tcp: simplify tcp_mark_skb_lost

This patch consolidates and simplifes the loss marking logic used
by a few loss detections (RACK, RFC6675, NewReno). Previously
each detection uses a subset of several intertwined subroutines.
This unncessary complexity has led to bugs (and fixes of bug fixes).

tcp_mark_skb_lost now is the single one routine to mark a packet loss
when a loss detection caller deems an skb ist lost:

   1. rewind tp->retransmit_hint_skb if skb has lower sequence or
      all lost ones have been retransmitted.

   2. book-keeping: adjust flags and counts depending on if skb was
      retransmitted or not.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1006,7 +1006,11 @@ static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
 		      ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
 }
 
-/* This must be called before lost_out is incremented */
+ /* This must be called before lost_out or retrans_out are updated
+  * on a new loss, because we want to know if all skbs previously
+  * known to be lost have already been retransmitted, indicating
+  * that this newly lost skb is our next skb to retransmit.
+  */
 static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
 {
 	if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
@@ -1018,32 +1022,25 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
 
 void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
 {
+	__u8 sacked = TCP_SKB_CB(skb)->sacked;
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	tcp_skb_mark_lost_uncond_verify(tp, skb);
-	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
-		/* Account for retransmits that are lost again */
-		TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
-		tp->retrans_out -= tcp_skb_pcount(skb);
-		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
-			      tcp_skb_pcount(skb));
-	}
-}
-
-/* Sum the number of packets on the wire we have marked as lost.
- * There are two cases we care about here:
- * a) Packet hasn't been marked lost (nor retransmitted),
- *    and this is the first loss.
- * b) Packet has been marked both lost and retransmitted,
- *    and this means we think it was lost again.
- */
-static void tcp_sum_lost(struct tcp_sock *tp, struct sk_buff *skb)
-{
-	__u8 sacked = TCP_SKB_CB(skb)->sacked;
+	if (sacked & TCPCB_SACKED_ACKED)
+		return;
 
-	if (!(sacked & TCPCB_LOST) ||
-	    ((sacked & TCPCB_LOST) && (sacked & TCPCB_SACKED_RETRANS)))
-		tp->lost += tcp_skb_pcount(skb);
+	tcp_verify_retransmit_hint(tp, skb);
+	if (sacked & TCPCB_LOST) {
+		if (sacked & TCPCB_SACKED_RETRANS) {
+			/* Account for retransmits that are lost again */
+			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
+			tp->retrans_out -= tcp_skb_pcount(skb);
+			NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
+				      tcp_skb_pcount(skb));
+		}
+	} else {
+		tp->lost_out += tcp_skb_pcount(skb);
+		TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
+	}
 }
 
 static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
@@ -1057,17 +1054,6 @@ static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
 	}
 }
 
-void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb)
-{
-	tcp_verify_retransmit_hint(tp, skb);
-
-	tcp_sum_lost(tp, skb);
-	if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
-		tp->lost_out += tcp_skb_pcount(skb);
-		TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
-	}
-}
-
 /* Updates the delivered and delivered_ce counts */
 static void tcp_count_delivered(struct tcp_sock *tp, u32 delivered,
 				bool ece_ack)
@@ -2688,8 +2674,7 @@ void tcp_simple_retransmit(struct sock *sk)
 	unsigned int mss = tcp_current_mss(sk);
 
 	skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
-		if (tcp_skb_seglen(skb) > mss &&
-		    !(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
+		if (tcp_skb_seglen(skb) > mss)
 			tcp_mark_skb_lost(sk, skb);
 	}