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68 lines
2.6 KiB
Diff
68 lines
2.6 KiB
Diff
From e1f31f3f20ab760ebb2425e67385b2c593c35be5 Mon Sep 17 00:00:00 2001
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From: Yuchung Cheng <ycheng@google.com>
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Date: Fri, 26 May 2017 15:45:15 +0530
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Subject: tcp: fix zero cwnd in tcp_cwnd_reduction
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[ Upstream commit: 8b8a321ff72c785ed5e8b4cf6eda20b35d427390]
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Patch 3759824da87b ("tcp: PRR uses CRB mode by default and SS mode
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conditionally") introduced a bug that cwnd may become 0 when both
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inflight and sndcnt are 0 (cwnd = inflight + sndcnt). This may lead
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to a div-by-zero if the connection starts another cwnd reduction
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phase by setting tp->prior_cwnd to the current cwnd (0) in
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tcp_init_cwnd_reduction().
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To prevent this we skip PRR operation when nothing is acked or
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sacked. Then cwnd must be positive in all cases as long as ssthresh
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is positive:
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1) The proportional reduction mode
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inflight > ssthresh > 0
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2) The reduction bound mode
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a) inflight == ssthresh > 0
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b) inflight < ssthresh
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sndcnt > 0 since newly_acked_sacked > 0 and inflight < ssthresh
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Therefore in all cases inflight and sndcnt can not both be 0.
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We check invalid tp->prior_cwnd to avoid potential div0 bugs.
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In reality this bug is triggered only with a sequence of less common
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events. For example, the connection is terminating an ECN-triggered
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cwnd reduction with an inflight 0, then it receives reordered/old
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ACKs or DSACKs from prior transmission (which acks nothing). Or the
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connection is in fast recovery stage that marks everything lost,
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but fails to retransmit due to local issues, then receives data
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packets from other end which acks nothing.
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Change-Id: I6edbd82492839ca86515c64ee22828f7582900aa
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Fixes: 3759824da87b ("tcp: PRR uses CRB mode by default and SS mode conditionally")
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Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
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Signed-off-by: Yuchung Cheng <ycheng@google.com>
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Signed-off-by: Neal Cardwell <ncardwell@google.com>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Signed-off-by: David S. Miller <davem@davemloft.net>
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Signed-off-by: Akshaya <akshayab@codeaurora.org>
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---
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net/ipv4/tcp_input.c | 3 +++
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1 file changed, 3 insertions(+)
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diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
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index cc7f940..c0be23d 100644
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--- a/net/ipv4/tcp_input.c
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+++ b/net/ipv4/tcp_input.c
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@@ -2516,6 +2516,9 @@ static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
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int newly_acked_sacked = prior_unsacked -
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(tp->packets_out - tp->sacked_out);
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+ if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
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+ return;
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+
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tp->prr_delivered += newly_acked_sacked;
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if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
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u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
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--
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1.9.1
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