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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>,
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Subject: Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching
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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().