Re: 9.0-RC1 panic in tcp_input: negative winow.

On Wednesday, October 26, 2011 3:54:31 am Pawel Jakub Dawidek wrote:
> On Mon, Oct 24, 2011 at 08:14:22AM -0400, John Baldwin wrote:
> > On Sunday, October 23, 2011 11:58:28 am Pawel Jakub Dawidek wrote:
> > > On Sun, Oct 23, 2011 at 11:44:45AM +0300, Kostik Belousov wrote:
> > > > On Sun, Oct 23, 2011 at 08:10:38AM +0200, Pawel Jakub Dawidek wrote:
> > > > > My suggestion would be that if we won't be able to fix it before 9.0,
> > > > > we should turn this assertion off, as the system seems to be able to
> > > > > recover.
> > > > 
> > > > Shipped kernels have all assertions turned off.
> > > 
> > > Yes, I'm aware of that, but many people compile their production kernels
> > > with INVARIANTS/INVARIANT_SUPPORT to fail early instead of eg.
> > > corrupting data. I'd be fine in moving this under DIAGNOSTIC or changing
> > > it into a printf, so it will be visible.
> > 
> > No, the kernel is corrupting things in other places when this is true, so
> > if you are running with INVARIANTS, we want to know about it.   Specifically,
> > in several places in TCP we assume that rcv_adv >= rcv_nxt, and depend on
> > being able to do 'rcv_adv - rcv_nxt'.
> > 
> > In this case, it looks like the difference is consistently less than one 
> > frame.  I suspect the other end of the connection is sending just beyond the 
> > end of the advertised window (it probably assumes it is better to send a full 
> > frame if it has that much pending data even though part of it is beyond the 
> > window edge vs sending a truncated packet that just fills the window) and that
> > that frame is accepted ok in the header prediction case and it's ACK is 
> > delayed, but the next packet to arrive then trips over this assumption.
> > 
> > Since 'win' is guaranteed to be non-negative and we explicitly cast
> > 'rcv_adv - rcv_nxt' to (int) in the following line that the assert is checking
> > for:
> > 
> > 	tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
> > 
> > I think we already handle this case ok and perhaps the assertion can just be
> > removed?  Not sure if others feel that it warrants a comment to note that this
> > is the case being handled.
> 
> I added debug to the places where rcv_adv and rcv_nxt are modified. Here
> is what happens before the panic occurs:
> 
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022361548 rcv_adv 4022360100 diff -1448
> tcp_do_segment:2847 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022362298 rcv_adv 4022361548 diff -750
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022363746 rcv_adv 4022362298 diff -1448
> tcp_do_segment:2847 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022364836 rcv_adv 4022363746 diff -1090
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022366284 rcv_adv 4022364836 diff -1448
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022370628 rcv_adv 4022369690 diff -938
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022379140 rcv_adv 4022377692 diff -1448
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022387792 rcv_adv 4022386344 diff -1448
> tcp_do_segment:2847 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022388890 rcv_adv 4022387792 diff -1098
> tcp_do_segment:1722 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022390338 rcv_adv 4022388890 diff -1448
> tcp_do_segment:2847 negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022394563 rcv_adv 4022394342 diff -221
> panic: tcp_input negative window: tp 0xfffffe000dab1b70 rcv_nxt 4022394563 rcv_adv 4022394342 win=0 diff -221
> 
> I can send you the full log if you want, I've plenty of messages where
> rcv_adv < rcv_nxt, not all of them trigger this assertion.

The assertion would be triggered when the next packet arrives (as I said
above).  Try modifying your debugging output to also log if the ACK is
delayed.  I suspect it is not delayed until the last one.  (Pushing out an
ACK will reset rcv_adv to be beyond rcv_nxt in tcp_output(), so in the case
of an immediate ACK, rcv_nxt > rcv_adv is only a transient condition all
under a single lock invocation so never visible to other consumers of the
protocol control block.)  If that is what you see, then that confirms what
I guessed above and I will likely just remove the assertion in tcp_input()
and patch the timewait code to handle this case.

-- 
John Baldwin