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    Subject[PATCH 4.3 100/157] tcp_yeah: dont set ssthresh below 2
    Date
    4.3-stable review patch.  If anyone has any objections, please let me know.

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

    From: Neal Cardwell <ncardwell@google.com>

    [ Upstream commit 83d15e70c4d8909d722c0d64747d8fb42e38a48f ]

    For tcp_yeah, use an ssthresh floor of 2, the same floor used by Reno
    and CUBIC, per RFC 5681 (equation 4).

    tcp_yeah_ssthresh() was sometimes returning a 0 or negative ssthresh
    value if the intended reduction is as big or bigger than the current
    cwnd. Congestion control modules should never return a zero or
    negative ssthresh. A zero ssthresh generally results in a zero cwnd,
    causing the connection to stall. A negative ssthresh value will be
    interpreted as a u32 and will set a target cwnd for PRR near 4
    billion.

    Oleksandr Natalenko reported that a system using tcp_yeah with ECN
    could see a warning about a prior_cwnd of 0 in
    tcp_cwnd_reduction(). Testing verified that this was due to
    tcp_yeah_ssthresh() misbehaving in this way.

    Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_yeah.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/net/ipv4/tcp_yeah.c
    +++ b/net/ipv4/tcp_yeah.c
    @@ -219,7 +219,7 @@ static u32 tcp_yeah_ssthresh(struct sock
    yeah->fast_count = 0;
    yeah->reno_count = max(yeah->reno_count>>1, 2U);

    - return tp->snd_cwnd - reduction;
    + return max_t(int, tp->snd_cwnd - reduction, 2);
    }

    static struct tcp_congestion_ops tcp_yeah __read_mostly = {

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