Priority Inheritance on Condition Variables

In this paper, a mechanism is presented to deal with priority inversion in real-time systems when multiple threads of execution synchronize with each other by means of mutual exclusion semaphores coupled with the programming abstraction of condition variables. Traditional priority inheritance solutions focus on addressing priority or deadline inversion as due to the attempt to lock mutual exclusion semaphores, or deal exclusively with specific interaction patterns such as client-server ones. The mechanism proposed in this paper allows the programmer to explicitly declare to the run-time environment what tasks are able to perform a notify operation on a condition over which other tasks may be suspended through a wait operation. This enables developers of custom interaction models for real-time tasks to exploit their knowledge of the application-specific interaction so as to potentially reduce priority inversion. The paper discusses issues in the realization of the technique, and its integration with existing priority inheritance mechanisms on current operating systems. Also, the paper briefly presents the prototyping of the technique within the open-source RTSim real-time systems simulator, which is used to highlight the potential advantages of the exposed technique through a simple simulated scenario.