netdev - Re: [PATCH-2.4] Fix divide by 0 in vegas_cong

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Date:	Tue, 29 May 2007 15:02:16 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"Lior Dotan" <liodot@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH-2.4] Fix divide by 0 in vegas_cong_avoid()

On Tue, 29 May 2007 20:23:45 +0200
"Lior Dotan" <liodot@...il.com> wrote:

> NTP was not running. I'm not sure what do you mean by fixing the -1.
> The trace shows that vegas_cong_avoid() is called with -1, and the
> only way it can happen is from tcp_clean_rtx_queue() and the patch
> should eliminate this. Another way of solving this is by checking
> vegas_rtt_calc() and see if it gets -1 and handle it there.
> Another thing that I don't understand is that some places like
> tcp_ack() declare seq_rtt as signed and some vegas_cong_avoid()
> declare it as unsigned. Shouldn't it be declared always as signed?
> 
> On 5/29/07, Stephen Hemminger <shemminger@...ux-foundation.org> wrote:
> > On Tue, 29 May 2007 12:18:19 +0300
> > "Lior Dotan" <liodot@...il.com> wrote:
> >
> > > Hi,
> > >
> > > I had a divide by zero on kernel 2.4.33 running with Vegas enabled.

I don't think anyone has backported the other vegas fixes from 2.6.21
to 2.4.

> > > The KDB back trace is:
> > > kdb> bt
> > > Stack traceback for pid 0
> > > 0x403a6000        0        0  1    0   R  0x403a6370 *swapper
> > > EBP        EIP        Function (args)
> > > 0x403a7d48 0x4026ae51 vegas_cong_avoid+0x111 (0x5f3bb638, 0x73c92cbb, 0xffffffff
> > > , 0x73c92cbb, 0xf28d0275)
> > >                                kernel .text 0x40100000 0x4026ad40 0x4026aef0
> > > 0x403a7d8c 0x4026bb67 tcp_ack+0x307 (0x5f3bb560, 0x581985c0, 0x18e, 0x4023e765,
> > > 0x5c369044)
> > >                                kernel .text 0x40100000 0x4026b860 0x4026be20
> >
> >
> > Seems like fixing the -1 would be better.  Perhaps NTP reset
> > the clock?
> >
> >
> >
> > --
> > Stephen Hemminger <shemminger@...ux-foundation.org>
> >

For 2.6.22, rtt_calc doesn't exist, instead pkts_acked is called. The following
should be added to avoid getting bogus timestamp values from retransmits.

--- a/net/ipv4/tcp_vegas.c	2007-05-02 12:26:35.000000000 -0700
+++ b/net/ipv4/tcp_vegas.c	2007-05-29 14:06:26.000000000 -0700
@@ -117,6 +117,10 @@ void tcp_vegas_pkts_acked(struct sock *s
 	struct vegas *vegas = inet_csk_ca(sk);
 	u32 vrtt;
 
+	/* Ignore retransmits */
+	if (unlikely(cnt == 0))
+		return;
+
 	/* Never allow zero rtt or baseRTT */
 	vrtt = ktime_to_us(net_timedelta(last)) + 1;
 


-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
-
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