netdev - Re: [PATCH v2] neigh: set nud_state to NUD_INCOMPLETE when probing router reachability

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Message-Id: <20140512.143720.896863177762079657.davem@davemloft.net>
Date:	Mon, 12 May 2014 14:37:20 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	duanj.fnst@...fujitsu.com
Cc:	hannes@...essinduktion.org, netdev@...r.kernel.org,
	jbenc@...hat.com
Subject: Re: [PATCH v2] neigh: set nud_state to NUD_INCOMPLETE when probing
 router reachability

From: Duan Jiong <duanj.fnst@...fujitsu.com>
Date: Mon, 12 May 2014 11:26:10 +0800

> 于 2014年05月12日 06:04, Hannes Frederic Sowa 写道:
>> On Thu, May 8, 2014, at 22:16, Duan Jiong wrote:
>>>
>>> Since commit 7e98056964("ipv6: router reachability probing"), a router falls
>>> into NUD_FAILED will be probed.
>>>
>>> Now if function rt6_select() selects a router which neighbour state is NUD_FAILED,
>>> and at the same time function rt6_probe() changes the neighbour state to NUD_PROBE,
>>> then function dst_neigh_output() can directly send packets, but actually the
>>> neighbour still is unreachable. If we set nud_state to NUD_INCOMPLETE instead
>>> NUD_PROBE, packets will not be sent out until the neihbour is reachable.
>>>
>>> In addition, because the route should be probes with a single NS, so we must
>>> set neigh->probes to neigh_max_probes(), then the neigh timer timeout and function
>>> neigh_timer_handler() will not send other NS Messages.
>>>
>>> Signed-off-by: Duan Jiong <duanj.fnst@...fujitsu.com>
>>> ---
>>> Changes from v1:
>>> 	*modify changelog to explain in detail why use neigh_max_probes().
>>>
>>>  net/core/neighbour.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>>> index 8f8a96e..32d872e 100644
>>> --- a/net/core/neighbour.c
>>> +++ b/net/core/neighbour.c
>>> @@ -1248,8 +1248,8 @@ void __neigh_set_probe_once(struct neighbour *neigh)
>>>  	neigh->updated = jiffies;
>>>  	if (!(neigh->nud_state & NUD_FAILED))
>>>  		return;
>>> -	neigh->nud_state = NUD_PROBE;
>>> -	atomic_set(&neigh->probes, NEIGH_VAR(neigh->parms, UCAST_PROBES));
>>> +	neigh->nud_state = NUD_INCOMPLETE;
>>> +	atomic_set(&neigh->probes, neigh_max_probes(neigh));
>> 
>> Wouldn't it be better if we neigh_suspect the neighbour and leaving the state in NUD_PROBE? We call down to ->output in case neighbour is in NUD_PROBE state, so we must just disable connected 'fast-path'.
>> 
> 
> You can look into neigh_event_send() called in neigh_resolve_output(), and if neigh->nud_state
> still is NUD_PROBE, the neigh_event_send() will return 0, so the packet will still be sent out
> without probe.
> 
> So, using neigh_suspect is not a good idea.

If you set it to NUD_INCOMPLETE however, neigh_event_send() is going to add the packet
to the neigh's ARP queue and return '1'.

Is that really what you want to happen in this case?
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