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Subject: using ddb to debug a double-panic?
Date: Fri, 01 Mar 1996 13:53:27 -0800
From: Paul Traina <pst@shockwave.com>
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I'm seeing something -very- strange while working on a device driver.

When I add a delay to some code that's doing direct I/O to the parallel port,
I get a system hang (but I can pop out to ddb and look at things, however ddb
is showing that I'm just waiting in the idle loop).

It seems like something is going to sleep on a system resource and never
coming back.  So, I figured, what the heck, I'd add a couple of kernel printfs
in strategic places so I could find out where it was actually hanging
(I didn't feel like doing breakpoints).

I -thought- our kernel printf was supposed to be safe to use.  I'm not at
raised IPL and I don't believe I'm overflowing the stack anywhere, but
without fail, I get a perfectly reproducable double-panic which takes me
out to DDB.

Now, the question I'm asking is:

Could someone give me a leg-up on tracking the original problem that caused
the first panic to occur?  Obviously, the stack/frame pointer from the
double panic point to the panic code,  but the original stuff does (?) get
saved somewhere.

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?

Thanks,

a confused Paul