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To: Paul Traina <pst@shockwave.com>
cc: questions@freebsd.org, hackers@freebsd.org
Subject: Re: using ddb to debug a double-panic? 
In-reply-to: Your message of "Fri, 01 Mar 1996 13:53:27 PST."
             <199603012153.NAA00721@precipice.shockwave.com> 
From: David Greenman <davidg@Root.COM>
Reply-To: davidg@Root.COM
Date: Sat, 02 Mar 1996 00:59:04 -0800
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>Is there a simple sequence I can type into ddb to switch stack pointers and
>frames so I can do a "where" to see where I was when the first panic occured?

   I thought about writing some extensions to "trace" to allow it to apply a
(operator supplied) 'stack offset' that would be used to adjust the pushed FPs
(for precisely the purpose of what you're requesting above). ...but I haven't
gotten around to this yet. For now, I just use 'curpcb' to find the stack that
[might] have been in use at the time of the double fault, and then munge
around in the stack manually (yes, I know, yuck). Of course if there was no
process running at the time, you'll want to look at tmpstk instead.
   This code definately code use some work - I just wanted to catch the case
in the first place so that machines didn't just "wedge"...and then have at
least a snowball's chance of figuring out the cause. I'm not happy with the
way that the double fault TSS currently works (using IdlePTD, etc), but I
haven't had any time to implement it better.

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project