Garbled output from kgdb?

Clifton Royston cliftonr at
Wed Apr 29 21:21:29 UTC 2009

On Wed, Apr 29, 2009 at 03:28:09PM -0500, Alan Amesbury wrote:
> One of my systems (FreeBSD 7.1-RELEASE-p3/amd64) has panicked a couple
> times recently without an identified cause.  This most recent time I was
> able to obtain a crash dump from the system, but output from kgdb is
> garbled.
> Fatal trap 12: page fault while in kernel mode
> cpuid = 3; apic id = 07
> fault virtual addresske rn= el0 xt60r
> afapul t 1co2de                 w= isutpehrv isiorn twerritreu pdtasta
> d,i spaagbe lnoet dpres
> ent
> instruction pointer     = 0x8:0xffffffff80424561
> s
> tack
>  pFoianttera     l        = 0x10t:0xfffrffaffpfac057af0
> ------------------ End output #1 ------------------
> I've seen a thread on garbled serial console output and have seen
> symptoms of this on several of my systems (including this one), but
> that's been more of an annoyance than anything about which I'm actually
> worried... until now.  (I've actually suspected hardware/BIOS issues
> relating to serial port access, which is why I've stayed out of that
> thread.)  That the crash dump includes similar corruption suggests to me
> that it's not the serial device, but something a bit closer to FreeBSD
> itself.
> Any ideas what's causing the garbled output in kgdb?

Lack of buffering within kernel threads; two or more CPUs/threads are
trying to log messages concurrently, one character at a time.  You can
actually decode some of it with a bit of attention:

  virtual address    = 0 x 601
                 kernel   t  r a p
                              f a u l t 

or something of the kind.  The issue is not really to do with serial
console vs. video console, which is why you can see it even in the
kernel message buffer following the crash dump.

  -- Clifton

    Clifton Royston  --  cliftonr at / cliftonr at
       President  - I and I Computing *
 Custom programming, network design, systems and network consulting services

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