Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sat, 12 Jun 2021 01:51:46 +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
Message-ID: <YMQTMnfmOfdv2DpA@zeniv-ca.linux.org.uk>
References: <162322846765.361452.17051755721944717990.stgit@web.messagingengine.com>
 <162322862726.361452.10114120072438540655.stgit@web.messagingengine.com>
 <YMP6topegaTXGNgC@zeniv-ca.linux.org.uk>
 <2ee74cbed729d66a38a5c7de9c4608d02fb89f26.camel@themaw.net>
 <ab91ce6f0c1b2e9549fc6e966db7514a988d0bf1.camel@themaw.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ab91ce6f0c1b2e9549fc6e966db7514a988d0bf1.camel@themaw.net>
Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk

On Sat, Jun 12, 2021 at 09:08:05AM +0800, Ian Kent wrote:

> But if I change to take the read lock to ensure there's no operation
> in progress for the revision check I would need the dget_parent(), yes?

WTF for?  ->d_parent can change *ONLY* when ->d_lock is held on all
dentries involved (including old and new parents).

And it very definitely does *not* change for negative dentries.  I mean,
look at the very beginning of __d_move().