Fix to l4_mutex_control for the unexpected sleeping with mutex held

when preemption occurs after call to wait_on_prepare

src/api/mutex.c

@@ -54,7 +54,8 @@ void mutex_queue_head_lock()
 
 void mutex_queue_head_unlock()
 {
-	mutex_unlock(&mutex_queue_head.mutex_control_mutex);
+	/* Async unlock because in some cases preemption may be disabled here */
+	mutex_unlock_async(&mutex_queue_head.mutex_control_mutex);
 }
 
 
@@ -170,11 +171,18 @@ int mutex_control_lock(unsigned long mutex_address)
 
 	/* Prepare to wait on the contenders queue */
 	CREATE_WAITQUEUE_ON_STACK(wq, current);
+
+	/* Disable to protect from sleeping by preemption */
+	preempt_disable();
+
 	wait_on_prepare(&mutex_queue->wqh_contenders, &wq);
 
 	/* Release lock */
 	mutex_queue_head_unlock();
 
+	/* Now safe to sleep voluntarily or by preemption */
+	preempt_enable();
+
 	/* Initiate prepared wait */
 	return wait_on_prepared_wait();
 }
@@ -213,11 +221,19 @@ int mutex_control_unlock(unsigned long mutex_address)
 
 		/* Prepare to wait on the lock holders queue */
 		CREATE_WAITQUEUE_ON_STACK(wq, current);
+
+		/* Disable to protect from sleeping by preemption */
+		preempt_disable();
+
+		/* Prepare to wait */
 		wait_on_prepare(&mutex_queue->wqh_holders, &wq);
 
-		/* Release lock */
+		/* Release lock first */
 		mutex_queue_head_unlock();
 
+		/* Now safe to sleep voluntarily or by preemption */
+		preempt_enable();
+
 		/* Initiate prepared wait */
 		return wait_on_prepared_wait();
 	}