dm: delay registering the gendisk

device mapper is currently the only outlier that tries to call
register_disk after add_disk, leading to fairly inconsistent state
of these block layer data structures.  Instead change device-mapper
to just register the gendisk later now that the holder mechanism
can cope with that.

Note that this introduces a user visible change: the dm kobject is
now only visible after the initial table has been loaded.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20210804094147.459763-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-rq.c

@@ -559,7 +559,6 @@ int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t)
 	err = blk_mq_init_allocated_queue(md->tag_set, md->queue);
 	if (err)
 		goto out_tag_set;
-	elevator_init_mq(md->queue);
 	return 0;
 
 out_tag_set:

drivers/md/dm.c

@@ -1693,7 +1693,10 @@ static void cleanup_mapped_device(struct mapped_device *md)
 		spin_lock(&_minor_lock);
 		md->disk->private_data = NULL;
 		spin_unlock(&_minor_lock);
-		del_gendisk(md->disk);
+		if (dm_get_md_type(md) != DM_TYPE_NONE) {
+			dm_sysfs_exit(md);
+			del_gendisk(md->disk);
+		}
 		dm_queue_destroy_keyslot_manager(md->queue);
 		blk_cleanup_disk(md->disk);
 	}
@@ -1788,7 +1791,6 @@ static struct mapped_device *alloc_dev(int minor)
 			goto bad;
 	}
 
-	add_disk_no_queue_reg(md->disk);
 	format_dev_t(md->name, MKDEV(_major, minor));
 
 	md->wq = alloc_workqueue("kdmflush", WQ_MEM_RECLAIM, 0);
@@ -1989,19 +1991,12 @@ static struct dm_table *__unbind(struct mapped_device *md)
  */
 int dm_create(int minor, struct mapped_device **result)
 {
-	int r;
 	struct mapped_device *md;
 
 	md = alloc_dev(minor);
 	if (!md)
 		return -ENXIO;
 
-	r = dm_sysfs_init(md);
-	if (r) {
-		free_dev(md);
-		return r;
-	}
-
 	*result = md;
 	return 0;
 }
@@ -2081,10 +2076,15 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
 	r = dm_table_set_restrictions(t, md->queue, &limits);
 	if (r)
 		return r;
+
+	add_disk(md->disk);
+
+	r = dm_sysfs_init(md);
+	if (r) {
+		del_gendisk(md->disk);
+		return r;
+	}
 	md->type = type;
-
-	blk_register_queue(md->disk);
-
 	return 0;
 }
 
@@ -2190,7 +2190,6 @@ static void __dm_destroy(struct mapped_device *md, bool wait)
 		DMWARN("%s: Forcibly removing mapped_device still in use! (%d users)",
 		       dm_device_name(md), atomic_read(&md->holders));
 
-	dm_sysfs_exit(md);
 	dm_table_destroy(__unbind(md));
 	free_dev(md);
 }