mptcp: Retransmit DATA_FIN

With this change, the MPTCP-level retransmission timer is used to resend
DATA_FIN. The retranmit timer is not stopped while waiting for a
MPTCP-level ACK of DATA_FIN, and retransmitted DATA_FINs are sent on all
subflows. The retry interval starts at TCP_RTO_MIN and then doubles on
each attempt, up to TCP_RTO_MAX.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/146
Fixes: 43b54c6ee3 ("mptcp: Use full MPTCP-level disconnect state machine")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -399,6 +399,14 @@ static bool mptcp_pending_data_fin(struct sock *sk, u64 *seq)
 	return false;
 }
 
+static void mptcp_set_datafin_timeout(const struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	mptcp_sk(sk)->timer_ival = min(TCP_RTO_MAX,
+				       TCP_RTO_MIN << icsk->icsk_retransmits);
+}
+
 static void mptcp_set_timeout(const struct sock *sk, const struct sock *ssk)
 {
 	long tout = ssk && inet_csk(ssk)->icsk_pending ?
@@ -1052,7 +1060,7 @@ out:
 	}
 
 	if (snd_una == READ_ONCE(msk->snd_nxt)) {
-		if (msk->timer_ival)
+		if (msk->timer_ival && !mptcp_data_fin_enabled(msk))
 			mptcp_stop_timer(sk);
 	} else {
 		mptcp_reset_timer(sk);
@@ -2276,8 +2284,19 @@ static void __mptcp_retrans(struct sock *sk)
 
 	__mptcp_clean_una_wakeup(sk);
 	dfrag = mptcp_rtx_head(sk);
-	if (!dfrag)
+	if (!dfrag) {
+		if (mptcp_data_fin_enabled(msk)) {
+			struct inet_connection_sock *icsk = inet_csk(sk);
+
+			icsk->icsk_retransmits++;
+			mptcp_set_datafin_timeout(sk);
+			mptcp_send_ack(msk);
+
+			goto reset_timer;
+		}
+
 		return;
+	}
 
 	ssk = mptcp_subflow_get_retrans(msk);
 	if (!ssk)
@@ -2460,6 +2479,8 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how)
 			pr_debug("Sending DATA_FIN on subflow %p", ssk);
 			mptcp_set_timeout(sk, ssk);
 			tcp_send_ack(ssk);
+			if (!mptcp_timer_pending(sk))
+				mptcp_reset_timer(sk);
 		}
 		break;
 	}