Hands-On RTOS with Microcontrollers

Mutexes minimize priority inversion

Earlier, we had said that, in FreeRTOS, mutexes were binary semaphores with one important additional feature. That important feature is priority inheritance – mutexes have the ability to temporarily change the priority of a task to avoid causing major delays in the system. This plays out when the scheduler finds that a high priority task is attempting to acquire a mutex already held by a lower priority task. In this specific case, the scheduler will temporarily increase the priority of the lower task until it releases the mutex. At this point, the priority of the lower task will be set back to what it was prior to the priority inheritance. Let's take a look at the exact same example from the preceding diagram ...