Re: [PATCH 7/10]: Avoid `division by zero' errors

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

Quoting Eddie Kohler:
|  Same comment (it's probably safe to use the Req-Response exchange for an 
|  initial RTT estimate).
Same answer: this is part of draft-ietf-dccp-rfc3448bis-00.txt, but not part of
RFC 3448; therefore I am against it.
|  
|  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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