Re: [PATCH v2 4/6] fs: Establish locking order for unrelated directories

[Christian Brauner <brauner@kernel.org>]

On Thu, Jun 01, 2023 at 12:58:24PM +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@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/inode.c    | 42 ++++++++++++++++++++++++++++++++++++++++++
>  fs/internal.h |  2 ++
>  fs/namei.c    |  4 ++--
>  3 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 577799b7855f..4000ab08bbc0 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1103,6 +1103,48 @@ 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)

I think you forgot the opening bracket...
I can just fix this up for you though.

> +		/*
> +		 * Make sure @subclass1 will be used for the acquired lock.
> +		 * This is not strictly necessary (no current caller cares) but
> +		 * let's keep things consistent.
> +		 */
> +		if (!inode1)
> +			swap(inode1, inode2);
> +		goto lock;
> +	}