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Date:   Mon, 6 May 2019 15:26:41 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Jan Kara <jack@suse.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>, LKP <lkp@01.org>
Subject: Re: [PATCH] fsnotify: fix unlink performance regression
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On Mon, May 06, 2019 at 03:43:24PM +0300, Amir Goldstein wrote:
> OK. What do you have to say about this statement?
> 
>     Because fsnotify_nameremove() is called from d_delete() with negative
>     or unhashed dentry, d_move() is not expected on this dentry, so it is
>     safe to use d_parent/d_name without take_dentry_name_snapshot().
> 
> I assume it is not correct, but cannot figure out why.
> Under what circumstances is d_move() expected to move an unhashed
> dentry and hash it?

For starters, d_splice_alias() picking an exising alias for given directory
inode.

> My other thought is why is fsnotify_nameremove() in d_delete() and
> not in vfs_unlink()/vfs_rmdir() under parent inode lock like the rest
> of the fsnotify_create/fsnotify_move hooks?
> 
> In what case would we need the fsnotify event that is not coming
> from vfs_unlink()/vfs_rmdir()?

*snort*

You can thank those who whine about notifications on sysfs/devpts/whatnot.
Go talk to them if you wish, but don't ask me to translate what you'll get
into something coherent - I'd never been able to.