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    Subject[PATCH 3.16 09/55] tcp: fix ssthresh and undo for consecutive short FRTO episodes
    Date
    3.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: Neal Cardwell <ncardwell@google.com>

    [ Upstream commit 0c9ab09223fe9922baeb22546c9a90d774a4bde6 ]

    Fix TCP FRTO logic so that it always notices when snd_una advances,
    indicating that any RTO after that point will be a new and distinct
    loss episode.

    Previously there was a very specific sequence that could cause FRTO to
    fail to notice a new loss episode had started:

    (1) RTO timer fires, enter FRTO and retransmit packet 1 in write queue
    (2) receiver ACKs packet 1
    (3) FRTO sends 2 more packets
    (4) RTO timer fires again (should start a new loss episode)

    The problem was in step (3) above, where tcp_process_loss() returned
    early (in the spot marked "Step 2.b"), so that it never got to the
    logic to clear icsk_retransmits. Thus icsk_retransmits stayed
    non-zero. Thus in step (4) tcp_enter_loss() would see the non-zero
    icsk_retransmits, decide that this RTO is not a new episode, and
    decide not to cut ssthresh and remember the current cwnd and ssthresh
    for undo.

    There were two main consequences to the bug that we have
    observed. First, ssthresh was not decreased in step (4). Second, when
    there was a series of such FRTO (1-4) sequences that happened to be
    followed by an FRTO undo, we would restore the cwnd and ssthresh from
    before the entire series started (instead of the cwnd and ssthresh
    from before the most recent RTO). This could result in cwnd and
    ssthresh being restored to values much bigger than the proper values.

    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_input.c | 8 +++-----
    1 file changed, 3 insertions(+), 5 deletions(-)

    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -2680,7 +2680,6 @@ static void tcp_enter_recovery(struct so
    */
    static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
    {
    - struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    bool recovered = !before(tp->snd_una, tp->high_seq);

    @@ -2706,12 +2705,9 @@ static void tcp_process_loss(struct sock

    if (recovered) {
    /* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
    - icsk->icsk_retransmits = 0;
    tcp_try_undo_recovery(sk);
    return;
    }
    - if (flag & FLAG_DATA_ACKED)
    - icsk->icsk_retransmits = 0;
    if (tcp_is_reno(tp)) {
    /* A Reno DUPACK means new data in F-RTO step 2.b above are
    * delivered. Lower inflight to clock out (re)tranmissions.
    @@ -3393,8 +3389,10 @@ static int tcp_ack(struct sock *sk, cons
    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
    tcp_rearm_rto(sk);

    - if (after(ack, prior_snd_una))
    + if (after(ack, prior_snd_una)) {
    flag |= FLAG_SND_UNA_ADVANCED;
    + icsk->icsk_retransmits = 0;
    + }

    prior_fackets = tp->fackets_out;




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