Priority inversion

In computer science, priority inversion is a scenario in scheduling in which a high priority task is indirectly superseded by a lower priority task effectively inverting the assigned priorities of the tasks. This violates the priority model that high-priority tasks can only be prevented from running by higher-priority tasks. Inversion occurs when there is a resource contention with a low-priority task that is then preempted by a medium-priority task. Consider two tasks H and L, of high and low priority respectively, either of which can acquire exclusive use of a shared resource R. If H attempts to acquire R after L has acquired it, then H becomes blocked until L relinquishes the resource. Sharing an exclusive-use resource (R in this case) in a well-designed system typically involves L relinquishing R promptly so that H (a higher priority task) does not stay blocked for excessive periods of time. Despite good design, however, it is possible that a third task M of medium priority becomes runnable during L's use of R. At this point, M being higher in priority than L, preempts L (since M does not depend on R), causing L to not be able to relinquish R promptly, in turn causing H—the highest priority process—to be unable to run (that is, H suffers unexpected blockage indirectly caused by lower priority tasks like M). In some cases, priority inversion can occur without causing immediate harm—the delayed execution of the high-priority task goes unnoticed, and eventually, the low-priority task releases the shared resource. However, there are also many situations in which priority inversion can cause serious problems. If the high-priority task is left starved of the resources, it might lead to a system malfunction or the triggering of pre-defined corrective measures, such as a watchdog timer resetting the entire system. The trouble experienced by the Mars Pathfinder lander in 1997 is a classic example of problems caused by priority inversion in realtime systems. Priority inversion can also reduce the perceived performance of the system.