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

On 10/26/11 12:54 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.
>
Might be a good place to use the new sifter tool.