callout_cpu_switch() allows preemption when dropping the outcoming callout cpu lock (and after having dropped it). If the newly scheduled thread wants to acquire the old queue it will just spin forever.
callout_cpu_switch() allows preemption when dropping the outcoming
callout cpu lock (and after having dropped it).
If the newly scheduled thread wants to acquire the old queue it will
just spin forever.

Fix this by disabling preemption and interrupts entirely (because fast
interrupt handlers may incur in the same problem too) while switching
locks.

Reported by:	hrs, Mike Tancsa <mike AT sentex DOT net>,
		Chip Camden <sterling AT camdensoftware DOT com>
Tested by:	hrs, Mike Tancsa <mike AT sentex DOT net>,
		Chip Camden <sterling AT camdensoftware DOT com>,
		Nicholas Esborn <nick AT desert DOT net>
Approved by:	re (kib)
MFC after:	10 days

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