Re: rcu locking issue in mpls output code?

On 6/20/16 12:30 AM, Lennert Buytenhek wrote:

On Sun, Jun 19, 2016 at 08:19:20PM -0600, David Ahern wrote:

quoted

quoted

diff --git a/net/mpls/mpls_iptunnel.c b/net/mpls/mpls_iptunnel.c
index fb31aa8..802956b 100644
--- a/net/mpls/mpls_iptunnel.c
+++ b/net/mpls/mpls_iptunnel.c

@@ -105,12 +105,15 @@ static int mpls_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		bos = false;
 	}

+	rcu_read_lock_bh();
 	if (rt)
 		err = neigh_xmit(NEIGH_ARP_TABLE, out_dev, &rt->rt_gateway,
 				 skb);
 	else if (rt6)
 		err = neigh_xmit(NEIGH_ND_TABLE, out_dev, &rt6->rt6i_gateway,
 				 skb);
+	rcu_read_unlock_bh();
+
 	if (err)
 		net_dbg_ratelimited("%s: packet transmission failed: %d\n",
 				    __func__, err);

I think those need to be added to neigh_xmit in the

	if (likely(index < NEIGH_NR_TABLES)) {

	}

That'll force callers that don't need the extra protection (i.e.
mpls_forward(), since that always runs from softirq and it's enough
to protect the neigh state with rcu_read_lock() from softirq and we're
already running under rcu_read_lock() when we get to neigh_xmit()) to
eat the useless overhead of an extra rcu_read_{,un}lock_bh() pair, but
sure, functionally that's correct, I think, and in my workload I don't
care about MPLS forwarding performance anyway. ;-)

__neigh_lookup_noref expects bh level protection. Since the if block in 
neigh_xmit requires the locking seems like this the appropriate place 
for it.

Want me to send a patch moving it to neigh_xmit() ?

Roopa/Robert: agree?