neighbour: Convert RTM_SETNEIGHTBL to RCU.

neightbl_set() fetches neigh_tables[] and updates attributes under
write_lock_bh(&tbl->lock), so RTNL is not needed.

neigh_table_clear() synchronises RCU only, and rcu_dereference_rtnl()
protects nothing here.

If we released RCU after fetching neigh_tables[], there would be no
synchronisation to block neigh_table_clear() further, so RCU is held
until the end of the function.

Another option would be to protect neigh_tables[] user with SRCU
and add synchronize_srcu() in neigh_table_clear().

But, holding RCU should be fine as we hold write_lock_bh() for the
rest of neightbl_set() anyway.

Let's perform RTM_SETNEIGHTBL under RCU and drop RTNL.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/neighbour.c

···
 			struct netlink_ext_ack *extack)
 {
 	struct net *net = sock_net(skb->sk);
+	struct nlattr *tb[NDTA_MAX + 1];
 	struct neigh_table *tbl;
 	struct ndtmsg *ndtmsg;
-	struct nlattr *tb[NDTA_MAX+1];
 	bool found = false;
 	int err, tidx;
 
···
 
 	ndtmsg = nlmsg_data(nlh);
 
+	rcu_read_lock();
+
 	for (tidx = 0; tidx < NEIGH_NR_TABLES; tidx++) {
-		tbl = rcu_dereference_rtnl(neigh_tables[tidx]);
+		tbl = rcu_dereference(neigh_tables[tidx]);
 		if (!tbl)
 			continue;
+
 		if (ndtmsg->ndtm_family && tbl->family != ndtmsg->ndtm_family)
 			continue;
+
 		if (nla_strcmp(tb[NDTA_NAME], tbl->id) == 0) {
 			found = true;
 			break;
 		}
 	}
 
-	if (!found)
-		return -ENOENT;
+	if (!found) {
+		rcu_read_unlock();
+		err = -ENOENT;
+		goto errout;
+	}
 
 	/*
 	 * We acquire tbl->lock to be nice to the periodic timers and
···
 
 errout_tbl_lock:
 	write_unlock_bh(&tbl->lock);
+	rcu_read_unlock();
 errout:
 	return err;
 }
···
 	 .flags = RTNL_FLAG_DOIT_UNLOCKED | RTNL_FLAG_DUMP_UNLOCKED},
 	{.msgtype = RTM_GETNEIGHTBL, .dumpit = neightbl_dump_info,
 	 .flags = RTNL_FLAG_DUMP_UNLOCKED},
-	{.msgtype = RTM_SETNEIGHTBL, .doit = neightbl_set},
+	{.msgtype = RTM_SETNEIGHTBL, .doit = neightbl_set,
+	 .flags = RTNL_FLAG_DOIT_UNLOCKED},
 };
 
 static int __init neigh_init(void)