An anonymous reader writes "Sixteen years ago, the Mars Pathfinder lander touched down on Mars and began collecting about the atmosphere and geology of the Red Planet. Its original mission was planned to last somewhere between a week and a month, but it only took a few days for software problems to crop up. The engineers responsible for the system were forced to diagnose the problem and issue a patch for a device that was millions of miles away. From the article: 'The Pathfinder's applications were scheduled by the VxWorks RTOS. Since VxWorks provides pre-emptive priority scheduling of threads, tasks were executed as threads with priorities determined by their relative urgency. The meteorological data gathering task ran as an infrequent, low priority thread, and used the information bus synchronized with mutual exclusion locks (mutexes). Other higher priority threads took precedence when necessary, including a very high priority bus management task, which also accessed the bus with mutexes. Unfortunately in this case, a long-running communications task, having higher priority than the meteorological task, but lower than the bus management task, prevented it from running. Soon, a watchdog timer noticed that the bus management task had not been executed for some time, concluded that something had gone wrong, and ordered a total system reset.'"
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