Re: [PATCH v3 2/4] kernfs: use VFS negative dentry caching

[Ian Kent]

On Fri, 2021-04-09 at 01:35 +0000, Al Viro wrote:
> On Fri, Apr 09, 2021 at 09:15:06AM +0800, Ian Kent wrote:
> > +		parent = kernfs_dentry_node(dentry->d_parent);
> > +		if (parent) {
> > +			const void *ns = NULL;
> > +
> > +			if (kernfs_ns_enabled(parent))
> > +				ns = kernfs_info(dentry->d_parent-
> > >d_sb)->ns;
> 
> 	For any dentry d, we have d->d_parent->d_sb == d->d_sb.  All
> the time.
> If you ever run into the case where that would not be true, you've
> found
> a critical bug.

Right, yes.

> 
> > +			kn = kernfs_find_ns(parent, dentry-
> > >d_name.name, ns);
> > +			if (kn)
> > +				goto out_bad;
> > +		}
> 
> Umm...  What's to prevent a race with successful rename(2)?  IOW,
> what's
> there to stabilize ->d_parent and ->d_name while we are in that
> function?

Indeed, glad you looked at this.

Now I'm wondering how kerfs_iop_rename() protects itself from
concurrent kernfs_rename_ns() ...