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Re: can't enter KDB after panic?

To: Ken Preslan <kpreslan@xxxxxxxxxxx>
Subject: Re: can't enter KDB after panic?
From: Keith Owens <kaos@xxxxxxxxxxxxxxxxx>
Date: Thu, 22 Feb 2001 15:13:28 +1100
Cc: kdb@xxxxxxxxxxx
In-reply-to: Your message of "Wed, 21 Feb 2001 20:37:37 MDT." <20010221203737.A12811@xxxxxxxxxxx>
Sender: owner-kdb@xxxxxxxxxxx
On Wed, 21 Feb 2001 20:37:37 -0600, 
Ken Preslan <kpreslan@xxxxxxxxxxx> wrote:
>I'm using kdb-v1.7-2.4.0.gz off the FTP site on a 2.4.1 kernel.  I have
>a serial console for my machine setup (with mingetty on the tty device to
>keep it open).  When the machine is running, I can use ctrl-A to break into
>KDB.  For some reason, ctrl-A doesn't work after machine panics (right when
>I need it :-).

If the machine is spinning with interrupts disabled then the IRQ from
the serial line is not being processed so ctrl-A does not get in.  This
is a standard problem with the serial line and keyboard, you need an
initial interrupt to get the kernel's attention.  Once kdb is running
it uses polled I/O but the first input needs a working IRQ.

The solution is to use the NMI watchdog which always interrupts, as
long as the hardware is not totally frozen.  SMP boxes automatically
have the NMI watchdog.  The kdb patch includes NMI for UP boxes, under
the processor type menu.

Mind you, a panic should drop into kdb after printing the registers.
Did you select "kdb off by default" in the config or boot with
"kdb=off"?


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