030901@linux.intel.com>
Date:	Thu, 19 Jul 2012 16:22:07 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: 3.5-rc6 futex_wait_requeue_pi oops.



On 07/13/2012 11:54 AM, Dave Jones wrote:
> On Fri, Jul 13, 2012 at 08:47:38PM +0200, Thomas Gleixner wrote:
>  > On Fri, 13 Jul 2012, Dave Jones wrote:
>  > 
>  > > Looks like calling futex() with garbage makes things unhappy.
>  > 
>  >                 WARN_ON(!&q.pi_state);
>  >                 pi_mutex = &q.pi_state->pi_mutex;
>  >                 ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1);
>  >                 debug_rt_mutex_free_waiter(&rt_waiter);
>  > 
>  > So there is some weird way which causes q.pi_state = NULL. Dave, did
>  > you see the warning before the oops happened ?
> 
> No, that didn't seem to trigger.

Well I don't have a fix yet, but I can explain this not triggering.

q is on the stack, so the ADDRESS for q.pi_state is never going to be
NULL. However, properly instrumented, we do see this:

[   23.621501] ---[ end trace 20bdfb44db182a17 ]---
[   23.622425] q.pi_state @           (null)
[   23.623272] &q.pi_state @ ffff880185e2dca8
[   23.624119] ------------[ cut here ]------------

Duh.

I'll add a fix to that WARN_ON in my futex-fixes branch along with the
fix for the bug Dan found.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel


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