Revision 309657 to stack_machdep.c renders unbootable system

[Steven G. Kargl]

Well, after 3 days of bisection, I finally found the commit
that renders my system unbootable.  The system does not panic.
It simply gets stuck in some state.  Nonfunctional keyboard,
so can't break into debugger.  No serial console available.
The verbose dmesg.boot for a working kernel from revision
309656 is at

http://troutmask.apl.washington.edu/~kargl/freebsd/dmesg.309656.txt

The kernel config file is at

http://troutmask.apl.washington.edu/~kargl/freebsd/SPEW.txt

In looking at /usr/src/UPDATING, there is no warning that one
can create a boat anchor by upgrading to 309657.  If compiling
a kernel with 'options DDB' is no longer supported, this should
be stated in UPDATING.  Or, UPDATING should state that 'options
DDB' requires 'options STACK'.  Or, 'options DDB' should simply
to the right thing and pull in whatever 'option STACK' does. 

-- 
Steve
http://troutmask.apl.washington.edu/~kargl/
https://www.youtube.com/watch?v=6hwgPfCcpyQ