Dead lock occur when assign multiple conditions to lock

Question

My program require to print backward from 10 to 1. Each number print from one thread. It uses one lock with multiple condition objects. But the program cause a dead lock when I run it. This is my program.

This class working fine

import java.util.concurrent.*;
import java.util.*;

public class Backward {
   public static void main(String[] args) {
      BackwardThread[] threads = new BackwardThread[10];
      MonitorArray monitorArray = new MonitorArray(10);
      for(int i = 0; i < 10; ++i) {
         threads[i] = new BackwardThread("thread" + i, i, monitorArray);
         threads[i].start();
      }
   }
}

This class is working fine

import java.util.concurrent.*;
import java.util.*;

public class BackwardThread extends Thread {
   private int id;
   private MonitorArray monitorArray;

   public BackwardThread(String name, int id, MonitorArray monitorArray) {
       super(name);
       this.id = id;
       this.monitorArray = monitorArray;
   }

  public void run() {
      monitorArray.waitTurn(id);
      System.out.println("hello world thread id = " + id);
      monitorArray.signalDone(id);
  }
}

It seem all thread is lock forever on and conditions[id].signal() doesn't work.

import java.util.concurrent.locks.*;
import java.util.concurrent.locks.Condition;

public class MonitorArray {

    private Lock lockvar;
    private Condition[] conditions;
    private int turn;

    public MonitorArray(int num) {
        this.lockvar = new ReentrantLock();
        this.conditions = new Condition[num];
        turn = num;
        for (int i = 0; i < num; ++i) {
            conditions[i] = lockvar.newCondition();
        }
        // TODO: need to initialize new variable here
    }
    public void waitTurn(int id) {
        lockvar.lock();
        while (id != turn) {
            try {
                conditions[id].await();
            } catch (Exception exception) {
                exception.printStackTrace();
            }
        }
        lockvar.unlock();
    }

    public void signalDone(int id) {
        lockvar.lock();
        // TODO: Need to modify new variable here to allow one of the threads
        // blocked on the while to continue
        turn--;
        if (id != 0) {
            conditions[id].signal();
        }

        lockvar.unlock();
    }
}

Are you getting errors or all your threads are in waiting ? I suppose since the id[0-9] and turn(10) are never matching, latter is happening. — happyHelper, Oct 07 '17 at 12:09
Thank you, happyHelper! All of my threads are in waiting. But I think I reduce "turn" in signalDone() so It should match the condition and release the threads one by one. — vtc, Oct 07 '17 at 12:20
Seemed like an interesting problem so I tried running it with minor tweaks, but similar thing is happening for me as well. The threads are not released. Instead of signal if I use `signalAll()`, it works though. The signal is waking up a thread that does not match the firstwaiter thread id I guess. — happyHelper, Oct 07 '17 at 13:25

score 0 · Answer 1 · answered Oct 08 '17 at 09:27

I have found the solution for this, I will give my explanation in the comment of my code

import java.util.concurrent.locks.*;
import java.util.concurrent.locks.Condition;

public class MonitorArray {

    private Lock lockvar;
    private Condition[] conditions;
    private int turn;

    public MonitorArray(int num) {
        this.lockvar = new ReentrantLock();
        this.conditions = new Condition[num];

        /*
        / the turn value should should be num-1(9) instead of num(10) 
        / because if turn = 10 so all threads will be in waiting section,
        / so there will be no thread to send the signal to wake up other threads
        */
        turn = num-1;
        for (int i = 0; i < num; ++i) {
            conditions[i] = lockvar.newCondition();
        }

    }
    public void waitTurn(int id) {
        lockvar.lock();
        while (id != turn) {
            try {
                conditions[id].await();
            } catch (Exception exception) {
                exception.printStackTrace();
            }
        }
        lockvar.unlock();
    }

    public void signalDone(int id) {
        lockvar.lock();
        // TODO: Need to modify new variable here to allow one of the threads
        // blocked on the while to continue
        turn--;
        if (id != 0) {

        /* 
        / this should be conditions[turn].signal(), not conditions[id] 
        / because a thread cannot wake up themself,
        / when we use condition[turn] we are in thread 9 and we sent signal to wake up thread 8, 
        / and furthermore, turn is reduced value and wake up the next smaller thread.
        */
            conditions[turn].signal();
        }

        lockvar.unlock();
    }
}

Dead lock occur when assign multiple conditions to lock

1 Answers1