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

[Al Viro]

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.

> +			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?