panic logging out on serial console

[Gavin Atkinson]

On Mon, 2005-12-05 at 11:47 +0000, Bjoern A. Zeeb wrote:
> I had been logged in on serial console and typed 'exit' and the
> RELENG_6 machine went *kaboom*. I hadn't seen sth like this befire on
> any of my other machines:
> 
> i386/RELENG_6 from around 2005-11-17 11:00 UTC.
> 
> --- 8< 8< 8< ---
> foo# exit
> logout
> 
> Fatal trap 12: page fault while in kernel mode
> fault virtual address   = 0x0
> fault code              = supervisor read, page not present
> instruction pointer     = 0x20:0x0
> stack pointer           = 0x28:0xef699954
> frame pointer           = 0x28:0xef699968
> code segment            = base 0x0, limit 0xfffff, type 0x1b
>                          = DPL 0, pres 1, def32 1, gran 1
> processor eflags        = interrupt enabled, resume, IOPL = 0
> current process         = 70703 (getty)
> trap number             = 12
> panic: page fault
> 
> I have the core file and can save it for some days but it won't
> help a lot unless someone tells me how I can skip the frame with
> the null pointer in kgdb.

I've never had a problem with backtraces (even when IP=0x0) but don't
forget you can always look at the stack with

(gdb) x/40xw 0xef699954

And then look for addresses that may be within the kernel (probably
start with 0xc)

I'd be interested to know if there is a possibilituy that your panic may
have anything to do with one that I saw
http://lists.freebsd.org/pipermail/freebsd-current/2004-December/043944.html
which seems to be a race in the tty closedown code (which I guess may
also be executed during a logout)

Gavin