block: delay freeing the gendisk

blkdev_get_no_open acquires a reference to the block_device through
the block device inode and then tries to acquire a device model
reference to the gendisk.  But at this point the disk migh already
be freed (although the race is free).  Fix this by only freeing the
gendisk from the whole device bdevs ->free_inode callback as well.

Fixes: 22ae8ce8 ("block: simplify bdev/disk lookup in blkdev_get")
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210722075402.983367-2-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -1079,10 +1079,9 @@ static void disk_release(struct device *dev)
 	disk_release_events(disk);
 	kfree(disk->random);
 	xa_destroy(&disk->part_tbl);
-	bdput(disk->part0);
 	if (test_bit(GD_QUEUE_REF, &disk->state) && disk->queue)
 		blk_put_queue(disk->queue);
-	kfree(disk);
+	bdput(disk->part0);	/* frees the disk */
 }
 struct class block_class = {
 	.name		= "block",

fs/block_dev.c

@@ -812,6 +812,8 @@ static void bdev_free_inode(struct inode *inode)
 	free_percpu(bdev->bd_stats);
 	kfree(bdev->bd_meta_info);
 
+	if (!bdev_is_partition(bdev))
+		kfree(bdev->bd_disk);
 	kmem_cache_free(bdev_cachep, BDEV_I(inode));
 }