How to debug a double fault? (Re: Could MSGBUF_SIZE be made a loader tunable?)

[Eugene Grosbein]

On 15.12.2010 16:37, perryh at pluto.rain.com wrote:
> Andriy Gapon <avg at freebsd.org> wrote:
>> on 06/12/2010 07:20 perryh at pluto.rain.com said the following:
>>> Would there be some fundamental problem in changing MSGBUF_SIZE
>>> from a compiled-in constant to a tunable that could be set at the
>>> loader prompt?
>>> I didn't see any obvious downside from examining the 8.1-RELEASE
>>> code ...
>> I also don't immediately see why that wouldn't work.
>> Can you try to come up with a patch?
> 
> I came up with what I think is a start, but it has led to:
> How do I go about debugging a "Fatal double fault"?

You also may checkup the code to not abuse stack.
Kernel threads have not much of free stack space and allocating
increasing length arrays in stack will lead to double fault.

Eugene Grosbein