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Date:   Sat, 12 Jun 2021 00:07:18 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Ian Kent <raven@themaw.net>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, Eric Sandeen <sandeen@sandeen.net>,
        Fox Chen <foxhlchen@gmail.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching
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On Wed, Jun 09, 2021 at 04:50:27PM +0800, Ian Kent wrote:

> +	if (d_really_is_negative(dentry)) {
> +		struct dentry *d_parent = dget_parent(dentry);
> +		struct kernfs_node *parent;

What the hell is dget_parent() for?  You don't do anything blocking
here, so why not simply grab dentry->d_lock - that'll stabilize
the value of ->d_parent just fine.  Just don't forget to drop the
lock before returning and that's it...

> +		/* If the kernfs parent node has changed discard and
> +		 * proceed to ->lookup.
> +		 */
> +		parent = kernfs_dentry_node(d_parent);
> +		if (parent) {
> +			if (kernfs_dir_changed(parent, dentry)) {
> +				dput(d_parent);
> +				return 0;
> +			}
> +		}
> +		dput(d_parent);
> +
> +		/* The kernfs node doesn't exist, leave the dentry
> +		 * negative and return success.
> +		 */
> +		return 1;
> +	}