tcp: check undo conditions before detecting losses

yuchungcheng · davem330 · commit 98e36d449cc6 · 2017-01-13T22:37:16.000-05:00
Currently RACK would mark loss before the undo operations in TCP loss recovery. This could incorrectly identify real losses as spurious. For example a sender first experiences a delay spike and then eventually some packets were lost due to buffer overrun. In this case, the sender should perform fast recovery b/c not all the packets were lost. But the sender may first trigger a (spurious) RTO and reset cwnd to 1. The following ACKs may used to mark real losses by tcp_rack_mark_lost. Then in tcp_process_loss this ACK could trigger F-RTO undo condition and unmark real losses and revert the cwnd reduction. If there are no more ACKs coming back, eventually the sender would timeout again instead of performing fast recovery. The patch fixes this incorrect process by always performing the undo checks before detecting losses. Fixes: 4f41b1c ("tcp: use RACK to detect losses") Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Acked-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
@@ -2801,6 +2801,21 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked)
 	return false;
 }
 
+static void tcp_rack_identify_loss(struct sock *sk, int *ack_flag,
+				   const struct skb_mstamp *ack_time)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Use RACK to detect loss */
+	if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS) {
+		u32 prior_retrans = tp->retrans_out;
+
+		tcp_rack_mark_lost(sk, ack_time);
+		if (prior_retrans > tp->retrans_out)
+			*ack_flag |= FLAG_LOST_RETRANS;
+	}
+}
+
 /* Process an event, which can update packets-in-flight not trivially.
  * Main goal of this function is to calculate new estimate for left_out,
  * taking into account both packets sitting in receiver's buffer and
@@ -2866,17 +2881,6 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
 		}
 	}
 
-	/* Use RACK to detect loss */
-	if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS) {
-		u32 prior_retrans = tp->retrans_out;
-
-		tcp_rack_mark_lost(sk, ack_time);
-		if (prior_retrans > tp->retrans_out) {
-			flag |= FLAG_LOST_RETRANS;
-			*ack_flag |= FLAG_LOST_RETRANS;
-		}
-	}
-
 	/* E. Process state. */
 	switch (icsk->icsk_ca_state) {
 	case TCP_CA_Recovery:
@@ -2894,11 +2898,13 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
 			tcp_try_keep_open(sk);
 			return;
 		}
+		tcp_rack_identify_loss(sk, ack_flag, ack_time);
 		break;
 	case TCP_CA_Loss:
 		tcp_process_loss(sk, flag, is_dupack, rexmit);
-		if (icsk->icsk_ca_state != TCP_CA_Open &&
-		    !(flag & FLAG_LOST_RETRANS))
+		tcp_rack_identify_loss(sk, ack_flag, ack_time);
+		if (!(icsk->icsk_ca_state == TCP_CA_Open ||
+		      (*ack_flag & FLAG_LOST_RETRANS)))
 			return;
 		/* Change state if cwnd is undone or retransmits are lost */
 	default:
@@ -2912,6 +2918,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
 		if (icsk->icsk_ca_state <= TCP_CA_Disorder)
 			tcp_try_undo_dsack(sk);
 
+		tcp_rack_identify_loss(sk, ack_flag, ack_time);
 		if (!tcp_time_to_recover(sk, flag)) {
 			tcp_try_to_open(sk, flag);
 			return;
