How did a low-priority task freeze a spacecraft on Mars?

A low-priority task froze a spacecraft on Mars because it held something an important task needed, while busier medium-priority tasks kept the low-priority one from finishing. This is called priority inversion, and it happened to Mars Pathfinder, which landed on 4 July 1997.

Priority inversion is a scenario in which a high-priority task is indirectly held up by a lower-priority one, so that the priorities are effectively turned upside down. Here is how. Call the important task H and a humble one L. If H needs a resource that L has already taken, H must wait until L lets go. Normally that is short. But a third task, M, of medium priority, can then preempt L, since M does not need the resource. L cannot finish and release it, so H, the most important task, cannot run.

On Mars the same thing happened. A much lower-priority weather-instrument task held a shared resource, and several medium-priority tasks preempted it. The high-priority data-handling task then blocked waiting for that resource and missed its deadline. The spacecraft is designed to react to a missed deadline by resetting its computer, and so it did. A reset lost no data already collected, but the rest of that day's activities waited until the next day.

The fix was one small setting, priority inheritance: a low-priority task holding a resource temporarily takes the priority of the highest-priority task waiting for it. Engineers sent a patch to Mars on 21 July.