Monday, January 9, 2012

Priority Inversion in Mars Probe Pathfinder in 1997

On 4 July 1997, after a seven-month voyage,  the spacecraft landed on Mars. A few days into the mission, the spacecraft began experiencing system resets, which was caused by Priority Inversion design issues.

To know more about Priority Inversion -
So we should never feel nervous when there are bugs reported for our software, even Mars project can have such critical bug as well!

There are a few solutions to the Priority-inversion Problem, all of which involve increasing the priority of tasks during their accesses to shared resources. The variation lies in when to increase priority and to what value. Assume that a task A gets a shared resource R. Except for PI, A’s priority is increased to the priority ceiling when A acquires R. The difference lies in the way the priority ceiling is computed for R.

  • For NPCS, the priority ceiling is equal to highest priority of all tasks in the system.
  • For CP and PC, the priority ceiling is equal to the highest priority of all tasks requiring R. The difference is in allowing or denying access to R.
  • For MB-CP, the priority ceiling is equal to the priority ceiling of the monitor, which contains the critical section of R.
  • Assume that a task A holds R. In PI, whenever a higher priority task B requests R, A inherits the priority of B and B is blocked.  

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