Re: [patch 5/5] futex: Simplify futex_lock_pi_atomic() and make it more robust

[Thomas Gleixner]

On Fri, 13 Jun 2014, Thomas Gleixner wrote:
> 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.

I'm a moron. Ran out of brown paperbags ....
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