Thread Preemption Disabling

The DKI API provides a means for a driver to disable/enable the preemption of the current thread. These services may be useful for a driver to prevent the current thread being preempted while interrupts are masked at bus/device level. Note that these services are implemented as macros.

DISABLE_PREEMPT()

disables preemption of the currently executed thread. Basically, this macro increments a per-processor preemption mask count. When the preemption mask count is not zero, the ChorusOS scheduler is locked, such as when there is a preemption request, the scheduler just raises a pending preemption flag deferring the real thread preemption until the preemption mask count drops to zero

ENABLE_PREEMPT()

enables preemption of the currently executed thread which has been previously disabled by DISABLE_PREEMPT(). Basically, this macro decrements the preemption mask count and, if it drops to zero, checks whether the current thread should be preempted because the pending preemption flag is raised.

Note that, as DISABLE_PREEMPT()/ENABLE_PREEMPT() rely on the preemption mask count, a driver may issue nested calls to these services.