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    Subject[PATCH 68/81] tcp: fix for zero packets_in_flight was too broad
    Date
    3.5.7.6 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= <ilpo.jarvinen@helsinki.fi>

    commit 6731d2095bd4aef18027c72ef845ab1087c3ba63 upstream.

    There are transients during normal FRTO procedure during which
    the packets_in_flight can go to zero between write_queue state
    updates and firing the resulting segments out. As FRTO processing
    occurs during that window the check must be more precise to
    not match "spuriously" :-). More specificly, e.g., when
    packets_in_flight is zero but FLAG_DATA_ACKED is true the problematic
    branch that set cwnd into zero would not be taken and new segments
    might be sent out later.

    Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
    Tested-by: Eric Dumazet <edumazet@google.com>
    Acked-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
    ---
    net/ipv4/tcp_input.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
    index 6affa92..3b14d81 100644
    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -3695,8 +3695,7 @@ static bool tcp_process_frto(struct sock *sk, int flag)
    ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
    tp->undo_marker = 0;

    - if (!before(tp->snd_una, tp->frto_highmark) ||
    - !tcp_packets_in_flight(tp)) {
    + if (!before(tp->snd_una, tp->frto_highmark)) {
    tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
    return true;
    }
    @@ -3716,6 +3715,11 @@ static bool tcp_process_frto(struct sock *sk, int flag)
    }
    } else {
    if (!(flag & FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
    + if (!tcp_packets_in_flight(tp)) {
    + tcp_enter_frto_loss(sk, 2, flag);
    + return true;
    + }
    +
    /* Prevent sending of new data. */
    tp->snd_cwnd = min(tp->snd_cwnd,
    tcp_packets_in_flight(tp));
    --
    1.7.9.5
    --
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