IOWorkLoop

command gate event source:
ex:

this->testCommandGate->runAction(OSMemberFunctionCast(IOCommandGate::Action,

                                                          this, &com_peter_driver_MyDriver::myCommandGateAction),

this->getProvider(), NULL, NULL, NULL);

the command gate action, myCommandGateAction, is executed by thread that call runAction
if A call testCommandGate's runAction, and B call testCommandGate's runAction later,
B will wait until A finishes

subclass IOEventSource:
is executed by newly created thread
checkForWork will repeatedly be called until it return false
do the task in checkForWork
use enable() to trigger checkForWork to be called