linux-ext4 - Re: [PATCH 4/6] fs: Establish locking order for unrelated directories

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Message-ID: <20230526-polarstern-herrichten-32fc46c63bfc@brauner>
Date:   Fri, 26 May 2023 11:45:15 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Al Viro <viro@...IV.linux.org.uk>, linux-fsdevel@...r.kernel.org,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J. Wong" <djwong@...nel.org>, Ted Tso <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 4/6] fs: Establish locking order for unrelated directories

On Thu, May 25, 2023 at 12:16:10PM +0200, Jan Kara wrote:
> Currently the locking order of inode locks for directories that are not
> in ancestor relationship is not defined because all operations that
> needed to lock two directories like this were serialized by
> sb->s_vfs_rename_mutex. However some filesystems need to lock two
> subdirectories for RENAME_EXCHANGE operations and for this we need the
> locking order established even for two tree-unrelated directories.
> Provide a helper function lock_two_inodes() that establishes lock
> ordering for any two inodes and use it in lock_two_directories().
> 
> CC: stable@...r.kernel.org
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/inode.c    | 34 ++++++++++++++++++++++++++++++++++
>  fs/internal.h |  2 ++
>  fs/namei.c    |  4 ++--
>  3 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 577799b7855f..2015fa50d34a 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1103,6 +1103,40 @@ void discard_new_inode(struct inode *inode)
>  }
>  EXPORT_SYMBOL(discard_new_inode);
>  
> +/**
> + * lock_two_inodes - lock two inodes (may be regular files but also dirs)
> + *
> + * Lock any non-NULL argument. The caller must make sure that if he is passing
> + * in two directories, one is not ancestor of the other.  Zero, one or two
> + * objects may be locked by this function.
> + *
> + * @inode1: first inode to lock
> + * @inode2: second inode to lock
> + * @subclass1: inode lock subclass for the first lock obtained
> + * @subclass2: inode lock subclass for the second lock obtained
> + */
> +void lock_two_inodes(struct inode *inode1, struct inode *inode2,
> +		     unsigned subclass1, unsigned subclass2)
> +{
> +	if (!inode1 || !inode2)
> +		goto lock;

Before this change in

lock_two_nondirectories(struct inode *inode1, struct inode *inode2)

the swap() would cause the non-NULL inode to always be locked with
I_MUTEX_NONDIR2. Now it can be either I_MUTEX_NORMAL or I_MUTEX_NONDIR2.
Is that change intentional?