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Date: Fri, 03 Jun 2011 18:13:50 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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Subject: [poll / rfc] kdb_stop_cpus
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I wonder if anybody uses kdb_stop_cpus with non-default value.
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