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tcp: bug fix in proportional rate reduction.

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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: Nandita Dukkipati <nanditad@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Nandita Dukkipati 2013-05-21 15:12:07 +00:00 committed by David S. Miller
parent d02cea0f43
commit 35f079ebbc
1 changed files with 13 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
net/ipv4/tcp_input.c
View File

@ -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);