[poll / rfc] kdb_stop_cpus
avg at FreeBSD.org
Fri Jun 3 15:13:53 UTC 2011
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).
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