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

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Message-ID: <CAPjX3Fem5E26+Fj537zcOXk9YZum2pDdcY9SAwCOr12wrGrroA@mail.gmail.com>
Date: Tue, 8 Apr 2025 19:36:35 +0200
From: Daniel Vacek <neelx@...e.com>
To: Filipe Manana <fdmanana@...nel.org>
Cc: Yangtao Li <frank.li@...o.com>, 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, 8 Apr 2025 at 16:47, Filipe Manana <fdmanana@...nel.org> wrote:
>
> 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()).

I do not follow. This actually looks good to me.
Is there really any functional change? What am I missing?

--nX

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