[PATCH] rtmutex: ensure we wake up the top waiter

[Wander Lairson Costa <wander@redhat.com>]

In task_blocked_on_lock() we save the owner, release the wait_lock and
call rt_mutex_adjust_prio_chain(). Before we acquire the wait_lock
again, the owner may release the lock and deboost.

rt_mutex_adjust_prio_chain() acquires the wait_lock. In the requeue
phase, waiter may be initially in the top of the queue, but after
dequeued and requeued it may no longer be true.

This scenario ends up waking the wrong task, which will verify it is no
the top waiter and comes back to sleep. Now we have a situation in which
no task is holding the lock but no one acquires it.

We can reproduce the bug in PREEMPT_RT with stress-ng:

while true; do
    stress-ng --sched deadline --sched-period 1000000000 \
    	    --sched-runtime 800000000 --sched-deadline \
    	    1000000000 --mmapfork 23 -t 20
done

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
---
 kernel/locking/rtmutex.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 010cf4e6d0b8..728f434de2bb 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -901,8 +901,9 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 		 * then we need to wake the new top waiter up to try
 		 * to get the lock.
 		 */
-		if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
-			wake_up_state(waiter->task, waiter->wake_state);
+		top_waiter = rt_mutex_top_waiter(lock);
+		if (prerequeue_top_waiter != top_waiter)
+			wake_up_state(top_waiter->task, top_waiter->wake_state);
 		raw_spin_unlock_irq(&lock->wait_lock);
 		return 0;
 	}
-- 
2.39.0