Interruptible Critical Sections

We present a new approach to synchronization on uniprocessors with special applicability to embedded and real-time systems. Existing methods for synchronization in real-time systems are pessimistic, and use blocking to enforce concurrency control. While protocols to bound the blocking of high priority tasks exist, high priority tasks can still be blocked by low priority tasks. In addition, these protocols require a complex interaction with the scheduler. We propose interruptible critical sections (i.e., optimistic synchronization) as an alternative to purely blocking methods. Practical optimistic synchronization requires techniques for writing interruptible critical sections, and system support for detecting critical section access connicts. We discuss our implementation of an interruptible lock on a system running the pSOS+ real time operating system. Our experimental performance results show that interruptible locks reduce the variance in the response time of the highest priority task with only a small impact on the performance of the low priority tasks. We show how interruptible critical sections can be combined with the Priority Ceiling Protocol, and present an analysis which shows that interruptible locks improve the schedulability of task sets that have high priority tasks with tight deadlines.