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

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Message-ID: <8304e49310b16b865868982802bd2344d076c974.camel@themaw.net>
Date:   Sun, 13 Jun 2021 09:57:10 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...iv.linux.org.uk>
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, 2021-06-12 at 01:51 +0000, Al Viro wrote:
> 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).

Understood, thanks.

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