block: move queue freezing & elevator_lock into elevator_change()

JIRA: https://issues.redhat.com/browse/RHEL-106845

commit 20117b5
Author: Ming Lei <ming.lei@redhat.com>
Date:   Mon May 5 22:17:52 2025 +0800

    block: move queue freezing & elevator_lock into elevator_change()

    Move queue freezing & elevator_lock into elevator_change(), and prepare
    for using elevator_change() for setting up & tearing down default elevator
    too.

    Also add lockdep_assert_held() in __elevator_change() because either
    read or write lock is required for changing elevator.

    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Reviewed-by: Hannes Reinecke <hare@suse.de>
    Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Link: https://lore.kernel.org/r/20250505141805.2751237-15-ming.lei@redhat.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/elevator.c

@@ -676,10 +676,19 @@ static int elevator_switch(struct request_queue *q, const char *name)
  */
 static int elevator_change(struct request_queue *q, const char *elevator_name)
 {
-	if (q->elevator && elevator_match(q->elevator->type, elevator_name))
-		return 0;
+	unsigned int memflags;
+	int ret = 0;
 
-	return elevator_switch(q, elevator_name);
+	lockdep_assert_held(&q->tag_set->update_nr_hwq_lock);
+
+	memflags = blk_mq_freeze_queue(q);
+	mutex_lock(&q->elevator_lock);
+	if (!(q->elevator && elevator_match(q->elevator->type,
+				elevator_name)))
+		ret = elevator_switch(q, elevator_name);
+	mutex_unlock(&q->elevator_lock);
+	blk_mq_unfreeze_queue(q, memflags);
+	return ret;
 }
 
 /*
@@ -718,7 +727,6 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	char elevator_name[ELV_NAME_MAX];
 	char *name;
 	int ret;
-	unsigned int memflags;
 	struct request_queue *q = disk->queue;
 	struct blk_mq_tag_set *set = q->tag_set;
 
@@ -737,13 +745,9 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	elv_iosched_load_module(name);
 
 	down_read(&set->update_nr_hwq_lock);
-	memflags = blk_mq_freeze_queue(q);
-	mutex_lock(&q->elevator_lock);
 	ret = elevator_change(q, name);
 	if (!ret)
 		ret = count;
-	mutex_unlock(&q->elevator_lock);
-	blk_mq_unfreeze_queue(q, memflags);
 	up_read(&set->update_nr_hwq_lock);
 	return ret;
 }