mpls: Do not decrement alive counter for unregister events

Multipath routes can be rendered usesless when a device in one of the
paths is deleted. For example:

$ ip -f mpls ro ls
100
	nexthop as to 200 via inet 172.16.2.2  dev virt12
	nexthop as to 300 via inet 172.16.3.2  dev br0
101
	nexthop as to 201 via inet6 2000:2::2  dev virt12
	nexthop as to 301 via inet6 2000:3::2  dev br0

$ ip li del br0

When br0 is deleted the other hop is not considered in
mpls_select_multipath because of the alive check -- rt_nhn_alive
is 0.

rt_nhn_alive is decremented once in mpls_ifdown when the device is taken
down (NETDEV_DOWN) and again when it is deleted (NETDEV_UNREGISTER). For
a 2 hop route, deleting one device drops the alive count to 0. Since
devices are taken down before unregistering, the decrement on
NETDEV_UNREGISTER is redundant.

Fixes: c89359a42e2a4 ("mpls: support for dead routes")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index a147798..33211f9 100644 (file)
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -1288,7 +1288,8 @@ static void mpls_ifdown(struct net_device *dev, int event)
                                /* fall through */
                        case NETDEV_CHANGE:
                                nh->nh_flags |= RTNH_F_LINKDOWN;
-                               ACCESS_ONCE(rt->rt_nhn_alive) = rt->rt_nhn_alive - 1;
+                               if (event != NETDEV_UNREGISTER)
+                                       ACCESS_ONCE(rt->rt_nhn_alive) = rt->rt_nhn_alive - 1;
                                break;
                        }
                        if (event == NETDEV_UNREGISTER)