block: take bd_mutex around delete_partitions in del_gendisk

There is nothing preventing an ioctl from trying do delete partition
concurrenly with del_gendisk, so take open_mutex to serialize against
that.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406062303.811835-6-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -680,7 +680,11 @@ void del_gendisk(struct gendisk *disk)
 	 * disk is marked as dead (GENHD_FL_UP cleared).
 	 */
 	down_write(&bdev_lookup_sem);
+
+	mutex_lock(&disk->part0->bd_mutex);
 	blk_drop_partitions(disk);
+	mutex_unlock(&disk->part0->bd_mutex);
+
 	fsync_bdev(disk->part0);
 	__invalidate_device(disk->part0, true);
 

block/partitions/core.c

@@ -531,6 +531,8 @@ void blk_drop_partitions(struct gendisk *disk)
 	struct disk_part_iter piter;
 	struct block_device *part;
 
+	lockdep_assert_held(&disk->part0->bd_mutex);
+
 	disk_part_iter_init(&piter, disk, DISK_PITER_INCL_EMPTY);
 	while ((part = disk_part_iter_next(&piter)))
 		delete_partition(part);