Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement

Fox Chen
2020-12-21T10:07:49Z

On Sun, Dec 20, 2020 at 7:52 AM Ian Kent <raven@themaw.net> wrote:
>
> On Sat, 2020-12-19 at 11:23 -0500, Tejun Heo wrote:
> > Hello,
> >
> > On Sat, Dec 19, 2020 at 03:08:13PM +0800, Ian Kent wrote:
> > > And looking further I see there's a race that kernfs can't do
> > > anything
> > > about between kernfs_refresh_inode() and fs/inode.c:update_times().
> >
> > Do kernfs files end up calling into that path tho? Doesn't look like
> > it to
> > me but if so yeah we'd need to override the update_time for kernfs.
>
> Sorry, the below was very hastily done and not what I would actually
> propose.
>
> The main point of it was the question
>
> + /* Which kernfs node attributes should be updated from
> + * time?
> + */
>
> but looking at it again this morning I think the node iattr fields
> that might need to be updated would be atime, ctime and mtime only,
> maybe not ctime ... not sure.
>
> What do you think?
>
> Also, if kn->attr == NULL it should fall back to what the VFS
> currently does.
>
> The update_times() function is one of the few places where the
> VFS updates the inode times.
>
> The idea is that the reason kernfs needs to overwrite the inode
> attributes is to reset what the VFS might have done but if kernfs
> has this inode operation they won't need to be overwritten since
> they won't have changed.
>
> There may be other places where the attributes (or an attribute)
> are set by the VFS, I haven't finished checking that yet so my
> suggestion might not be entirely valid.
>
> What I need to do is work out what kernfs node attributes, if any,
> should be updated by .update_times(). If I go by what
> kernfs_refresh_inode() does now then that would be none but shouldn't
> atime at least be updated in the node iattr.
>
> > > +static int kernfs_iop_update_time(struct inode *inode, struct
> > > timespec64 *time, int flags)
> > > {
> > > - struct inode *inode = d_inode(path->dentry);
> > > struct kernfs_node *kn = inode->i_private;
> > > + struct kernfs_iattrs *attrs;
> > >
> > > mutex_lock(&kernfs_mutex);
> > > + attrs = kernfs_iattrs(kn);
> > > + if (!attrs) {
> > > + mutex_unlock(&kernfs_mutex);
> > > + return -ENOMEM;
> > > + }
> > > +
> > > + /* Which kernfs node attributes should be updated from
> > > + * time?
> > > + */
> > > +
> > > kernfs_refresh_inode(kn, inode);
> > > mutex_unlock(&kernfs_mutex);
> >
> > I don't see how this would reflect the changes from kernfs_setattr()
> > into
> > the attached inode. This would actually make the attr updates
> > obviously racy
> > - the userland visible attrs would be stale until the inode gets
> > reclaimed
> > and then when it gets reinstantiated it'd show the latest
> > information.
>
> Right, I will have to think about that, but as I say above this
> isn't really what I would propose.
>
> If .update_times() sticks strictly to what kernfs_refresh_inode()
> does now then it would set the inode attributes from the node iattr
> only.
>
> >
> > That said, if you wanna take the direction where attr updates are
> > reflected
> > to the associated inode when the change occurs, which makes sense,
> > the right
> > thing to do would be making kernfs_setattr() update the associated
> > inode if
> > existent.
>
> Mmm, that's a good point but it looks like the inode isn't available
> there.
>
Is it possible to embed super block somewhere, then we can call
kernfs_get_inode to get inode in kernfs_setattr???

thanks,
fox