Книга: Real-Time Concepts for Embedded Systems

15.4.2 Preemption Locks

15.4.2 Preemption Locks

Preemption locking (disabling the kernel scheduler) is another method used in resource synchronization. Many RTOS kernels support priority-based, preemptive task scheduling. A task disables the kernel preemption when it enters its critical section and re-enables the preemption when finished. The executing task cannot be preempted while the preemption lock is in effect.

On the surface, preemption locking appears to be more acceptable than interrupt locking. Closer examination reveals that preemption locking introduces the possibility for priority inversion. Even though interrupts are enabled while preemption locking is in effect, actual servicing of the event is usually delayed to a dedicated task outside the context of the ISR. The ISR must notify that task that such an event has occurred.

This dedicated task usually executes at a high priority. This higher priority task, however, cannot run while another task is inside a critical region that a preemption lock is guarding. In this case, the result is not much different from using an interrupt lock. The priority inversion, however, is bounded. Chapter 16 discusses priority inversion in detail.

The problem with preemption locking is that higher priority tasks cannot execute, even when they are totally unrelated to the critical section that the preemption lock is guarding. This process can introduce indeterminism in a similar manner to that caused by the interrupt lock. This indeterminism is unacceptable to many systems requiring consistent real-time response.

For example, consider two medium-priority tasks that share a critical section and that use preemption locking as the synchronization primitive. An unrelated print server daemon task runs at a much higher priority; however, the printer daemon cannot send a command to the printer to eject one page and feed the next while either of the medium tasks is inside the critical section. This issue results in garbled output or output mixed from multiple print jobs.

The benefit of preemption locking is that it allows the accumulation of asynchronous events instead of deleting them. The I/O device is maintained in a consistent state because its ISR can execute. Unlike interrupt locking, preemption locking can be expensive, depending on its implementation.

In the majority of RTOSes when a task makes a blocking call while preemption is disabled, another task is scheduled to run, and the scheduler disables preemption after the original task is ready to resume execution.

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