[PATCH net] net/smc: fix netdev refcnt leak in smc_ib_find_route()

[Wen Gu <guwen@linux.alibaba.com>]

A netdev refcnt leak issue was found when unregistering netdev after
using SMC. It can be reproduced as follows.

- run tests based on SMC.
- unregister the net device.

The following error message can be observed.

'unregister_netdevice: waiting for ethx to become free. Usage count = x'

With CONFIG_NET_DEV_REFCNT_TRACKER set, more detailed error message can
be provided by refcount tracker:

 unregister_netdevice: waiting for eth1 to become free. Usage count = 2
 ref_tracker: eth%d@ffff9cabc3bf8548 has 1/1 users at
      ___neigh_create+0x8e/0x420
      neigh_event_ns+0x52/0xc0
      arp_process+0x7c0/0x860
      __netif_receive_skb_list_core+0x258/0x2c0
      __netif_receive_skb_list+0xea/0x150
      netif_receive_skb_list_internal+0xf2/0x1b0
      napi_complete_done+0x73/0x1b0
      mlx5e_napi_poll+0x161/0x5e0 [mlx5_core]
      __napi_poll+0x2c/0x1c0
      net_rx_action+0x2a7/0x380
      __do_softirq+0xcd/0x2a7

It is because in smc_ib_find_route(), neigh_lookup() takes a netdev
refcnt but does not release. So fix it.

Fixes: e5c4744cfb59 ("net/smc: add SMC-Rv2 connection establishment")
Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
---
 net/smc/smc_ib.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/smc/smc_ib.c b/net/smc/smc_ib.c
index 97704a9e84c7..b431bd8a5172 100644
--- a/net/smc/smc_ib.c
+++ b/net/smc/smc_ib.c
@@ -210,10 +210,11 @@ int smc_ib_find_route(struct net *net, __be32 saddr, __be32 daddr,
 		goto out;
 	if (rt->rt_uses_gateway && rt->rt_gw_family != AF_INET)
 		goto out;
-	neigh = rt->dst.ops->neigh_lookup(&rt->dst, NULL, &fl4.daddr);
+	neigh = dst_neigh_lookup(&rt->dst, &fl4.daddr);
 	if (neigh) {
 		memcpy(nexthop_mac, neigh->ha, ETH_ALEN);
 		*uses_gateway = rt->rt_uses_gateway;
+		neigh_release(neigh);
 		return 0;
 	}
 out:
-- 
2.32.0.3.g01195cf9f