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

Christian Brauner
2023-05-30T12:42:53Z

On Mon, May 29, 2023 at 02:41:31PM +0200, Jan Kara wrote:
> On Fri 26-05-23 11:45:15, Christian Brauner wrote:
> > 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@vger.kernel.org
> > > Signed-off-by: Jan Kara <jack@suse.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?
>
> Kind of. I don't think we really care so I didn't bother to complicate the
> code for this. If you think keeping the lockdep class consistent is worth
> it, I can modify the patch...

Either a short comment or consistent lockdep class would be nice. I know
it probably doesn't matter much but otherwise someone may end up
wondering whether that's ok or not.