linux-kernel - Re: 2.6.23-rc8-mm2 - tcp_fastretrans

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Date:	Sat, 29 Sep 2007 15:44:49 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Cedric Le Goater <clg@...ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: 2.6.23-rc8-mm2 - tcp_fastretrans_alert() WARNING

On Fri, 28 Sep 2007, Ilpo Järvinen wrote:
> On Fri, 28 Sep 2007, Cedric Le Goater wrote:
>
> > I just found that warning in my logs. It seems that it's been 
> > happening since rc7-mm1 at least. 
> > 
> > WARNING: at /home/legoater/linux/2.6.23-rc8-mm2/net/ipv4/tcp_input.c:2314 tcp_fastretrans_alert()
> >
> > Call Trace:
> >  <IRQ>  [<ffffffff8040fdc3>] tcp_ack+0xcd6/0x1894
> > ...snip...
> 
> ...Thanks for the report, I'll have look what could still break 
> fackets_out...

I think this one is now clear to me, tcp_fragment/collapse adjusts 
fackets_out (incorrectly) also for reno flow when there were some dupACKs 
that made sacked_out != 0. Could you please try if patch below proves all 
them to be of non-SACK origin... In case that's true, it's rather 
harmless, I'll send a fix on Monday or so (this would anyway be needed)... 
If you find out that them occur with SACK enabled flow, that would be
more interesting and requires more digging...

-- 
 i.

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2286361..e642779 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2311,8 +2311,10 @@ tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
 	if (!tp->packets_out)
 		tp->sacked_out = 0;

-	if (WARN_ON(!tp->sacked_out && tp->fackets_out))
+	if (WARN_ON(!tp->sacked_out && tp->fackets_out)) {
+		printk(KERN_ERR "TCP %d\n", tcp_is_reno(tp));
 		tp->fackets_out = 0;
+	}

 	/* Now state machine starts.
 	 * A. ECE, hence prohibit cwnd undoing, the reduction is required. */