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

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Date:   Fri, 9 Apr 2021 01:35:05 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Brice Goglin <brice.goglin@...il.com>,
        Fox Chen <foxhlchen@...il.com>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>,
        Eric Sandeen <sandeen@...deen.net>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 2/4] kernfs: use VFS negative dentry caching

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?