[PATCH][RT] futex: protect against pi_blocked_on corruption duringrequeue PI

[Darren Hart]

Thanks to Thomas, Steven, and Mike for hashing this over me. After an
IRC discussion with Thomas, I put the following together. It resolves
the issue for me, Mike please test and let us know if it fixes it for
you. A couple of points of discussion before we commit this:

The use of the new state flag, PI_WAKEUP_INPROGRESS, is pretty ugly.
Would a new task_pi_blocked_on_valid() method be preferred (in
rtmutex.c)? 

The new WARN_ON() in task_blocks_on_rt_mutex() is complex. It didn't
exist before and we've now closed this gap, should we just drop it?

I've added a couple BUG_ON()s in futex_wait_requeue_pi() dealing with
the race with requeue and q.lock_ptr. I'd like to leave this for the
time being if nobody strongly objects.

Thanks,

Darren