neigh: RCU conversion of struct neighbour

This is the second step for neighbour RCU conversion.

(first was commit d6bf7817 : RCU conversion of neigh hash table)

neigh_lookup() becomes lockless, but still take a reference on found
neighbour. (no more read_lock()/read_unlock() on tbl->lock)

struct neighbour gets an additional rcu_head field and is freed after an
RCU grace period.

Future work would need to eventually not take a reference on neighbour
for temporary dst (DST_NOCACHE), but this would need dst->_neighbour to
use a noref bit like we did for skb->_dst.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -91,7 +91,7 @@ struct neigh_statistics {
 #define NEIGH_CACHE_STAT_INC(tbl, field) this_cpu_inc((tbl)->stats->field)
 
 struct neighbour {
-	struct neighbour	*next;
+	struct neighbour __rcu	*next;
 	struct neigh_table	*tbl;
 	struct neigh_parms	*parms;
 	struct net_device	*dev;
@@ -111,6 +111,7 @@ struct neighbour {
 	struct sk_buff_head	arp_queue;
 	struct timer_list	timer;
 	const struct neigh_ops	*ops;
+	struct rcu_head		rcu;
 	u8			primary_key[0];
 };
 
@@ -139,7 +140,7 @@ struct pneigh_entry {
  */
 
 struct neigh_hash_table {
-	struct neighbour	**hash_buckets;
+	struct neighbour __rcu	**hash_buckets;
 	unsigned int		hash_mask;
 	__u32			hash_rnd;
 	struct rcu_head		rcu;