futex_requeue() can acquire the lock on behalf of a waiter
early on or during the requeue loop if it is uncontended or in
the event of a lock steal or owner died. On wakeup, the waiter
(in futex_wait_requeue_pi()) cleans up the pi_state owner using
the lock_ptr to protect against concurrent access to the
pi_state. The pi_state is hung off futex_q's on the requeue
target futex hash bucket so the lock_ptr needs to be updated
accordingly.

The problem manifested by triggering the WARN_ON in
lookup_pi_state() about the pid != pi_state->owner->pid.  With
this patch, the pi_state is properly guarded against concurrent
access via the requeue target hb lock.

The astute reviewer may notice that there is a window of time
between when futex_requeue() unlocks the hb locks and when
futex_wait_requeue_pi() will acquire hb2->lock.  During this
time the pi_state and uval are not in sync with the underlying
rtmutex owner (but the uval does indicate there are waiters, so
no atomic changes will occur in userspace).  However, this is
not a problem. Should a contending thread enter
lookup_pi_state() and acquire hb2->lock before the ownership is
fixed up, it will find the pi_state hung off a waiter's
(possibly the pending owner's) futex_q and block on the
rtmutex.  Once futex_wait_requeue_pi() fixes up the owner, it
will also move the pi_state from the old owner's
task->pi_state_list to its own.

v3: Fix plist lock name for application to mainline (rather
    than -rt) Compile tested against tip/v2.6.31-rc5.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@linux.vnet.ibm.com>
LKML-Reference: <4A7F4EFF.6090903@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>