[Programming Problem] The Dining Philosophers

[Problem Link]

  • Solution here is to add some `ordering` on the resources (i.e. forks) requested by the processes (i.e. the philosophers).
  • Identify the resource id’s (or fork id’s) that a philosopher is surround with
  • We make sure the process requests the `lower` id fork before the `higher` id fork to avoid deadlocks
  • Identify which fork (left or right) is the lower id fork and request in that order (i.e. first left then right first right then left)
  • Make sure mutex’s are held until forks are released
class DiningPhilosophers {
 
    private static ReentrantLock[] mutex = new ReentrantLock[] {
        new ReentrantLock(),
        new ReentrantLock(),
        new ReentrantLock(),
        new ReentrantLock(),
        new ReentrantLock(),
    };
 
    public DiningPhilosophers() {
 
    }
 
    // call the run() method of any runnable to execute its code
    public void wantsToEat(int philosopher,
                           Runnable pickLeftFork,
                           Runnable pickRightFork,
                           Runnable eat,
                           Runnable putLeftFork,
                           Runnable putRightFork) throws InterruptedException {
 
        int left = philosopher;
        int right = (philosopher-1) == -1 ? 4 : philosopher-1;
 
        if (Math.min(left, right) == left) {
 
            DiningPhilosophers.mutex[left].lock();
            DiningPhilosophers.mutex[right].lock();
            pickLeftFork.run();
            pickRightFork.run();
 
        } else {
 
            DiningPhilosophers.mutex[right].lock();
            DiningPhilosophers.mutex[left].lock();
            pickRightFork.run();
            pickLeftFork.run();
 
        }
 
 
        eat.run();
 
        putLeftFork.run();
        putRightFork.run();
        DiningPhilosophers.mutex[left].unlock();
        DiningPhilosophers.mutex[right].unlock();
    }
}

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193 thoughts on “[Programming Problem] The Dining Philosophers”

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