diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 1c20d773dd0c..6beb1f5b71ac 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -340,7 +340,15 @@ static ssize_t nullb_device_bool_attr_store(bool *val, const char *page,
 	return count;
 }
 
-/* The following macro should only be used with TYPE = {uint, ulong, bool}. */
+/*
+ * The following macro should only be used with TYPE = {uint, ulong, bool}.
+ *
+ * The device configuration is modified under the global lock to serialize
+ * attribute changes against null_add_dev() and null_del_dev(): without this,
+ * an attribute could be changed while null_add_dev() is running, that is,
+ * before NULLB_DEV_FL_CONFIGURED is set, which would let null_add_dev()
+ * observe inconsistent values for the device configuration.
+ */
 #define NULLB_DEVICE_ATTR(NAME, TYPE, APPLY)				\
 static ssize_t								\
 nullb_device_##NAME##_show(struct config_item *item, char *page)	\
@@ -381,6 +389,8 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 	struct blk_mq_tag_set *set;
 	int ret, nr_hw_queues;
 
+	lockdep_assert_held(&lock);
+
 	if (!dev->nullb)
 		return 0;
 
@@ -2205,8 +2215,6 @@ static void __exit null_exit(void)
 
 	if (tag_set.ops)
 		blk_mq_free_tag_set(&tag_set);
-
-	mutex_destroy(&lock);
 }
 
 module_init(null_init);