drm/sched: Lock drm_sched_entity_is_idle()

drm_sched_entity_is_idle() contains a badly documented memory barrier
and an invalid lockless access to entity->stopped. Moreover, it also
accesses entity->list which is at all other places protected by
entity->lock.

This function is in no way performance critical, so it is safer, more
readable and more maintainable to take the spinlock.

Add locking to drm_sched_entity_is_idle().

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260813094539.2407911-2-phasta@kernel.org

drivers/gpu/drm/scheduler/sched_entity.c

@@ -179,14 +179,18 @@ EXPORT_SYMBOL(drm_sched_entity_modify_sched);
 
 static bool drm_sched_entity_is_idle(struct drm_sched_entity *entity)
 {
-	rmb(); /* for list_empty to work without lock */
+	bool idle = false;
+
+	spin_lock(&entity->lock);
 
 	if (list_empty(&entity->list) ||
 	    spsc_queue_count(&entity->job_queue) == 0 ||
 	    entity->stopped)
-		return true;
+		idle = true;
 
-	return false;
+	spin_unlock(&entity->lock);
+
+	return idle;
 }
 
 /**