In computer science, priority inversion is a scenario in scheduling in which a high priority task is indirectly preempted by a lower priority task effectively inverting the relative priorities of the two tasks. This violates the priority model that high priority tasks can only be prevented from running by higher priority tasks and briefly by low priority tasks which will quickly complete their use of a resource shared by the high and low priority tasks.
Example of a priority inversion
Consider two tasks H and L, of high and low priority respectively, either of which can acquire exclusive use of a shared resourceR. 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 in a well-designed system typically involves L relinquishing R promptly so that H does not stay blocked for excessive periods of time. Despite good design, however, it is possible that a third task M of medium priority < p < p, where p represents the priority for task ) becomes runnable during L's use of R. At this point, M being higher in priority than L, preempts L, causing L to not be able to relinquish R promptly, in turn causing H—the highest priority process—to be unable to run.
Consequences
In some cases, priority inversion can occur without causing immediate harm—the delayedexecution 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. Low priority tasks usually have a low priority because it is not important for them to finish promptly. Similarly, a high priority task has a high priority because it is more likely to be subject to strict time constraints—it may be providing data to an interactive user, or acting subject to realtime response guarantees. Because priority inversion results in the execution of a lower priority task blocking the high priority task, it can lead to reduced system responsiveness, or even the violation of response time guarantees. A similar problem called deadline interchange can occur within earliest deadline first scheduling.
Solutions
The existence of this problem has been known since the 1970s. Lampson and Redell published one of the first papers to point out the priority inversion problem. Systems such as the UNIX kernel were already addressing the problem with the splx primitive. There is no foolproof method to predict the situation. There are however many existing solutions, of which the most common ones are: ;Disabling all interrupts to protect critical sections ;Priority ceiling protocol ;Priority inheritance ;Random boosting ;Avoid blocking
