HEADSUP: New pts code triggers panics on amd64 systems.

Giorgos Keramidas keramida at ceid.upatras.gr
Fri Feb 10 15:10:19 PST 2006

On 2006-02-07 13:26, Robert Watson <rwatson at FreeBSD.org> wrote:
>On Tue, 7 Feb 2006, Giorgos Keramidas wrote:
>>On 2006-02-01 15:55, Steve Kargl <sgk at troutmask.apl.washington.edu> wrote:
>>>After a binary search, I have determined that the new pts code is
>>>triggering kernel panics on an AMD64 system.
>> It also makes syscons unusable here.
>> I just rebuilt a HEAD snapshot from today's latest CVSup, installed it
>> in /dev/ad0s1a (my test partition), and the behavior is still the same
>> as a few days ago:
>>    - single user mode shell works fine
>>    - in multiuser mode, when syscons reaches a login prompt
>>      i have to press RET twice to see the last line
>> It seems that something is broken in the way syscons detects whether an
>> output line should be flushed out, but I'm not sure.
>> A snapshot from -D '2006/01/26 01:30:00 UTC' works fine (just before the
>> first pts change).
>> I don't know how to debug this or provide more useful feedback, but I'll
>> look at the diffs later today, when I'm done with $REALJOB stuff.
> Does the instability occur if kern.pts.enable=0, or only when
> kern.pts.enable=1?  If 0, if you back out the user space changes but leave
> tty_pts.c compiled into the kernel, do the instability issues persist?  How
> about with the kernel code compiled out, but the user space code in place?
> Basically, it would be good to know if what you're seeing is a property of
> the pts code being in the kernel at all, or a property of it actually in
> use.

I'm not sure if what I see is a pts side-effect.  I rebuilt a snapshot
of HEAD again today, and discovered something else that may help us
track this down.

A few more observations, in case they prove helpful to someone more
acquainted with the way syscons output is supposed to work:

  - If I keep hitting Scroll-Lock again and again, then syscons output
    *does* eventually appear.

  - If I type stuff without seeing it and then press RETURN twice, the
    previous from last line *does* appear in my terminal.

  - When pressing CTRL-ALT-ESC, the debugger starts normally, but after
    typing many times commands that have large output, i.e.:

	show witness

    the console locks up entirely.

This looks like a locking problem, instead of a pts/syscons one :-/

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