Panic when logging out from serial console

Robert Watson rwatson at
Sun Apr 10 04:42:14 PDT 2005

On Sun, 10 Apr 2005 ltning at wrote:

> warning: This report might be somewhat vague. For quite a while now I`ve 
> been plagued with the problem that logging out from a serial console 
> causes the box to panic. For a while I`ve been sure this was isolated to 
> one of my boxen, because it`s been acting up in other ways as well, but 
> today it happened on two other boxes too! And these boxes have been rock 
> stable for the last two years.
> I`m running a fairly recent variation of RELENG-5 on all the boxes; one 
> of them is amd64, the two others - including the one I`ve pasted from - 
> are plain old p3 machines. They are all dual-CPU though.

I've seen precisely this panic -- in fact, I saw it yesterday on a 
RELENG_5 box, and under identical circumstances -- it looks like it 
happens if a last process in a login session on a serial console closes 
the tty, and then getty re-opens it while there's console output coming 
from syslog.  I was able to get a core dump, but haven't made much headway 
on it yet.  It looks like the tty structure has been released -- the 
refcount on the tty is 0, and the mutex pointers in the kqueue state have 
been cleared (hence the null pointer dereference you see).  Now, the 
question is why -- I've added some debugging output to the local box I saw 
it on, and will see if I can reproduce it.

Robert N M Watson

> I have no clue what I can do from here; has anyone seen this before? I can`t
> always reproduce it, but the risk is fairly high - around 33% I`d say.
> Anyone?
> Thanks for your attention, details below.
> Fatal trap 12: page fault while in kernel mode
> cpuid = 1; apic id = 00
> fault virtual address   = 0x1c
> fault code              = supervisor write, page not present
> instruction pointer     = 0x8:0xc0620b5f
> stack pointer           = 0x10:0xdadbd988
> frame pointer           = 0x10:0xdadbd994
> 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         = 51999 (getty)
> trap number             = 12
> panic: page fault
> cpuid = 1
> boot() called on cpu#0
> Uptime: 66d11h24m50s
> /Eirik
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