netdev - Re: Possible networking regression in 3.6.0

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Message-Id: <20121002.225535.1684530348122683084.davem@davemloft.net>
Date:	Tue, 02 Oct 2012 22:55:35 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	chris2553@...glemail.com, netdev@...r.kernel.org, gpiez@....de,
	davej@...hat.com
Subject: Re: Possible networking regression in 3.6.0

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 02 Oct 2012 17:48:39 +0200

> [PATCH] ipv4: properly cache forward routes
> 
> commit d2d68ba9fe8 (ipv4: Cache input routes in fib_info nexthops.)
> introduced a regression for forwarding.
> 
> This was hard to reproduce but the symptom was that packets were
> delivered to local host instead of being forwarded.
> 
> Add a separate cache (nh_rth_forward) to solve the problem.
> 
> Many thanks to Chris Clayton for his patience and help.
> 
> Reported-by: Chris Clayton <chris2553@...glemail.com>
> Bisected-by: Chris Clayton <chris2553@...glemail.com>
> Reported-by: Dave Jones <davej@...hat.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

I'm still having trouble understanding how this can happen,
which is probably why I introduced this bug in the first
place :-)

Only INPUT routes created by ip_route_input_slow() cache using
nh_rth_input.

Routes for locally destinations vs. forwarded destinations will
resolve to different fib_info objects.

If at some point a new route is added which turns a local destination
into one for which we forward, normal invalidation of cached routes
ought to fix it.

There's some sequence of events I don't understand that causes the
corrupt route cache, can you show it to me?

Thanks.
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