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SubjectRe: [patch 5/5] futex: Simplify futex_lock_pi_atomic() and make it more robust
On Thu, 12 Jun 2014, Darren Hart wrote:

> On Wed, 2014-06-11 at 20:45 +0000, Thomas Gleixner wrote:
> > futex_lock_pi_atomic() is a maze of retry hoops and loops.
> >
> > Reduce it to simple and understandable states:
>
> Heh... well...
>
> With this patch applied (1-4 will not reproduce without 5), if userspace
> wrongly sets the uval to 0, the pi_state can end up being NULL for a
> subsequent requeue_pi operation:
>
> [ 10.426159] requeue: 00000000006022e0 to 00000000006022e4
> [ 10.427737] this:ffff88013a637da8
> [ 10.428749] waking:ffff88013a637da8
> fut2 = 0
> [ 10.429994] comparing requeue_pi_key
> [ 10.431034] prepare waiter to take the rt_mutex
> [ 10.432344] pi_state: (null)
> [ 10.433414] BUG: unable to handle kernel NULL pointer dereference at
> 0000000000000038
>
> This occurs in the requeue loop, in the requeue_pi block at:
>
> atomic_inc(&pi_state->refcount);

Hmm. Took me some time to reproduce. Digging into it now.

Thanks,

tglx


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