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    Subject[PATCH 4.9 18/33] tcp_bbr: fix to zero idle_restart only upon S/ACKed data
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

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

    From: Neal Cardwell <ncardwell@google.com>

    [ Upstream commit e6e6a278b1eaffa19d42186bfacd1ffc15a50b3f ]

    Previously the bbr->idle_restart tracking was zeroing out the
    bbr->idle_restart bit upon ACKs that did not SACK or ACK anything,
    e.g. receiving incoming data or receiver window updates. In such
    situations BBR would forget that this was a restart-from-idle
    situation, and if the min_rtt had expired it would unnecessarily enter
    PROBE_RTT (even though we were actually restarting from idle but had
    merely forgotten that fact).

    The fix is simple: we need to remember we are restarting from idle
    until we receive a S/ACK for some data (a S/ACK for the first flight
    of data we send as we are restarting).

    This commit is a stable candidate for kernels back as far as 4.9.

    Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
    Signed-off-by: Priyaranjan Jha <priyarjha@google.com>
    Signed-off-by: Yousuk Seung <ysseung@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_bbr.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    --- a/net/ipv4/tcp_bbr.c
    +++ b/net/ipv4/tcp_bbr.c
    @@ -773,7 +773,9 @@ static void bbr_update_min_rtt(struct so
    }
    }
    }
    - bbr->idle_restart = 0;
    + /* Restart after idle ends only once we process a new S/ACK for data */
    + if (rs->delivered > 0)
    + bbr->idle_restart = 0;
    }

    static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)

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