udp: call dst_hold_safe() in udp_sk_rx_set_dst()

In udp_v4/6_early_demux() code, we try to hold dst->__refcnt for
dst with DST_NOCACHE flag. This is because later in udp_sk_rx_dst_set()
function, we will try to cache this dst in sk for connected case.
However, a better way to achieve this is to not try to hold dst in
early_demux(), but in udp_sk_rx_dst_set(), call dst_hold_safe(). This
approach is also more consistant with how tcp is handling it. And it
will make later changes simpler.
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1977,9 +1977,10 @@ static void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 {
 	struct dst_entry *old;
 
-	dst_hold(dst);
+	if (dst_hold_safe(dst)) {
 		old = xchg(&sk->sk_rx_dst, dst);
 		dst_release(old);
+	}
 }
 
 /*
@@ -2303,14 +2304,12 @@ void udp_v4_early_demux(struct sk_buff *skb)
 	if (dst)
 		dst = dst_check(dst, 0);
 	if (dst) {
-		/* DST_NOCACHE can not be used without taking a reference */
-		if (dst->flags & DST_NOCACHE) {
-			if (likely(atomic_inc_not_zero(&dst->__refcnt)))
-				skb_dst_set(skb, dst);
-		} else {
+		/* set noref for now.
+		 * any place which wants to hold dst has to call
+		 * dst_hold_safe()
+		 */
 		skb_dst_set_noref(skb, dst);
 	}
-	}
 }
 
 int udp_rcv(struct sk_buff *skb)

net/ipv6/udp.c

@@ -920,13 +920,12 @@ static void udp_v6_early_demux(struct sk_buff *skb)
 	if (dst)
 		dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);
 	if (dst) {
-		if (dst->flags & DST_NOCACHE) {
-			if (likely(atomic_inc_not_zero(&dst->__refcnt)))
-				skb_dst_set(skb, dst);
-		} else {
+		/* set noref for now.
+		 * any place which wants to hold dst has to call
+		 * dst_hold_safe()
+		 */
 		skb_dst_set_noref(skb, dst);
 	}
-	}
 }
 
 static __inline__ int udpv6_rcv(struct sk_buff *skb)