netdev - Re: [PATCH net] net: ipv4: Multipath needs to handle unreachable nexthops

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Message-ID: <56F4274C.70707@cumulusnetworks.com>
Date:	Thu, 24 Mar 2016 11:43:40 -0600
From:	David Ahern <dsa@...ulusnetworks.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: ipv4: Multipath needs to handle unreachable
 nexthops

On 3/24/16 10:45 AM, Eric Dumazet wrote:
> On Thu, 2016-03-24 at 08:25 -0700, David Ahern wrote:
>> Multipath route lookups should consider knowledge about next hops and not
>> select a hop that is known to be failed.
>
> Does not look a net candidate to me.

you don't consider this a bug? certainly not a feature.

>
>> Signed-off-by: David Ahern <dsa@...ulusnetworks.com>
>> ---
>>   net/ipv4/fib_semantics.c | 34 ++++++++++++++++++++++++++++++++--
>>   1 file changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
>> index d97268e8ff10..28fc6700c2b1 100644
>> --- a/net/ipv4/fib_semantics.c
>> +++ b/net/ipv4/fib_semantics.c
>> @@ -1563,13 +1563,43 @@ int fib_sync_up(struct net_device *dev, unsigned int nh_flags)
>>   void fib_select_multipath(struct fib_result *res, int hash)
>>   {
>>   	struct fib_info *fi = res->fi;
>> +	struct neighbour *n;
>> +	int state;
>>
>>   	for_nexthops(fi) {
>>   		if (hash > atomic_read(&nh->nh_upper_bound))
>>   			continue;
>>
>> -		res->nh_sel = nhsel;
>> -		return;
>> +		state = NUD_NONE;
>> +		n = neigh_lookup(&arp_tbl, &nh->nh_gw, fi->fib_dev);
>> +		if (n) {
>> +			state = n->nud_state;
>> +			neigh_release(n);
>> +		}
>
> This looks like something that could use RCU to avoid expensive
> refcounting ?

Yes, good point.