Re: Stop scheduler on panic

2011/11/21 John Baldwin <jhb_at_freebsd.org>:
> On Friday, November 18, 2011 4:59:32 pm Andriy Gapon wrote:
>> on 17/11/2011 23:38 John Baldwin said the following:
>> > On Thursday, November 17, 2011 4:35:07 pm John Baldwin wrote:
>> >> Hmmm, you could also make critical_exit() not perform deferred preemptions
>> >> if SCHEDULER_STOPPED?  That would fix the recursion and still let the
>> >> preemption "work" when resuming from the debugger?
>>
>> Yes, that's a good solution, I think.  I just didn't want to touch such a "low
>> level" code, but I guess there is no rational reason for that.
>>
>> > Or you could let the debugger run in a permament critical section (though
>> > perhaps that breaks too many other things like debugger routines that try
>> > to use locks like the 'kill' command (which is useful!)).  Effectively what you
>> > are trying to do is having the debugger run in a critical section until the
>> > debugger is exited.  It would be cleanest to let it run that way explicitly
>> > if possible since then you don't have to catch as many edge cases.
>>
>> I like this idea, but likely it would take some effort to get done.
>
> Yes, it would take some effort, so checking SCHEDULER_STOPPED in
> critical_exit() is probably good for the short term.  Would be nice to fix
> it properly some day.
>
>> Related to this is something that I attempted to discuss before.  I think that
>> because the debugger acts on a frozen system image (the debugger is a sole actor
>> and observer), the debugger should try to minimize its interaction with the
>> debugged system.  In this vein I think that the debugger should also bypass any
>> locks just like with SCHEDULER_STOPPED.  The debugger should also be careful to
>> note a state of any subsystems that it uses (e.g. the keyboard) and return them
>> to the initial state if it had to be changed.  But that's a bit different story.
>>  And I really get beyond my knowledge zone when I try to think about things like
>> handling 'call func_xxxx' in the debugger where func_xxxx may want to acquire
>> some locks or noticeably change some state within a system.
>
> I think to some extent, I think if a user calls a function from the debugger
> they better know what they are doing.  However, I think it can also be useful
> to have the debugger modify the system in some cases if it can safely do so
> (e.g. the 'kill' command from DDB can be very useful, and IIRC, it is careful
> to only use try locks and just fail if it can't acquire the needed locks).

I would be very in favor about having a 'thread trampoline for KDB',
thus that it can use locks.

The only downside is that it assume an healthy state of the switching
infrastructure, so maybe it would be fine to wrapper this under a
proper compile-time option (KDB_LITE -> it will run in interrupt
context and the users will better know what they do).

Attilio


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