block: call bdev_add later in device_add_disk

Once bdev_add is called userspace can open the block device.  Ensure
that the struct device, which is used for refcounting of the disk
besides various other things, is fully setup at that point.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20210818144542.19305-4-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -466,29 +466,14 @@ void device_add_disk(struct device *parent, struct gendisk *disk,
 
 	disk_alloc_events(disk);
 
-	if (disk->flags & GENHD_FL_HIDDEN) {
-		/*
-		 * Don't let hidden disks show up in /proc/partitions,
-		 * and don't bother scanning for partitions either.
-		 */
-		disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
-		disk->flags |= GENHD_FL_NO_PART_SCAN;
-	} else {
-		/* Register BDI before referencing it from bdev */
-		ddev->devt = MKDEV(disk->major, disk->first_minor);
-		ret = bdi_register(disk->bdi, "%u:%u",
-				   disk->major, disk->first_minor);
-		WARN_ON(ret);
-		bdi_set_owner(disk->bdi, ddev);
-		bdev_add(disk->part0, ddev->devt);
-	}
-
 	/* delay uevents, until we scanned partition table */
 	dev_set_uevent_suppress(ddev, 1);
 
 	ddev->parent = parent;
 	ddev->groups = groups;
 	dev_set_name(ddev, "%s", disk->disk_name);
+	if (!(disk->flags & GENHD_FL_HIDDEN))
+		ddev->devt = MKDEV(disk->major, disk->first_minor);
 	if (device_add(ddev))
 		return;
 	if (!sysfs_deprecated) {
@@ -521,12 +506,25 @@ void device_add_disk(struct device *parent, struct gendisk *disk,
 		disk->slave_dir = NULL;
 	}
 
-	if (!(disk->flags & GENHD_FL_HIDDEN)) {
+	if (disk->flags & GENHD_FL_HIDDEN) {
+		/*
+		 * Don't let hidden disks show up in /proc/partitions,
+		 * and don't bother scanning for partitions either.
+		 */
+		disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
+		disk->flags |= GENHD_FL_NO_PART_SCAN;
+	} else {
+		ret = bdi_register(disk->bdi, "%u:%u",
+				   disk->major, disk->first_minor);
+		WARN_ON(ret);
+		bdi_set_owner(disk->bdi, ddev);
+		bdev_add(disk->part0, ddev->devt);
+
 		disk_scan_partitions(disk);
 
 		/*
 		 * Announce the disk and partitions after all partitions are
-		 * created.
+		 * created. (for hidden disks uevents remain suppressed forever)
 		 */
 		dev_set_uevent_suppress(ddev, 0);
 		disk_uevent(disk, KOBJ_ADD);