Make two operations atomic

Question

I'm implementing a custom datastructure that should follow these requirements:

1. Elements should be searchable (ie, locate and retrieve an element according to .equals() )
2. When I try to retrieve an element and it is not found the method should block until such an element is available. If never, the method blocks forever or for a given timeout.

I have implemented my own class - I started modifying LinkedBlockingQueue first - but I have a point where I can have a bad interleaving and I can't figure out how to avoid one.

To test the datastructure I created a method with 3 threads. Each of these threads will try and receive a unique string (eg, "Thread1" for the first thread etc..). If the string is found in the datastore, it will insert a string for the next thread (eg, Thread 1 will insert "Thread2").

This way, a thread will block and only send a message if it's message is in the store. To start things off, after I started my threads, I manually add "Thread1" to the store. This should trigger Thread 1 to get his value out of the store and insert a value for thread 2. Thread 2 should then be notified and get his value and insert a value for thread 3 etc..

However, I notice that the cycle stops after a random times of passing the message around. This is - I think - due to a bad interleaving that is possible in the takeOrWait() method (as indicated there). I have tried several solutions but I can't figure out a way.

The problem is - I think - that I have to release the lock to modify, followed by a call to sync.wait() . In between those calls a thread can already insert an element in the queue which would cause all the waiting threads to miss the notification.

Is it possible to initiate the wait() before I release the lock?

For completness' sake I have added the Main.java class I use to test.

BlockingDataStore

public class BlockingStore<T> {

    // Linked list node.
    static class Node<E> {
        /**
         * The item, volatile to ensure barrier separating write and read
         */
        volatile E item;
        Node<E> next;

        Node(E x) {
            item = x;
        }
    }


    private Node<T> _head;
    private Node<T> _lastPtr;
    private int _size;

    // Locks
    private Lock changeLock = new ReentrantLock();
    private final Object sync = new Object();

    //////////////////////////////////
    //  CONSTRUCTOR                 //
    //////////////////////////////////
    public BlockingStore() {
        _head = null;
        _lastPtr = null;
    }

    //////////////////////////////////
    //  INTERNAL MODIFICATION       //
    //////////////////////////////////

    /**
     * Locates an element in the storage and removes it.
     * Returns null if the element is not found in the list.
     *
     * @param toRemove Element to remove.
     * @return Returns the removed element.
     */
    private T findAndRemove(T toRemove) {
        T result = null;

        // Empty queue.
        if (_head == null)
            return result;

        // Do we want the head?
        if (_head.item.equals(toRemove)) {
            result = _head.item;
            _head = _head.next;
            this._size--;
            return result;
        }

        Node<T> previous = _head;
        Node<T> current = previous.next;

        while (current != null) {
            if (current.item.equals(toRemove)) {
                // Take the element out of the list.
                result = current.item;

                // we have to update the last pointer.
                if (current == _lastPtr)
                    _lastPtr = previous.next;
                else
                    previous.next = current.next;
                this._size--;
                return result;
            }
            previous = current;
            current = current.next;
        }
        return result;
    }

    /**
     * Adds an element to the end of the list.
     *
     * @param toAdd Element to add to the end of the list.
     */
    private void addToEnd(T toAdd) {
        // If the queue is empty
        if (_head == null) {
            _head = new Node<T>(toAdd);
            _lastPtr = _head;
        } else {
            _lastPtr.next = new Node<T>(toAdd);
            _lastPtr = _lastPtr.next;
        }
        this._size++;
    }

    /**
     * Takes an element from the front of the list.
     * Returns null if list is empty.
     *
     * @return Element taken from the front of the list.
     */
    private T takeFromFront() {
        // Check for empty queue.
        if (_head == null)
            return null;

        T result = _head.item;
        _head = _head.next;
        this._size--;
        return result;
    }

    //////////////////////////////////
    //  API METHODS                 //
    //////////////////////////////////

    /**
     * Takes an element from the datastore,
     * if it is not found the method blocks
     * and retries every time a new object
     * is inserted into the store.
     *
     * @param toTake
     * @return
     */
    public T takeOrWait(T toTake) {
        T value;
        changeLock.lock();
        value = findAndRemove(toTake);

        // Wait until somebody adds to the store
        // and then try again.
        while (value == null)
            // Sync on the notification object
            // such that we are waken up when there
            // is a new element.
            synchronized (sync) {
                changeLock.unlock(); // allow writes.
                // I think I can have bad inter-leavings here.
                // If an insert is interleaved here, the thread
                // will never be notified..
                try {
                    sync.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                changeLock.lock();
                value = findAndRemove(toTake);
            }

        changeLock.unlock();
        return value;
    }

    public T dequeue() {
        T value;
        changeLock.lock();
        value = takeFromFront();
        changeLock.unlock();
        return value;
    }

    public void enqueue(T toPut) {
        changeLock.lock();
        addToEnd(toPut);

        // Notify about new element in queue.
        synchronized (sync) {
            sync.notifyAll();
            changeLock.unlock();
        }

    }

    public int size() {
        return _size;
    }
}

** Main.java **

public class Main {
    public static void main(String[] args) throws InterruptedException {
        final BlockingStore<String> q = new BlockingStore<String>();

        new Thread(new Runnable() {
            public String name = "Thread 1: ";
            public String to   = "Thread 2: ";

            public void print(String message) {
                System.out.println(name + message);
            }

            @Override
            public void run() {
                while (true) {
                    String value = q.takeOrWait(name);
                    print("Got: " + value);
                    q.enqueue(to);
                }
            }
        }).start();

        new Thread(new Runnable() {
            public String name = "Thread 2: ";
            public String to   = "Thread 3: ";

            public void print(String message) {
                System.out.println(name + message);
            }

            @Override
            public void run() {
                while (true) {
                    String value = q.takeOrWait(name);
                    print("Got: " + value);
                    q.enqueue(to);
                }
            }
        }).start();

        new Thread(new Runnable() {
            public String name = "Thread 3: ";
            public String to   = "Thread 1: ";

            public void print(String message) {
                System.out.println(name + message);
            }

            @Override
            public void run() {
                while (true) {
                    String value = q.takeOrWait(name);
                    print("Got: " + value);
                    q.enqueue(to);
                }
            }
        }).start();

        Thread.sleep(1000);
        System.out.println("Main: Sending new message to queue for thread 1");
        q.enqueue("Thread 1: ");

    }

}

Answer 1

The problem is - I think - that I have to release the lock to modify, followed by a call to sync.wait()

Sounds like lost notification . Understand that sync.notify() does nothing at all if there is not some other thread already blocked in sync.wait() . The sync object does not remember that it was notified.

This doesn't work (based on your example):

public void waitForFlag() {
    ...
    while (! flag) {
        synchronized (sync) {
            try {
              sync.wait();
            } catch (InterruptedException e) { ... }
        }
    }
}

public void setFlag() {
    flag = true;
    synchronized (sync) {
        sync.notifyAll();
    }
}

Suppose thread A calls waitForFlag(), finds flag to be false, and then is preempted. Then, thread B calls setFlag(), notifying no-one. Finally, thread A calls sync.wait().

Now you have the flag set, and thread A blocked in a wait() call, waiting for somebody to set the flag. That's what a lost notification looks like.

Here's how it should look:

public void waitForFlag() {
    ...
    synchronized(sync) {
        while (! flag) {
            try {
              sync.wait();
            } catch (InterruptedException e) { ... }
        }
    }
}


public void setFlag() {
    synchronized (sync) {
        flag = true;
        sync.notifyAll();
    }
}

This way, the notification can not be lost because the statement that sets the flag and the statement that tests the flag are both inside synchronized blocks.