[poll / rfc] kdb_stop_cpus

Andriy Gapon avg at FreeBSD.org
Wed Jun 22 15:51:14 UTC 2011

on 03/06/2011 18:13 Andriy Gapon said the following:
> I wonder if anybody uses kdb_stop_cpus with non-default value.

I would like to go ahead and remove kdb_stop_cpus tunable/sysctl if nobody objects.

> If, yes, I am very interested to learn about your usecase for it.
> I think that the default kdb behavior is the correct one, so it doesn't make sense
> to have a knob to turn on incorrect behavior.
> But I may be missing something obvious.
> The comment in the code doesn't really satisfy me:
> /*
>  * Flag indicating whether or not to IPI the other CPUs to stop them on
>  * entering the debugger.  Sometimes, this will result in a deadlock as
>  * stop_cpus() waits for the other cpus to stop, so we allow it to be
>  * disabled.  In order to maximize the chances of success, use a hard
>  * stop for that.
>  */
> The hard stop should be sufficiently mighty.
> Yes, I am aware of supposedly extremely rare situations where a deadlock could
> happen even when using hard stop.  But I'd rather fix that than have this switch.
> Oh, the commit message (from 2004) explains it:
>> Add a new sysctl, debug.kdb.stop_cpus, which controls whether or not we
>> attempt to IPI other cpus when entering the debugger in order to stop
>> them while in the debugger.  The default remains to issue the stop;
>> however, that can result in a hang if another cpu has interrupts disabled
>> and is spinning, since the IPI won't be received and the KDB will wait
>> indefinitely.  We probably need to add a timeout, but this is a useful
>> stopgap in the mean time.
> But that was before we started using hard stop in this context (in 2009).

Andriy Gapon

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