Mars Pathfinder Example

• In July 1997, Pathfinder’s computer reset itself several times during data collection and transmission from Mars. One of its processes failed to complete by a deadline, triggering the reset.
• Priority Inversion Problem.
  • A low priority process held a mutual exclusion semaphore on a shared data structure, but was preempted to let higher priority processes run.
  • The higher priority process which failed to complete in time was blocked on this semaphore.
  • Meanwhile a bunch of medium priority processes ran, until finally the deadline ran out. The low priority semaphore-holding process never got the chance to run again in that time to get to the point of releasing the semaphore
  • Priority inheritance had not been enabled on semaphore.

Dealing with Deadlock

• It can be prevented by breaking one of the prerequisite conditions (review):
  • Mutually exclusive use of resources Example: Allowing shared access to read-only files (readers/writers problem from readers point of view)
  • circular waiting Example: Define an ordering on resources and acquire them in order (lower numbered fork first)
  • hold and wait
  • no pre-emption
• Let it happen, then detect it and recover via externally-imposed preemption of resources
• Avoid dynamically by monitoring resource requests and denying some. Banker’s Alg ...

Deadlock Theory

• Bipartite graph model with a set P of vertices representing processes and a set R for resources.
• Directed edges
  • Ri -> Pj means Ri alloc to Pj
  • Pj -> Ri means Pj requests Ri
• Resource vertices contain units of the resource

State transitions by operations:

• Granting a request
• Making a new request if all outstanding requests satisfied

Deadlock defined on graph:

• Pi is blocked in state S if there is no operation Pi can perform
• Pi is deadlocked if it is blocked in all reachable states from S
• S is safe if no reachable state is a deadlock state (i.e., having some deadlocked process)

Cycle in graph is a necessary condition

• no cycle -> no deadlock.

No deadlock iff graph is completely reducible

• Intuition: Analyze graph, asking if deadlock is inevitable from this state by simulating most favorable state transitions.

Deadlock Detection Algorithm

while (there exist unblocked processes in U)
	{ Remove unblocked Pi from U; 
	   Cancel Pi’s outstanding requests;
	   Release Pi’s allocated resources; 
	    /* possibly unblocking other Pk in U */}
if ( U != l) signal deadlock;

Consumable Resources

• Not a fixed number of units, operations of producing and consuming (e.g. messages)
• Ordering matters on applying reductions
  • Reducing by producer makes “enough” units, w
  • Start with P2
  • Start with P1

Deadlock Detection & Recovery

• Continuous monitoring and running this algorithm are expensive.
• What to do when a deadlock is detected?
  • Abort deadlocked processes (will result in restarts).
  • Preempt resources from selected processes, rolling back the victims to a previous state (undoing effects of work that has been done)
  • Watch out for starvation.