linux-kernel - Re: [PATCH 1/4] btrfs: use BTRFS_PATH_AUTO_FREE in insert_balance

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Message-ID: <CAL3q7H6ysGxpXs8P9iPY-Y1KNKPggGSFHR_tMv-34Q+Qf6PZTQ@mail.gmail.com>
Date: Tue, 8 Apr 2025 15:44:57 +0100
From: Filipe Manana <fdmanana@...nel.org>
To: Yangtao Li <frank.li@...o.com>
Cc: clm@...com, josef@...icpanda.com, dsterba@...e.com, 
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] btrfs: use BTRFS_PATH_AUTO_FREE in insert_balance_item()

On Tue, Apr 8, 2025 at 1:18 PM Yangtao Li <frank.li@...o.com> wrote:
>
> All cleanup paths lead to btrfs_path_free so we can define path with the
> automatic free callback.
>
> Signed-off-by: Yangtao Li <frank.li@...o.com>
> ---
>  fs/btrfs/volumes.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index c8c21c55be53..a962efaec4ea 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3730,7 +3730,7 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>         struct btrfs_trans_handle *trans;
>         struct btrfs_balance_item *item;
>         struct btrfs_disk_balance_args disk_bargs;
> -       struct btrfs_path *path;
> +       BTRFS_PATH_AUTO_FREE(path);
>         struct extent_buffer *leaf;
>         struct btrfs_key key;
>         int ret, err;
> @@ -3740,10 +3740,8 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>                 return -ENOMEM;
>
>         trans = btrfs_start_transaction(root, 0);
> -       if (IS_ERR(trans)) {
> -               btrfs_free_path(path);
> +       if (IS_ERR(trans))
>                 return PTR_ERR(trans);
> -       }
>
>         key.objectid = BTRFS_BALANCE_OBJECTID;
>         key.type = BTRFS_TEMPORARY_ITEM_KEY;
> @@ -3767,7 +3765,6 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>         btrfs_set_balance_sys(leaf, item, &disk_bargs);
>         btrfs_set_balance_flags(leaf, item, bctl->flags);
>  out:
> -       btrfs_free_path(path);
>         err = btrfs_commit_transaction(trans);

This isn't a good idea at all.
We're now committing a transaction while holding a write lock on some
leaf of the tree root - this can result in a deadlock as the
transaction commit needs to update the tree root (see
update_cowonly_root()).

Thanks.


>         if (err && !ret)
>                 ret = err;
> --
> 2.39.0
>
>