Re: [PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2020/2/21 下午9:11, Nikolay Borisov wrote:
> __del_reloc_root is called from:
>
> a) Transaction commit via:
> btrfs_transaction_commit
>  commit_fs_roots
>   btrfs_update_reloc_root
>    __del_reloc_root
>
> b) From the relocation thread with the following call chains:
>
> relocate_block_group
>  merge_reloc_roots
>   insert_dirty_subvol
>    btrfs_update_reloc_root
>     __del_reloc_root
>
> c) merge_reloc_roots
>     free_reloc_roots
>      __del_reloc_roots
>
> (The above call chain can called from btrfs_recover_relocation as well
> but for the purpose of this fix this is irrelevant).
>
> The commont data structure that needs protecting is
> fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list.
> Turns out it's no needed to hold the trans_lock in __del_reloc_root
> since consistency is already guaranteed by call chain b) above holding
> a transaction while calling insert_dirty_subvol, meaning we cannot have
> a concurrent transaction commit. For call chain c) above a snapshot of
> the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and
> free_reloc_roots is called on this local snapshot.
>
> Those invariants are sufficient to prevent racing calls to
> __del_reloc_root alongside other users of the list, as such it's fine
> to drop the lock acquisition.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>  fs/btrfs/relocation.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 8076c340749f..e5cb64409f7c 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root)
>  		BUG_ON((struct btrfs_root *)node->data != root);
>  	}
>
> -	spin_lock(&fs_info->trans_lock);
>  	list_del_init(&root->root_list);
> -	spin_unlock(&fs_info->trans_lock);
>  	kfree(node);
>  }
>
>