Stephen Lindholm wrote in a message to Rob Landley:

 SL> I'm curious about SMP:

 SL> Why the heck doesn't this smash all of the device drivers
 SL> which assume that they will have complete control of the
 SL> system without having something else run at the same time
 SL> except when they permit it (almost all of them?) to itty
 SL> bitty pieces?

 SL> Surely the other CPU's don't block while a device driver is
 SL> called! 

The DevHelp_AcquireSpinLock() call allows a driver to freeze all of the
other CPUs in the system in order to perform some sort of extremely
critical operation, using a handle created earlier with
DosCreateSpinlock().  The system requires that such "spin locks"
be released as quickly as possible after being acquired.

It is very rare that such a "spin lock" will be needed, since
device drivers are ordinarily written to be fully re-entrant anyway.  OS/2
makes sure that there are not multiple copies of a device driver running on
different CPUs at the same time, so the ordinary constraints of re-entrancy
usually suffice.
 
-- Mike


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