[PATCH] locking/mutex: Clear mutex-handoff flag on interrupt

[Chris Wilson <chris@chris-wilson.co.uk>]

If we abort the mutex_lock() due to an interrupt, or other error from
ww_mutex, we need to relinquish the handoff flag if we applied it.
Otherwise, we may cause missed wakeups as the current owner may try to
handoff to a new thread that is not expecting the handoff and so sleep
thinking the lock is already claimed (and since the owner unlocked there
may never be a new wakeup).

Fixes: 9d659ae14b54 ("locking/mutex: Add lock handoff to avoid starvation")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <Waiman.Long@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
---
 kernel/locking/mutex.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 9b349619f431..087bbc082ad7 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -738,6 +738,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 err:
 	__set_task_state(task, TASK_RUNNING);
 	mutex_remove_waiter(lock, &waiter, task);
+	if (first)
+		__mutex_clear_flag(lock, MUTEX_FLAG_HANDOFF);
 	spin_unlock_mutex(&lock->wait_lock, flags);
 	debug_mutex_free_waiter(&waiter);
 	mutex_release(&lock->dep_map, 1, ip);
-- 
2.11.0