netdev - Re: [PATCH] various dst_ifdown routines to catch refcounting bugs

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Message-ID: <m1myv8kr9c.fsf@ebiederm.dsl.xmission.com>
Date:	Thu, 27 Sep 2007 10:27:43 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Denis V. Lunev" <den@...nvz.org>
Cc:	davem@...emloft.net, devel@...nvz.org, netdev@...r.kernel.org,
	containers@...ts.osdl.org, ebiederm@...ssion.com
Subject: Re: [PATCH] various dst_ifdown routines to catch refcounting bugs

"Denis V. Lunev" <den@...nvz.org> writes:

> Moving dst entries into init_net.loopback_dev is not a good thing.
> This hides obvious and non-obvious ref-counting bugs.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

To be clear using init_net.loopback is currently safe because we don't
have any destination cache entries for anything except the initial
network namespace.

I have not yet made this change simply because I haven't gotten around
to this part in my patches.

I do have a question I would like to bring up, because I like avoiding
explicit references to loopback_dev when I can.

/* Dirty hack. We did it in 2.2 (in __dst_free),
 * we have _very_ good reasons not to repeat
 * this mistake in 2.3, but we have no choice
 * now. _It_ _is_ _explicit_ _deliberate_
 * _race_ _condition_.
 *
 * Commented and originally written by Alexey.
 */

What is the race that is talked about in that comment.  Can we just
assign NULL instead of the loopback device when we bring a route down.
My gut feeling is that something like:
	dst->input = dst->output = dst_discard;
may be enough.    But I don't know where the deliberate race is.

I haven't traced this all of the way through but from the obvious
parts I just get this nagging feeling that something isn't quite
right.

Eric
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