==================
Priority Inversion
==================

Priority Inversion

* Can occur in multi-tasking systems
* It is a condition where a lower priority task effectively blocks a higher priority task from accomplishing its job


Priority Inversion example
==========================

.. code-block:: c

    // Data acquisition thread
    ImportantTask() 
    { 
        SSetPriority(HIGHEST) 
        While(TRUE) 
        {
            //...
            Sleep(1 second)
            AcquireSemaphore(pDatabaseSem) 
            // ... work on database 
            ReleaseSemaphore(pDatabaseSem)
        }
    }
    
    // Database statistics thread
    MenialTask() 
    {
        SetPriority(LOWEST) 
        While(TRUE) 
        { 
            //...
            Sleep(5 minutes) 
            AcquireSemaphore(pDatabaseSem) 
            //... check database statistics 
            ReleaseSemaphore(pDatabaseSem)
        }
    }

* Can be difficult to detect
* Solutions - Don't share semaphores in this fashion by design
* Priority Inheritance - supplied by some OSs

  * OS bumps low-priority tasks priority to that of high priority task for 
    duration of inversion. (Sometimes only one-level deep, can be costly and non-deterministic)