| Certainly some chunks of kernel code do not allow for arbitrary
| pre-emption of processes.
|
| (The SVR4 kernel was changed to allow process pre-emption at any
| time as part of the SMP work).

SVR4 added "preemption points" strategically placed in the high
latency part of the kernel in order to have better response times
for the new "real-time" scheduling class.  However, this is not
real kernel preemption.

SunOS 5 added real kernel preemption (although old it only preempts
for interrupt or real-time threads), and I expect the mach-derived
operating systems also have real kernel preemption.

Eduardo