[UDP]: Defer InDataGrams increment until recvmsg() does checksum

Thanks dave, herbert, gerrit, andi and other people for your
discussion about this problem.

UdpInDatagrams can be confusing because it counts packets that
might be dropped later.
Move UdpInDatagrams into recvmsg() as allowed by the RFC.
Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/udplite.txt

@@ -236,7 +236,7 @@
 
   This displays UDP-Lite statistics variables, whose meaning is as follows.
 
-   InDatagrams:     Total number of received datagrams.
+   InDatagrams:     The total number of datagrams delivered to users.
 
    NoPorts:         Number of packets received to an unknown port.
                     These cases are counted separately (not as InErrors).

net/ipv4/udp.c

@@ -873,6 +873,8 @@ int udp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 	if (err)
 		goto out_free;
 
+	UDP_INC_STATS_USER(UDP_MIB_INDATAGRAMS, is_udplite);
+
 	sock_recv_timestamp(msg, sk, skb);
 
 	/* Copy the address. */
@@ -966,11 +968,9 @@ int udp_queue_rcv_skb(struct sock * sk, struct sk_buff *skb)
 			int ret;
 
 			ret = (*up->encap_rcv)(sk, skb);
-			if (ret <= 0) {
-				UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS, up->pcflag);
+			if (ret <= 0)
 				return -ret;
 		}
-		}
 
 		/* FALLTHROUGH -- it's a UDP Packet */
 	}
@@ -1023,7 +1023,6 @@ int udp_queue_rcv_skb(struct sock * sk, struct sk_buff *skb)
 		goto drop;
 	}
 
-	UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS, up->pcflag);
 	return 0;
 
 drop:

net/ipv6/udp.c

@@ -164,6 +164,8 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 	if (err)
 		goto out_free;
 
+	UDP6_INC_STATS_USER(UDP_MIB_INDATAGRAMS, is_udplite);
+
 	sock_recv_timestamp(msg, sk, skb);
 
 	/* Copy the address. */
@@ -292,7 +294,7 @@ int udpv6_queue_rcv_skb(struct sock * sk, struct sk_buff *skb)
 			UDP6_INC_STATS_BH(UDP_MIB_RCVBUFERRORS, up->pcflag);
 		goto drop;
 	}
-	UDP6_INC_STATS_BH(UDP_MIB_INDATAGRAMS, up->pcflag);
+
 	return 0;
 drop:
 	UDP6_INC_STATS_BH(UDP_MIB_INERRORS, up->pcflag);