mptcp: send explicit ack on delayed ack_seq incr

When the worker moves some bytes from the OoO queue into
the receive queue, the msk->ask_seq is updated, the MPTCP-level
ack carrying that value needs to wait the next ingress packet,
possibly slowing down or hanging the peer

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -407,13 +407,27 @@ static void mptcp_set_timeout(const struct sock *sk, const struct sock *ssk)
 	mptcp_sk(sk)->timer_ival = tout > 0 ? tout : TCP_RTO_MIN;
 }
 
-static void mptcp_check_data_fin(struct sock *sk)
+static void mptcp_send_ack(struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow;
+
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		lock_sock(ssk);
+		tcp_send_ack(ssk);
+		release_sock(ssk);
+	}
+}
+
+static bool mptcp_check_data_fin(struct sock *sk)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	u64 rcv_data_fin_seq;
+	bool ret = false;
 
 	if (__mptcp_check_fallback(msk) || !msk->first)
-		return;
+		return ret;
 
 	/* Need to ack a DATA_FIN received from a peer while this side
 	 * of the connection is in ESTABLISHED, FIN_WAIT1, or FIN_WAIT2.
@@ -429,8 +443,6 @@ static void mptcp_check_data_fin(struct sock *sk)
 	 */
 
 	if (mptcp_pending_data_fin(sk, &rcv_data_fin_seq)) {
-		struct mptcp_subflow_context *subflow;
-
 		WRITE_ONCE(msk->ack_seq, msk->ack_seq + 1);
 		WRITE_ONCE(msk->rcv_data_fin, 0);
 
@@ -454,17 +466,12 @@ static void mptcp_check_data_fin(struct sock *sk)
 			break;
 		}
 
+		ret = true;
 		mptcp_set_timeout(sk, NULL);
-		mptcp_for_each_subflow(msk, subflow) {
-			struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
-
-			lock_sock(ssk);
-			tcp_send_ack(ssk);
-			release_sock(ssk);
-		}
-
+		mptcp_send_ack(msk);
 		mptcp_close_wake_up(sk);
 	}
+	return ret;
 }
 
 static bool __mptcp_move_skbs_from_subflow(struct mptcp_sock *msk,
@@ -1547,7 +1554,8 @@ static bool __mptcp_move_skbs(struct mptcp_sock *msk)
 	} while (!done);
 
 	if (mptcp_ofo_queue(msk) || moved > 0) {
-		mptcp_check_data_fin((struct sock *)msk);
+		if (!mptcp_check_data_fin((struct sock *)msk))
+			mptcp_send_ack(msk);
 		return true;
 	}
 	return false;