Deadlock Review Questions

Topics

Coffman Conditions Resource Allocation Graphs Dining Philosophers

• Failed DP Solutions
• Livelocking DP Solutions
• Working DP Solutions: Benefits/Drawbacks

Questions

• What are the Coffman Conditions?
• What do each of the Coffman conditions mean? (e.g. can you provide a definition of each one)
• Give a real life example of breaking each Coffman condition in turn. A situation to consider: Painters, Paint, Paintbrushes etc. How would you assure that work would get done?
• Be able to identify when Dining Philosophers code causes a deadlock (or not). For example, if you saw the following code snippet which Coffman condition is not satisfied?

// Get both locks or none.
pthread_mutex_lock( a );
if( pthread_mutex_trylock( b ) ) { /*failed*/
   pthread_mutex_unlock( a );
   ...
}

• If one thread calls

  pthread_mutex_lock(m1) // success
  pthread_mutex_lock(m2) // blocks

and another threads calls

  pthread_mutex_lock(m2) // success
  pthread_mutex_lock(m1) // blocks

What happens and why? What happens if a third thread calls pthread_mutex_lock(m1) ?

• How many processes are blocked? As usual assume that a process is able to complete if it is able to acquire all of the resources listed below.
  • P1 acquires R1
  • P2 acquires R2
  • P1 acquires R3
  • P2 waits for R3
  • P3 acquires R5
  • P1 acquires R4
  • P3 waits for R1
  • P4 waits for R5
  • P5 waits for R1

(Draw out the resource graph!)