From: Eddie Kohler <kohler@cs.ucla.edu>
To: dccp@vger.kernel.org
Subject: Re: [PATCH 7/10]: Avoid `division by zero' errors
Date: Mon, 27 Nov 2006 16:16:06 +0000 [thread overview]
Message-ID: <456B0F46.5030304@cs.ucla.edu> (raw)
In-Reply-To: <200611241619.56426@strip-the-willow>
Same comment (it's probably safe to use the Req-Response exchange for an
initial RTT estimate).
E
Gerrit Renker wrote:
> [CCID 3]: Avoid `division by zero' errors
>
> Several places of the code divide by the current RTT value. A division-by-zero
> error results if this value is 0.
> To protect against this,
> * DCCP_BUG_ON conditions are added throughout the tx code;
> * in ccid3_hc_tx_packet_recv, a code block is shifted, to avoid not updating
> p and x_recv in case the RTT estimation results in an error;
>
> Since this could probably be done in a smarter way, a FIXME is added. Lastly,
> some minor code comment updates.
>
> Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
> ---
> net/dccp/ccids/ccid3.c | 53 ++++++++++++++++++++-----------------
> net/dccp/ccids/ccid3.h | 2 -
> net/dccp/ccids/lib/tfrc_equation.c | 7 +---
> 3 files changed, 33 insertions(+), 29 deletions(-)
>
> --- a/net/dccp/ccids/ccid3.c
> +++ b/net/dccp/ccids/ccid3.c
> @@ -117,7 +117,7 @@ static inline void ccid3_update_send_tim
> TFRC_OPSYS_HALF_TIME_GRAN);
> }
> /*
> - * Update X by
> + * Update X by (this implements step (4) of [RFC 3448, 4.3]):
> * If (p > 0)
> * x_calc = calcX(s, R, p);
> * X = max(min(X_calc, 2 * X_recv), s / t_mbi);
> @@ -125,12 +125,17 @@ static inline void ccid3_update_send_tim
> * If (now - tld >= R)
> * X = max(min(2 * X, 2 * X_recv), s / R);
> * tld = now;
> + *
> + * If X has changed, we also update the scheduled send time t_now,
> + * the inter-packet interval t_ipi, and the delta value.
> */
> static void ccid3_hc_tx_update_x(struct sock *sk)
> {
> struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
> const __u32 old_x = hctx->ccid3hctx_x;
>
> + DCCP_BUG_ON(hctx->ccid3hctx_rtt = 0);
> +
> /* To avoid large error in calcX */
> if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
> hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
> @@ -421,7 +426,6 @@ static void ccid3_hc_tx_packet_recv(stru
> unsigned long next_tmout;
> u32 t_elapsed;
> u32 pinv;
> - u32 x_recv;
> u32 r_sample;
>
> BUG_ON(hctx = NULL);
> @@ -434,15 +438,11 @@ static void ccid3_hc_tx_packet_recv(stru
> opt_recv = &hctx->ccid3hctx_options_received;
>
> t_elapsed = dp->dccps_options_received.dccpor_elapsed_time * 10;
> - x_recv = opt_recv->ccid3or_receive_rate;
> pinv = opt_recv->ccid3or_loss_event_rate;
>
> switch (hctx->ccid3hctx_state) {
> case TFRC_SSTATE_NO_FBACK:
> case TFRC_SSTATE_FBACK:
> - /* Calculate new round trip sample by
> - * R_sample = (now - t_recvdata) - t_delay */
> - /* get t_recvdata from history */
> packet = dccp_tx_hist_find_entry(&hctx->ccid3hctx_hist,
> DCCP_SKB_CB(skb)->dccpd_ack_seq);
> if (unlikely(packet = NULL)) {
> @@ -453,7 +453,25 @@ static void ccid3_hc_tx_packet_recv(stru
> return;
> }
>
> - /* Update RTT */
> + /* Update receive rate */
> + hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
> +
> + /* Update loss event rate */
> + if (pinv = ~0 || pinv = 0)
> + hctx->ccid3hctx_p = 0;
> + else {
> + hctx->ccid3hctx_p = 1000000 / pinv;
> +
> + if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
> + hctx->ccid3hctx_p = TFRC_SMALLEST_P;
> + ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
> + dccp_role(sk), sk);
> + }
> + }
> +
> + /* Calculate new round trip sample by
> + * R_sample = (t_now - t_recvdata) - t_delay
> + * t_recvdata is taken from packet history */
> dccp_timestamp(sk, &now);
> r_sample = timeval_delta(&now, &packet->dccphtx_tstamp);
> if (unlikely(r_sample <= t_elapsed))
> @@ -462,7 +480,7 @@ static void ccid3_hc_tx_packet_recv(stru
> else
> r_sample -= t_elapsed;
>
> - /* Update RTT estimate by
> + /* Update RTT estimate by (step (2) of [RFC 3448, 4.3]):
> * If (No feedback recv)
> * R = R_sample;
> * Else
> @@ -481,21 +499,10 @@ static void ccid3_hc_tx_packet_recv(stru
> "r_sample=%us\n", dccp_role(sk), sk,
> hctx->ccid3hctx_rtt, r_sample);
>
> - /* Update receive rate */
> - hctx->ccid3hctx_x_recv = x_recv;/* X_recv in bytes per sec */
> -
> - /* Update loss event rate */
> - if (pinv = ~0 || pinv = 0)
> - hctx->ccid3hctx_p = 0;
> - else {
> - hctx->ccid3hctx_p = 1000000 / pinv;
> -
> - if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
> - hctx->ccid3hctx_p = TFRC_SMALLEST_P;
> - ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
> - dccp_role(sk), sk);
> - }
> - }
> + /* Avoid `division-by zero' errors.
> + * FIXME: We need a smarter and more robust way to
> + * protect against this possibility */
> + DCCP_BUG_ON(hctx->ccid3hctx_rtt = 0);
>
> /* unschedule no feedback timer */
> sk_stop_timer(sk, &hctx->ccid3hctx_no_feedback_timer);
> --- a/net/dccp/ccids/lib/tfrc_equation.c
> +++ b/net/dccp/ccids/lib/tfrc_equation.c
> @@ -578,6 +578,8 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
> u32 f;
> u64 tmp1, tmp2;
>
> + DCCP_BUG_ON(R = 0); /* RTT can't be zero or else divide by zero */
> +
> if (p < TFRC_CALC_X_SPLIT)
> index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
> else
> @@ -587,11 +589,6 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
> /* p should be 0 unless there is a bug in my code */
> index = 0;
>
> - if (R = 0) {
> - DCCP_WARN("RTT=0, setting to 1\n");
> - R = 1; /* RTT can't be zero or else divide by zero */
> - }
> -
> BUG_ON(index >= TFRC_CALC_X_ARRSIZE);
>
> if (p >= TFRC_CALC_X_SPLIT)
> --- a/net/dccp/ccids/ccid3.h
> +++ b/net/dccp/ccids/ccid3.h
> @@ -78,7 +78,7 @@ enum ccid3_hc_tx_states {
> /** struct ccid3_hc_tx_sock - CCID3 sender half-connection socket
> *
> * @ccid3hctx_x - Current sending rate
> - * @ccid3hctx_x_recv - Receive rate
> + * @ccid3hctx_x_recv - Receive rate in bytes per second
> * @ccid3hctx_x_calc - Calculated send rate (RFC 3448, 3.1)
> * @ccid3hctx_rtt - Estimate of current round trip time in usecs
> * @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
> -
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next prev parent reply other threads:[~2006-11-27 16:16 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-11-24 16:19 [PATCH 7/10]: Avoid `division by zero' errors Gerrit Renker
2006-11-26 20:59 ` Ian McDonald
2006-11-27 10:24 ` Gerrit Renker
2006-11-27 16:16 ` Eddie Kohler [this message]
2006-11-27 17:15 ` Gerrit Renker
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