linux-kernel - Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching

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Message-ID: <YMQTMnfmOfdv2DpA@zeniv-ca.linux.org.uk>
Date:   Sat, 12 Jun 2021 01:51:46 +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>, Eric Sandeen <sandeen@...deen.net>,
        Fox Chen <foxhlchen@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching

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().