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

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Message-ID: <1458837934.12033.6.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Thu, 24 Mar 2016 09:45:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Ahern <dsa@...ulusnetworks.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: ipv4: Multipath needs to handle unreachable
 nexthops

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.

> 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 ?