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

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Date:   Mon,  9 Jan 2017 11:52:03 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     linux-kernel@...r.kernel.org
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <Waiman.Long@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] locking/mutex: Clear mutex-handoff flag on interrupt

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@...is-wilson.co.uk>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Waiman Long <Waiman.Long@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.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