From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eddie Kohler Date: Mon, 27 Nov 2006 16:16:06 +0000 Subject: Re: [PATCH 7/10]: Avoid `division by zero' errors Message-Id: <456B0F46.5030304@cs.ucla.edu> List-Id: References: <200611241619.56426@strip-the-willow> In-Reply-To: <200611241619.56426@strip-the-willow> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: dccp@vger.kernel.org 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 > --- > 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 > - > To unsubscribe from this list: send the line "unsubscribe dccp" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html