Commit 35f079eb authored by Nandita Dukkipati's avatar Nandita Dukkipati Committed by David S. Miller
Browse files

tcp: bug fix in proportional rate reduction. 



This patch is a fix for a bug triggering newly_acked_sacked < 0
in tcp_ack(.).

The bug is triggered by sacked_out decreasing relative to prior_sacked,
but packets_out remaining the same as pior_packets. This is because the
snapshot of prior_packets is taken after tcp_sacktag_write_queue() while
prior_sacked is captured before tcp_sacktag_write_queue(). The problem
is: tcp_sacktag_write_queue (tcp_match_skb_to_sack() -> tcp_fragment)
adjusts the pcount for packets_out and sacked_out (MSS change or other
reason). As a result, this delta in pcount is reflected in
(prior_sacked - sacked_out) but not in (prior_packets - packets_out).

This patch does the following:
1) initializes prior_packets at the start of tcp_ack() so as to
capture the delta in packets_out created by tcp_fragment.
2) introduces a new "previous_packets_out" variable that snapshots
packets_out right before tcp_clean_rtx_queue, so pkts_acked can be
correctly computed as before.
3) Computes pkts_acked using previous_packets_out, and computes
newly_acked_sacked using prior_packets.

Signed-off-by: default avatarNandita Dukkipati <nanditad@google.com>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent d02cea0f

net/ipv4/tcp_input.c

+13 −10
Original line number Diff line number Diff line
@@ -2743,8 +2743,8 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack) 
 * tcp_xmit_retransmit_queue().

 */

static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,

				  int prior_sacked, bool is_dupack,

				  int flag)

				  int prior_sacked, int prior_packets,

				  bool is_dupack, int flag)

{

	struct inet_connection_sock *icsk = inet_csk(sk);

	struct tcp_sock *tp = tcp_sk(sk);
@@ -2804,7 +2804,8 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, 
				tcp_add_reno_sack(sk);

		} else

			do_lost = tcp_try_undo_partial(sk, pkts_acked);

		newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;

		newly_acked_sacked = prior_packets - tp->packets_out +

				     tp->sacked_out - prior_sacked;

		break;

	case TCP_CA_Loss:

		tcp_process_loss(sk, flag, is_dupack);
@@ -2818,7 +2819,8 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, 
			if (is_dupack)

				tcp_add_reno_sack(sk);

		}

		newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;

		newly_acked_sacked = prior_packets - tp->packets_out +

				     tp->sacked_out - prior_sacked;



		if (icsk->icsk_ca_state <= TCP_CA_Disorder)

			tcp_try_undo_dsack(sk);
@@ -3330,9 +3332,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) 
	bool is_dupack = false;

	u32 prior_in_flight;

	u32 prior_fackets;

	int prior_packets;

	int prior_packets = tp->packets_out;

	int prior_sacked = tp->sacked_out;

	int pkts_acked = 0;

	int previous_packets_out = 0;



	/* If the ack is older than previous acks

	 * then we can probably ignore it.
@@ -3403,14 +3406,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) 
	sk->sk_err_soft = 0;

	icsk->icsk_probes_out = 0;

	tp->rcv_tstamp = tcp_time_stamp;

	prior_packets = tp->packets_out;

	if (!prior_packets)

		goto no_queue;



	/* See if we can take anything off of the retransmit queue. */

	previous_packets_out = tp->packets_out;

	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);



	pkts_acked = prior_packets - tp->packets_out;

	pkts_acked = previous_packets_out - tp->packets_out;



	if (tcp_ack_is_dubious(sk, flag)) {

		/* Advance CWND, if state allows this. */
@@ -3418,7 +3421,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) 
			tcp_cong_avoid(sk, ack, prior_in_flight);

		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));

		tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,

				      is_dupack, flag);

				      prior_packets, is_dupack, flag);

	} else {

		if (flag & FLAG_DATA_ACKED)

			tcp_cong_avoid(sk, ack, prior_in_flight);
@@ -3441,7 +3444,7 @@ no_queue: 
	/* If data was DSACKed, see if we can undo a cwnd reduction. */

	if (flag & FLAG_DSACKING_ACK)

		tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,

				      is_dupack, flag);

				      prior_packets, is_dupack, flag);

	/* If this ack opens up a zero window, clear backoff.  It was

	 * being used to time the probes, and is probably far higher than

	 * it needs to be for normal retransmission.
@@ -3464,7 +3467,7 @@ old_ack: 
	if (TCP_SKB_CB(skb)->sacked) {

		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);

		tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,

				      is_dupack, flag);

				      prior_packets, is_dupack, flag);

	}



	SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);

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