linux-kernel - Re: [PATCH] fs/namespace: don't clobber mnt

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Message-ID: <CA+55aFzsD3cvmQN_JFRkiNrQ=0G474_fi0Ki5hCyonxd8vw=pA@mail.gmail.com>
Date:	Wed, 19 Mar 2014 21:02:33 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Max Kellermann <mk@...all.com>, max@...mpel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs/namespace: don't clobber mnt_hash.next while umounting [v2]

On Wed, Mar 19, 2014 at 8:48 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> NAK.  Nice catch, the bug is real, but the fix is wrong.  For one thing,
> you have missed detach_mnt()/attach_mnt(), so you are not covering
> all places where the sucker might be removed from the list.  For another,
> I don't believe that this is the right approach.
>
> The *only* thing we care about is not getting stuck in __lookup_mnt().

Quite frankly, if that's the main issue, then may I suggest aiming to
use a 'hlist' instead of a doubly-linked list? Those have the
advantage that they are NULL-terminated.

Yeah, hlists have some disadvantages too, which might not make them
work in this case, but really, for mnt_hash? hlists are generally
*exactly* what you want for hash lists, because the head is smaller.
And because of the NULL termination rather than having the head used
in the middle of a circular list, you don't get the termination
problems when moving entries across chains.

I did not look whether there was some reason a hlist isn't appropriate
here. Maybe you can tell me. Or go "yeah, a hlist would work nicely".

                Linus
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