one of the following:
- Holding kernel_lock (indicating that the code is not MT safe).
- Bracketing critical sections with kpreempt_disable/kpreempt_enable.
- Holding the interrupt priority level above IPL_NONE.
Statistics on kernel preemption are reported via event counters, and
where preemption is deferred for some reason, it's also reported via
lockstat. The LWP priority at which preemption is triggered is tuneable
via sysctl.
there are no waiters. This gives a major boost to build.sh on larger
systems as directory vnode locks are exclusive for lookup, but are often
only held for a very short period of time.
This change has the potential to more readily expose lock order reversals
and other types of deadlock.
