TABLE OF CONTENTS Chapter 6 discusses semaphores and mutexes that can be used as resource synchronization primitives. Two other methods, interrupt locking and preemption locking, can also be deployed in accomplishing resource synchronization.
Interrupt locking (disabling system interrupts) is the method used to synchronize exclusive access to shared resources between tasks and ISRs. Some processor architecture designs allow for a fine-grained, interrupt-level lock, i.e., an interrupt lock level is specified so that asynchronous events at or below the level of the disabled interrupt are blocked for the duration of the lock. Other processor architecture designs allow only coarse-grained locking, i.e., all system interrupts are disabled.
When interrupts are disabled at certain levels, even the kernel scheduler cannot run because the system becomes non-responsive to those external events that can trigger task re-scheduling. This process guarantees that the current task continues to execute until it voluntarily relinquishes control. As such, interrupt locking can also be used to synchronize access to shared resources between tasks.
Interrupt locking is simple to implement and involves only a few instructions. However, frequent use of interrupt locks can alter overall system timing, with side effects including missed external events (resulting in data overflow) and clock drift (resulting in missed deadlines). Interrupt locks, although the most powerful and the most effective synchronization method, can introduce indeterminism into the system when used indiscriminately. Therefore, the duration of interrupt locks should be short, and interrupt locks should be used only when necessary to guard a...
