Fwd: stop scheduler in panic context

I would like to ask for testing of the following patch.
Since the patch affects panic(9) context, then obviously its testing requires
getting some sort of panic, and preferably some sort of "post-panic" activity too:
doing something in kdb, dumping a core (via debugger command or in unattended
mode), resetting a machine finally.

At minimum I hope that no regressions are introduced.
At maximum I hope that some things are improved like, e.g., crash dump succeeding
where it failed before (PR amd64/139614).

The patch is for recent head/CURRENT.  It mostly affects SMP systems, but also has
a smaller impact on UP systems.
I can try to adapt it to stable/8, if sufficient interest arises.
Please see the following message for the patch and some more details.

-------- Original Message --------

I would like to present the following diff for review and discussion:
http://people.freebsd.org/~avg/stop_scheduler_on_panic.diff

The idea is to stop scheduler in a panic context and to provide a special
environment for the only running thread, the one that called panic(9).

I tried to make this diff as minimal as possible, it doesn't include changes that
I consider to be useful improvements and [even] bug fixes, but which generated
controversy in non-public discussions.

If there is no negative feedback within next few days, then I plan to post the
patch to current_at_ to solicit some testing.  I will definitely wait for positive
feedback before committing this change.  I hope that I will be able to sneak it
into the 9 release (unless there are objections to this).

Thank you!
-- 
Andriy Gapon