block: add proper helpers for elevator_type module refcount management

Make sure we have helpers for all relevant module refcount operations on
the elevator_type in elevator.h, and use them.  Move the call to the get
helper in blk_mq_elv_switch_none a bit so that it is obvious with a less
verbose comment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221020064819.1469928-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -4554,17 +4554,10 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 
 	INIT_LIST_HEAD(&qe->node);
 	qe->q = q;
+	/* keep a reference to the elevator module as we'll switch back */
+	__elevator_get(qe->type);
 	qe->type = q->elevator->type;
 	list_add(&qe->node, head);
-
-	/*
-	 * After elevator_switch, the previous elevator_queue will be
-	 * released by elevator_release. The reference of the io scheduler
-	 * module get by elevator_get will also be put. So we need to get
-	 * a reference of the io scheduler module here to prevent it to be
-	 * removed.
-	 */
-	__module_get(qe->type->elevator_owner);
 	elevator_switch(q, NULL);
 	mutex_unlock(&q->sysfs_lock);
 

block/elevator.c

@@ -132,11 +132,6 @@ static struct elevator_type *elevator_find(const char *name,
 	return NULL;
 }
 
-static void elevator_put(struct elevator_type *e)
-{
-	module_put(e->elevator_owner);
-}
-
 static struct elevator_type *elevator_get(struct request_queue *q,
 					  const char *name, bool try_loading)
 {
@@ -152,7 +147,7 @@ static struct elevator_type *elevator_get(struct request_queue *q,
 		e = elevator_find(name, q->required_elevator_features);
 	}
 
-	if (e && !try_module_get(e->elevator_owner))
+	if (e && !elevator_tryget(e))
 		e = NULL;
 
 	spin_unlock(&elv_list_lock);
@@ -659,7 +654,7 @@ static struct elevator_type *elevator_get_by_features(struct request_queue *q)
 		}
 	}
 
-	if (found && !try_module_get(found->elevator_owner))
+	if (found && !elevator_tryget(found))
 		found = NULL;
 
 	spin_unlock(&elv_list_lock);

block/elevator.h

@@ -84,6 +84,21 @@ struct elevator_type
 	struct list_head list;
 };
 
+static inline bool elevator_tryget(struct elevator_type *e)
+{
+	return try_module_get(e->elevator_owner);
+}
+
+static inline void __elevator_get(struct elevator_type *e)
+{
+	__module_get(e->elevator_owner);
+}
+
+static inline void elevator_put(struct elevator_type *e)
+{
+	module_put(e->elevator_owner);
+}
+
 #define ELV_HASH_BITS 6
 
 void elv_rqhash_del(struct request_queue *q, struct request *rq);