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Commit 973ec449 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: fix an infinite loop in tcp_slow_start() 



Since commit 9dc27415 (tcp: fix ABC in tcp_slow_start()),
a nul snd_cwnd triggers an infinite loop in tcp_slow_start()

Avoid this infinite loop and log a one time error for further
analysis. FRTO code is suspected to cause this bug.

Reported-by: default avatarPasi Kärkkäinen <pasik@iki.fi>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 59fa5348

net/ipv4/tcp_cong.c

+10 −4
Original line number Diff line number Diff line
@@ -310,6 +310,12 @@ void tcp_slow_start(struct tcp_sock *tp) 
{

	int cnt; /* increase in packets */

	unsigned int delta = 0;

	u32 snd_cwnd = tp->snd_cwnd;



	if (unlikely(!snd_cwnd)) {

		pr_err_once("snd_cwnd is nul, please report this bug.\n");

		snd_cwnd = 1U;

	}



	/* RFC3465: ABC Slow start

	 * Increase only after a full MSS of bytes is acked
@@ -324,7 +330,7 @@ void tcp_slow_start(struct tcp_sock *tp) 
	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)

		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */

	else

		cnt = tp->snd_cwnd;			/* exponential increase */

		cnt = snd_cwnd;				/* exponential increase */



	/* RFC3465: ABC

	 * We MAY increase by 2 if discovered delayed ack
@@ -334,11 +340,11 @@ void tcp_slow_start(struct tcp_sock *tp) 
	tp->bytes_acked = 0;



	tp->snd_cwnd_cnt += cnt;

	while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {

		tp->snd_cwnd_cnt -= tp->snd_cwnd;

	while (tp->snd_cwnd_cnt >= snd_cwnd) {

		tp->snd_cwnd_cnt -= snd_cwnd;

		delta++;

	}

	tp->snd_cwnd = min(tp->snd_cwnd + delta, tp->snd_cwnd_clamp);

	tp->snd_cwnd = min(snd_cwnd + delta, tp->snd_cwnd_clamp);

}

EXPORT_SYMBOL_GPL(tcp_slow_start);