[TCP]: Extract DSACK detection code from tcp_sacktag_write_queue().

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -960,6 +960,39 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
  * Both of these heuristics are not used in Loss state, when we cannot
  * account for retransmits accurately.
  */
+static int tcp_check_dsack(struct tcp_sock *tp, struct sk_buff *ack_skb,
+			   struct tcp_sack_block_wire *sp, int num_sacks,
+			   u32 prior_snd_una)
+{
+	u32 start_seq_0 = ntohl(get_unaligned(&sp[0].start_seq));
+	u32 end_seq_0 = ntohl(get_unaligned(&sp[0].end_seq));
+	int dup_sack = 0;
+
+	if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
+		dup_sack = 1;
+		tp->rx_opt.sack_ok |= 4;
+		NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
+	} else if (num_sacks > 1) {
+		u32 end_seq_1 = ntohl(get_unaligned(&sp[1].end_seq));
+		u32 start_seq_1 = ntohl(get_unaligned(&sp[1].start_seq));
+
+		if (!after(end_seq_0, end_seq_1) &&
+		    !before(start_seq_0, start_seq_1)) {
+			dup_sack = 1;
+			tp->rx_opt.sack_ok |= 4;
+			NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
+		}
+	}
+
+	/* D-SACK for already forgotten data... Do dumb counting. */
+	if (dup_sack &&
+	    !after(end_seq_0, prior_snd_una) &&
+	    after(end_seq_0, tp->undo_marker))
+		tp->undo_retrans--;
+
+	return dup_sack;
+}
+
 static int
 tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_una)
 {
@@ -985,27 +1018,10 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	}
 	prior_fackets = tp->fackets_out;
 
-	/* Check for D-SACK. */
-	if (before(ntohl(sp[0].start_seq), TCP_SKB_CB(ack_skb)->ack_seq)) {
-		flag |= FLAG_DSACKING_ACK;
-		found_dup_sack = 1;
-		tp->rx_opt.sack_ok |= 4;
-		NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
-	} else if (num_sacks > 1 &&
-			!after(ntohl(sp[0].end_seq), ntohl(sp[1].end_seq)) &&
-			!before(ntohl(sp[0].start_seq), ntohl(sp[1].start_seq))) {
+	found_dup_sack = tcp_check_dsack(tp, ack_skb, sp,
+					 num_sacks, prior_snd_una);
+	if (found_dup_sack)
 		flag |= FLAG_DSACKING_ACK;
-		found_dup_sack = 1;
-		tp->rx_opt.sack_ok |= 4;
-		NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
-	}
-
-	/* D-SACK for already forgotten data...
-	 * Do dumb counting. */
-	if (found_dup_sack &&
-			!after(ntohl(sp[0].end_seq), prior_snd_una) &&
-			after(ntohl(sp[0].end_seq), tp->undo_marker))
-		tp->undo_retrans--;
 
 	/* Eliminate too old ACKs, but take into
 	 * account more or less fresh ones, they can