Problem with WriteLock

[Erik Hetzner]

Hi,

I am attempting to use the WriteLock recipe provided with ZK 3.3.2.
Unfortunately, I seem to be deadlocking when I try to lock from
multiple threads. If I lock sequentially, I have no trouble. Can
somebody let me know what I am doing wrong here? I am attaching some
sample code; I can also provide a complete maven project to test if
necessary. (For the record, I have increased maxClientCnxns in my
zoo.cfg to 100.)

Here we see the results without threading:

  $ time java -cp "lib/*" Test nothread
  […]
  finished 9
  
  real  0m1.361s
  user  0m0.380s
  sys   0m0.050s

And here is the threaded version:

  $ time java -cp "lib/*" Test thread
  finished 9
  finished 8
  finished 1
  ^C
  real  3m4.627s
  user  0m4.600s
  sys   0m2.060s

On the other hand, it does not always deadlock:

  $ time java -cp "lib/*" Test thread
  finished 4
  finished 0
  finished 3
  finished 5
  finished 6
  finished 7
  finished 2
  finished 1
  finished 8
  finished 9
  
  real  0m7.824s
  user  0m1.030s
  sys   0m0.170s

Thanks for any help you can provide!

best, Erik Hetzner


import java.util.*;
import org.apache.zookeeper.*;
import java.lang.Math;
import org.apache.zookeeper.recipes.lock.*;
import java.io.IOException;

public class Test {
    public static class Ignorer implements Watcher {
        public void process(WatchedEvent event){}
    }

    public static void lockTest (int i) {
        try {
            ZooKeeper zk = new ZooKeeper("localhost:2181", 10000, 
                           new Ignorer());

            WriteLock lock = new WriteLock(zk, "/testLocker/mytest", null);
            lock.lock();
            while (!lock.isOwner()) {
                Thread.sleep(Math.round(Math.random() * 10));
            }
            lock.unlock();
            System.err.println(String.format("finished %d", i));
        } catch (Exception ex) {
            ex.printStackTrace(System.err);
        }
    }

    public static class MyThread extends Thread {
        int i;

        public MyThread (int i) {
            this.i = i;
        }

        public void run () {
            lockTest(this.i);
        }
    }

    public static void main (String[] args) throws InterruptedException {
        if (args.length > 0 && args[0].equals("thread")) {
            List<Thread> threads = new ArrayList<Thread>();
            for (int i = 0 ; i < 10 ; i++) {
                Thread thread = new MyThread(i);
                threads.add(thread);
            }
            for (Thread t : threads) {
                t.start();
            }
            for (Thread t : threads) {
                t.join();
            }
        } else {
            for (int i = 0 ; i < 10 ; i++) {
                lockTest(i);
            }
        }
    }
}
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