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

From: Duan Jiong <[email-masked]>
Date: Tue, 13 May 2014 09:07:12 +0800

于 2014年05月13日 02:37, David Miller 写道:
> From: Duan Jiong <[email-masked]>
> 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 <[email-masked]>
>>>> ---
>>>> 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?

Yes, packets should not be sent out until the neihbour is reachable.

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