Re: Stop scheduler on panic

on 01/12/2011 20:49 John Baldwin said the following:
> On Thursday, December 01, 2011 11:59:10 am Andriy Gapon wrote:
>>
>> [cc list trimmed]
>>
>> on 21/11/2011 18:32 John Baldwin said the following:
>>> 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?
>>
>>
>> Just to clarify, probably you are actually suggesting to not perform deferred
>> preemptions if kdb_active == TRUE.  Because that's where we get the recursion (via
>> kdb_switch).
>>
>> I think that if we get into the mi_switch in a state where !kdb_active &&
>> SCHEDULER_STOPPED(), then we probably should just - I don't know - panic again?
>>
>> [the following is preserved for context]
> 
> Hmmm.  I'd be tempted to just ignore pending preemptions anytime
> SCHEDULER_STOPPED() is true.  If it's stopped for a reason other than being
> in the debugger (e.g. panic), I'd rather make a best effort at getting a dump
> than panic again.

Yep, me too.  It's just that I assumed that ending up at mi_switch in the panic
thread/context meant that something had gone very wrong already.  But I am not
sure if this was a valid assumption.

Returning to critical_exit, what do you think about the following patch?
I guess that it could be committed independently of / before the
SCHEDULER_STOPPED thing.

commit ee3d1a04985e86911a68d854439ae8c5429b7bd5
Author: Andriy Gapon <avg_at_icyb.net.ua>
Date:   Thu Dec 1 18:53:36 2011 +0200

    critical_exit: ignore td_owepreempt if kdb_active

    calling mi_switch in such a context result in a recursion via
    kdb_switch

diff --git a/sys/kern/kern_switch.c b/sys/kern/kern_switch.c
index 93cbf7b..885dc22 100644
--- a/sys/kern/kern_switch.c
+++ b/sys/kern/kern_switch.c
_at__at_ -200,7 +200,7 _at__at_ critical_exit(void)

 	if (td->td_critnest == 1) {
 		td->td_critnest = 0;
-		if (td->td_owepreempt) {
+		if (td->td_owepreempt && !kdb_active) {
 			td->td_critnest = 1;
 			thread_lock(td);
 			td->td_critnest--;


Would it make sense wrap kdb_active check with __predict_false?

-- 
Andriy Gapon