[PATCH -v3 07/10] mutex: Restructure wait loop

[Peter Zijlstra <peterz@infradead.org>]

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Doesn't really matter yet, but pull the HANDOFF and trylock out from
under the wait_lock.

The intention is to add an optimistic spin loop here, which requires
we do not hold the wait_lock, so shuffle code around in preparation.

Also clarify the purpose of taking the wait_lock in the wait loop, its
tempting to want to avoid it altogether, but the cancellation cases
need to to avoid loosing wakeups.

Suggested-by: Waiman Long <waiman.long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/locking/mutex.c |   18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -627,10 +627,12 @@ __mutex_lock_common(struct mutex *lock,
 
 	lock_contended(&lock->dep_map, ip);
 
+	set_task_state(task, state);
 	for (;;) {
 		/*
-		 * got a signal? (This code gets eliminated in the
-		 * TASK_UNINTERRUPTIBLE case.)
+		 * Check for signals and wound conditions while holding
+		 * wait_lock. This ensures the lock cancellation is ordered
+		 * against mutex_unlock() and wake-ups do not go missing.
 		 */
 		if (unlikely(signal_pending_state(state, task))) {
 			ret = -EINTR;
@@ -643,20 +645,27 @@ __mutex_lock_common(struct mutex *lock,
 				goto err;
 		}
 
-		__set_task_state(task, state);
 		spin_unlock_mutex(&lock->wait_lock, flags);
 		schedule_preempt_disabled();
-		spin_lock_mutex(&lock->wait_lock, flags);
 
 		if (__mutex_waiter_is_first(lock, &waiter)) {
 			first = true;
 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 		}
 
+		set_task_state(task, state);
+		/*
+		 * Here we order against unlock; we must either see it change
+		 * state back to RUNNING and fall through the next schedule(),
+		 * or we must see its unlock and acquire.
+		 */
 		if (__mutex_trylock(lock, true))
 			break;
+
+		spin_lock_mutex(&lock->wait_lock, flags);
 	}
 	__set_task_state(task, TASK_RUNNING);
+	spin_lock_mutex(&lock->wait_lock, flags);
 
 remove_waiter:
 	mutex_remove_waiter(lock, &waiter, task);
@@ -681,6 +690,7 @@ __mutex_lock_common(struct mutex *lock,
 	return 0;
 
 err:
+	__set_task_state(task, TASK_RUNNING);
 	mutex_remove_waiter(lock, &waiter, task);
 	spin_unlock_mutex(&lock->wait_lock, flags);
 	debug_mutex_free_waiter(&waiter);