http://catless.ncl.ac.uk/Risks/19.49.htmlWhat really happened on Mars Rover Pathfinderhttp://catless.ncl.ac.uk/Risks/19.49.htmlWhat really happened on Mars Rover PathfinderMike Jones <[email protected]>Sunday, December 07, 1997 6:47 PM The Mars Pathfinder mission was widely proclaimed as "flawless" in the early days after its July 4th, 1997 landing on the Martian surface. Successes included its unconventional "landing" -- bouncing onto the Martian surface surrounded by airbags, deploying the Sojourner rover, and gathering and transmitting voluminous data back to Earth, including the panoramic pictures that were such a hit on the Web. But a few days into the mission, not long after Pathfinder started gathering meteorological data, the spacecraft began experiencing total system resets, each resulting in losses of data. The press reported these failures in terms such as "software glitches" and "the computer was trying to do too many things at once".This week at the IEEE Real-Time Systems Symposium I heard a fascinating keynote address by David Wilner, Chief Technical Officer of Wind River Systems. Wind River makes VxWorks, the real-time embedded systems kernel that was used in the Mars Pathfinder mission. In his talk, he explained in detail the actual software problems that caused the total system resets of the Pathfinder spacecraft, how they were diagnosed, andhow they were solved. I wanted to share his story with each of you.VxWorks provides preemptive priority scheduling of threads. Tasks on the Pathfinder spacecraft were executed as threads with priorities that were assigned in the usual manner reflecting the relative urgency of these tasks.Pathfinder contained an "information bus", which you can think of as a shared memory area used for passing information between different components of the spacecraft. A bus management task ran frequently withhigh priority to move certain kinds of data in and out of the information bus. Access to the bus was synchronized with mutual exclusion locks (mutexes).The meteorological data gathering task ran as an infrequent, low priority thread, and used the information bus to publish its data. Whenpublishing its data, it would acquire a mutex, do writes to the bus, andrelease the mutex. If an interrupt caused the information bus thread tobe scheduled while this mutex was held, and if the information bus thread then attempted to acquire this same mutex in order to retrieve published data, this would cause it to block on the mutex, waiting untilthe meteorological thread released the mutex before it could continue. The spacecraft also contained a communications task that ran with mediumpriority.Most of the time this combination worked fine. However, very infrequently it was possible for an interrupt to occur that caused the (medium priority) communications task to be scheduled during the short interval while the(high priority) information bus thread was blocked waiting for the (low priority) meteorological data thread. In this case, the long-running communications task, having higher priority thanthe meteorological task, would prevent it from running, consequently preventing the blocked information bus task from running. After some time had passed, a watchdog timer would go off, notice that the data bustask had not been executed for some time, conclude that something had gone drastically wrong, and initiate a total system reset.This scenario is a classic case of priority inversion.HOW WAS THIS DEBUGGED?VxWorks can be run in a mode where it records a total trace of all interesting system events, including context switches, uses of synchronization objects, and interrupts. After the failure, JPL engineers spent hours and hours running the system on the exact spacecraft replica in their lab with tracing turned on, attempting to replicate the precise conditions under which they believed that the reset occurred. Early in the morning, after all but one engineer had gone home, the engineer finally reproduced a system reset on the replica. Analysis of the trace revealed the priority inversion.HOW WAS THE PROBLEM CORRECTED?When created, a VxWorks mutex object accepts a boolean parameter that indicates whether priority inheritance should be performed by the mutex.The mutex in question had been initialized with the parameter off; had it been on, the low-priority meteorological thread would have inherited the priority of the high-priority data bus thread blocked on it while itheld the mutex, causing it be scheduled with higher priority than the medium-priority communications task, thus preventing the priority inversion.Once diagnosed, it was clear to the JPL engineers that using priority inheritance would prevent the resets they were seeing.VxWorks contains a C language interpreter intended to allow developers to type in C expressions and functions to be executed on the fly during system debugging. The JPL engineers fortuitously decided to launch the spacecraft with this feature still enabled. By coding convention, the initialization parameter for the mutex in question (and those for two others which could have caused the same problem) were stored in global variables, whose addresses were in symbol tables also included in the launch software, and available to the C interpreter. A short C program was uploaded to the spacecraft, which when interpreted, changed the values of these variables from FALSE to TRUE. No more system resets occurred.ANALYSIS AND LESSONSFirst and foremost, diagnosing this problem as a black box would have been impossible. Only detailed traces of actual system behavior enabledthe faulty execution sequence to be captured and identified.Secondly, leaving the "debugging" facilities in the system saved the day. Without the ability to modify the system in the field, the problem could not have been corrected.Finally, the engineer's initial analysis that "the data bus task executes very frequently and is time-critical -- we shouldn't spend theextra time in it to perform priority inheritance" was exactly wrong. Itis precisely in such time critical and important situations where correctness is essential, even at some additional performance cost.HUMAN NATURE, DEADLINE PRESSURESDavid told us that the JPL engineers later confessed that one or two system resets had occurred in their months of pre-flight testing. They had never been reproducible or explainable, and so the engineers, in a very human-nature response of denial, decided
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