rt-mutex: fix chain walk early wakeup bug

Alexey Kuznetsov found some problems in the pi-futex code.

One of the root causes is:

When a wakeup happens, we do not to stop the chain walk so we follow a not
longer relevant locking chain.

Drop out when this happens.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NIngo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/rtmutex.c

@@ -189,6 +189,19 @@ int rt_mutex_adjust_prio_chain(struct task_struct *task,
 	if (!waiter || !waiter->task)
 		goto out_unlock_pi;
 
+	/*
+	 * Check the orig_waiter state. After we dropped the locks,
+	 * the previous owner of the lock might have released the lock
+	 * and made us the pending owner:
+	 */
+	if (orig_waiter && !orig_waiter->task)
+		goto out_unlock_pi;
+
+	/*
+	 * Drop out, when the task has no waiters. Note,
+	 * top_waiter can be NULL, when we are in the deboosting
+	 * mode!
+	 */
 	if (top_waiter && (!task_has_pi_waiters(task) ||
 			   top_waiter != task_top_pi_waiter(task)))
 		goto out_unlock_pi;