Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read

[Ian Kent]

On 27/7/23 12:30, Imran Khan wrote:
> Hello Ian,
> Sorry for late reply. I was about to reply this week.
>
> On 27/7/2023 10:38 am, Ian Kent wrote:
> > On 20/7/23 10:03, Ian Kent wrote:
> > > On Wed, 2023-07-19 at 12:23 +0800, Ian Kent wrote:
> [...]
> > > I do see a problem with recent changes.
> > >
> > > I'll send this off to Greg after I've done some testing (primarily just
> > > compile and function).
> > >
> > > Here's a patch which describes what I found.
> > >
> > > Comments are, of course, welcome, ;)
> > Anders I was hoping you would check if/what lockdep trace
> >
> > you get with this patch.
> >
> >
> > Imran, I was hoping you would comment on my change as it
> >
> > relates to the kernfs_iattr_rwsem changes.
> >
> >
> > Ian
> >
> > > kernfs: fix missing kernfs_iattr_rwsem locking
> > >
> > > From: Ian Kent <raven@xxxxxxxxxx>
> > >
> > > When the kernfs_iattr_rwsem was introduced a case was missed.
> > >
> > > The update of the kernfs directory node child count was also protected
> > > by the kernfs_rwsem and needs to be included in the change so that the
> > > child count (and so the inode n_link attribute) does not change while
> > > holding the rwsem for read.
> kernfs direcytory node's child count changes in kernfs_(un)link_sibling and
> these are getting invoked while adding (kernfs_add_one),
> removing(__kernfs_remove) or moving (kernfs_rename_ns)a node. Each of these
> operations proceed under kernfs_rwsem and I see each invocation of
> kernfs_link/unlink_sibling during the above mentioned operations is happening
> under kernfs_rwsem.
> So the child count should still be protected by kernfs_rwsem and we should not
> need to acquire kernfs_iattr_rwsem in kernfs_link/unlink_sibling.

Yes, that's exactly what I intended (assuming you mean write lock in 
those cases)

when I did it so now I wonder what I saw that lead to my patch, I'll 
need to look

again ...


> Kindly let me know your thoughts. I would still like to see new lockdep traces
> with this change.

Indeed, I hope Anders can find time to get the trace.


Ian

> Thanks,
> Imran
>
> > > Fixes: 9caf696142 (kernfs: Introduce separate rwsem to protect inode
> > > attributes)
> > >
> > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
> > > Cc: Anders Roxell <anders.roxell@xxxxxxxxxx>
> > > Cc: Imran Khan <imran.f.khan@xxxxxxxxxx>
> > > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>
> > > ---
> > >    fs/kernfs/dir.c |    4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> > > index 45b6919903e6..6e84bb69602e 100644
> > > --- a/fs/kernfs/dir.c
> > > +++ b/fs/kernfs/dir.c
> > > @@ -383,9 +383,11 @@ static int kernfs_link_sibling(struct kernfs_node
> > > *kn)
> > >        rb_insert_color(&kn->rb, &kn->parent->dir.children);
> > >          /* successfully added, account subdir number */
> > > +    down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
> > >        if (kernfs_type(kn) == KERNFS_DIR)
> > >            kn->parent->dir.subdirs++;
> > >        kernfs_inc_rev(kn->parent);
> > > +    up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
> > >          return 0;
> > >    }
> > > @@ -408,9 +410,11 @@ static bool kernfs_unlink_sibling(struct
> > > kernfs_node *kn)
> > >        if (RB_EMPTY_NODE(&kn->rb))
> > >            return false;
> > >    +    down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
> > >        if (kernfs_type(kn) == KERNFS_DIR)
> > >            kn->parent->dir.subdirs--;
> > >        kernfs_inc_rev(kn->parent);
> > > +    up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
> > >          rb_erase(&kn->rb, &kn->parent->dir.children);
> > >        RB_CLEAR_NODE(&kn->rb);