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![Eric Dumazet](/assets/img/avatar_default.png)
We had various bugs over the years with code
breaking the assumption that tp->snd_cwnd is greater
than zero.
Lately, syzbot reported the WARN_ON_ONCE(!tp->prior_cwnd) added
in commit 8b8a321ff7
("tcp: fix zero cwnd in tcp_cwnd_reduction")
can trigger, and without a repro we would have to spend
considerable time finding the bug.
Instead of complaining too late, we want to catch where
and when tp->snd_cwnd is set to an illegal value.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/20220405233538.947344-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
238 lines
5.8 KiB
C
238 lines
5.8 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* TCP Veno congestion control
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*
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* This is based on the congestion detection/avoidance scheme described in
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* C. P. Fu, S. C. Liew.
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* "TCP Veno: TCP Enhancement for Transmission over Wireless Access Networks."
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* IEEE Journal on Selected Areas in Communication,
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* Feb. 2003.
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* See https://www.ie.cuhk.edu.hk/fileadmin/staff_upload/soung/Journal/J3.pdf
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/inet_diag.h>
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#include <net/tcp.h>
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/* Default values of the Veno variables, in fixed-point representation
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* with V_PARAM_SHIFT bits to the right of the binary point.
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*/
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#define V_PARAM_SHIFT 1
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static const int beta = 3 << V_PARAM_SHIFT;
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/* Veno variables */
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struct veno {
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u8 doing_veno_now; /* if true, do veno for this rtt */
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u16 cntrtt; /* # of rtts measured within last rtt */
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u32 minrtt; /* min of rtts measured within last rtt (in usec) */
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u32 basertt; /* the min of all Veno rtt measurements seen (in usec) */
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u32 inc; /* decide whether to increase cwnd */
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u32 diff; /* calculate the diff rate */
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};
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/* There are several situations when we must "re-start" Veno:
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*
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* o when a connection is established
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* o after an RTO
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* o after fast recovery
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* o when we send a packet and there is no outstanding
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* unacknowledged data (restarting an idle connection)
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*
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*/
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static inline void veno_enable(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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/* turn on Veno */
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veno->doing_veno_now = 1;
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veno->minrtt = 0x7fffffff;
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}
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static inline void veno_disable(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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/* turn off Veno */
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veno->doing_veno_now = 0;
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}
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static void tcp_veno_init(struct sock *sk)
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{
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struct veno *veno = inet_csk_ca(sk);
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veno->basertt = 0x7fffffff;
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veno->inc = 1;
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veno_enable(sk);
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}
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/* Do rtt sampling needed for Veno. */
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static void tcp_veno_pkts_acked(struct sock *sk,
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const struct ack_sample *sample)
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{
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struct veno *veno = inet_csk_ca(sk);
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u32 vrtt;
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if (sample->rtt_us < 0)
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return;
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/* Never allow zero rtt or baseRTT */
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vrtt = sample->rtt_us + 1;
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/* Filter to find propagation delay: */
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if (vrtt < veno->basertt)
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veno->basertt = vrtt;
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/* Find the min rtt during the last rtt to find
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* the current prop. delay + queuing delay:
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*/
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veno->minrtt = min(veno->minrtt, vrtt);
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veno->cntrtt++;
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}
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static void tcp_veno_state(struct sock *sk, u8 ca_state)
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{
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if (ca_state == TCP_CA_Open)
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veno_enable(sk);
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else
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veno_disable(sk);
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}
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/*
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* If the connection is idle and we are restarting,
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* then we don't want to do any Veno calculations
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* until we get fresh rtt samples. So when we
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* restart, we reset our Veno state to a clean
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* state. After we get acks for this flight of
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* packets, _then_ we can make Veno calculations
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* again.
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*/
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static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
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{
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if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
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tcp_veno_init(sk);
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}
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static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct veno *veno = inet_csk_ca(sk);
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if (!veno->doing_veno_now) {
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tcp_reno_cong_avoid(sk, ack, acked);
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return;
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}
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/* limited by applications */
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if (!tcp_is_cwnd_limited(sk))
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return;
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/* We do the Veno calculations only if we got enough rtt samples */
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if (veno->cntrtt <= 2) {
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/* We don't have enough rtt samples to do the Veno
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* calculation, so we'll behave like Reno.
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*/
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tcp_reno_cong_avoid(sk, ack, acked);
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} else {
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u64 target_cwnd;
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u32 rtt;
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/* We have enough rtt samples, so, using the Veno
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* algorithm, we determine the state of the network.
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*/
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rtt = veno->minrtt;
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target_cwnd = (u64)tcp_snd_cwnd(tp) * veno->basertt;
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target_cwnd <<= V_PARAM_SHIFT;
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do_div(target_cwnd, rtt);
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veno->diff = (tcp_snd_cwnd(tp) << V_PARAM_SHIFT) - target_cwnd;
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if (tcp_in_slow_start(tp)) {
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/* Slow start. */
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acked = tcp_slow_start(tp, acked);
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if (!acked)
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goto done;
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}
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/* Congestion avoidance. */
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if (veno->diff < beta) {
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/* In the "non-congestive state", increase cwnd
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* every rtt.
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*/
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tcp_cong_avoid_ai(tp, tcp_snd_cwnd(tp), acked);
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} else {
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/* In the "congestive state", increase cwnd
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* every other rtt.
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*/
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if (tp->snd_cwnd_cnt >= tcp_snd_cwnd(tp)) {
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if (veno->inc &&
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tcp_snd_cwnd(tp) < tp->snd_cwnd_clamp) {
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tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
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veno->inc = 0;
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} else
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veno->inc = 1;
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tp->snd_cwnd_cnt = 0;
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} else
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tp->snd_cwnd_cnt += acked;
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}
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done:
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if (tcp_snd_cwnd(tp) < 2)
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tcp_snd_cwnd_set(tp, 2);
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else if (tcp_snd_cwnd(tp) > tp->snd_cwnd_clamp)
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tcp_snd_cwnd_set(tp, tp->snd_cwnd_clamp);
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}
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/* Wipe the slate clean for the next rtt. */
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/* veno->cntrtt = 0; */
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veno->minrtt = 0x7fffffff;
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}
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/* Veno MD phase */
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static u32 tcp_veno_ssthresh(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct veno *veno = inet_csk_ca(sk);
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if (veno->diff < beta)
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/* in "non-congestive state", cut cwnd by 1/5 */
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return max(tcp_snd_cwnd(tp) * 4 / 5, 2U);
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else
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/* in "congestive state", cut cwnd by 1/2 */
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return max(tcp_snd_cwnd(tp) >> 1U, 2U);
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}
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static struct tcp_congestion_ops tcp_veno __read_mostly = {
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.init = tcp_veno_init,
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.ssthresh = tcp_veno_ssthresh,
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.undo_cwnd = tcp_reno_undo_cwnd,
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.cong_avoid = tcp_veno_cong_avoid,
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.pkts_acked = tcp_veno_pkts_acked,
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.set_state = tcp_veno_state,
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.cwnd_event = tcp_veno_cwnd_event,
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.owner = THIS_MODULE,
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.name = "veno",
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};
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static int __init tcp_veno_register(void)
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{
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BUILD_BUG_ON(sizeof(struct veno) > ICSK_CA_PRIV_SIZE);
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tcp_register_congestion_control(&tcp_veno);
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return 0;
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}
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static void __exit tcp_veno_unregister(void)
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{
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tcp_unregister_congestion_control(&tcp_veno);
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}
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module_init(tcp_veno_register);
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module_exit(tcp_veno_unregister);
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MODULE_AUTHOR("Bin Zhou, Cheng Peng Fu");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("TCP Veno");
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