Priority Inversion and Mutex Inheritance in FreeRTOS

Priority inversion is one of the sneakiest scheduling bugs in embedded systems: a high-priority task starts waiting on a low-priority one it should never have to wait for.

The scenario

Low-priority task L locks a resource. High-priority task H requests the same resource and blocks. Meanwhile medium-priority task M starts running and preempts L, so H effectively waits behind M.

The fix: priority inheritance

Mutexes created with xSemaphoreCreateMutex() in FreeRTOS support priority inheritance: task L holding the resource is temporarily raised to H's priority so it releases the resource quickly.

Binary semaphores (xSemaphoreCreateBinary) do not implement inheritance. Always use a mutex for mutual exclusion.

Practical advice

• Keep critical sections as short as possible.
• Never take a mutex from an interrupt context.
• Visualise timing issues with Tracealyzer / SystemView.