btrfs: explicitly ref count block_group on new_bgs list

All other users of the bg_list list_head increment the refcount when
adding to a list and decrement it when deleting from the list. Just for
the sake of uniformity and to try to avoid refcounting bugs, do it for
this list as well.

This does not fix any known ref-counting bug, as the reference belongs
to a single task (trans_handle is not shared and this represents
trans_handle->new_bgs linkage) and will not lose its original refcount
while that thread is running. And BLOCK_GROUP_FLAG_NEW protects against
ref-counting errors "moving" the block group to the unused list without
taking a ref.

With that said, I still believe it is simpler to just hold the extra ref
count for this list user as well.

Reviewed-by: Filipe Manana <[email protected]>
Signed-off-by: Boris Burkov <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: boryas

fs/btrfs/block-group.c

@@ -2801,6 +2801,7 @@ void btrfs_create_pending_block_groups(struct btrfs_trans_handle *trans)
 		spin_lock(&fs_info->unused_bgs_lock);
 		list_del_init(&block_group->bg_list);
 		clear_bit(BLOCK_GROUP_FLAG_NEW, &block_group->runtime_flags);
+		btrfs_put_block_group(block_group);
 		spin_unlock(&fs_info->unused_bgs_lock);
 
 		/*
@@ -2939,6 +2940,7 @@ struct btrfs_block_group *btrfs_make_block_group(struct btrfs_trans_handle *tran
 	}
 #endif
 
+	btrfs_get_block_group(cache);
 	list_add_tail(&cache->bg_list, &trans->new_bgs);
 	btrfs_inc_delayed_refs_rsv_bg_inserts(fs_info);
 

fs/btrfs/transaction.c

@@ -2108,6 +2108,7 @@ static void btrfs_cleanup_pending_block_groups(struct btrfs_trans_handle *trans)
 		*/
 	       spin_lock(&fs_info->unused_bgs_lock);
                list_del_init(&block_group->bg_list);
+	       btrfs_put_block_group(block_group);
 	       spin_unlock(&fs_info->unused_bgs_lock);
        }
 }