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Deadlock defintion

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Deadlock is a situation that can occur when two or more threads are blocked indefinitely trying to obtain access to a resource locked by the other threads in the deadlock. Thus, the deadlocked threads are blocked, waiting for each other to release a resource which neither of them can release because they are blocked waiting for the other thread to release its resource first.

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Such resources can be Java Locks, Java synchronized blocks (Java monitor objects), files, database tables and / or records etc.

Deadlock code example using two Lock instances in Java


public class DeadLockExample {

    public static void main(String[] args) {

        Lock lock1 = new ReentrantLock();
        Lock lock2 = new ReentrantLock();

        Thread thread1 = new Thread(new Runnable1(lock1, lock2));
        Thread thread2 = new Thread(new Runnable2(lock1, lock2));

        thread1.start();
        thread2.start();
    }
}


public class Runnable1 implements Runnable{

    private Lock lock1 = null;
    private Lock lock2 = null;

    public Runnable1(Lock lock1, Lock lock2) {
        this.lock1 = lock1;
        this.lock2 = lock2;
    }

    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName+ " attempt to lock Lock1");
        lock1.lock();
        System.out.println(threadName+ " lock Lock1");

        try {
sleep(3000);
        } catch (InterruptedException e) {
        }
        System.out.println(threadName+ " attempt to lock Lock2");
        lock2.lock();
        System.out.println(threadName+ " lock Lock2");

        //do the work - 두 lock 모두 lock함

        //unlock
        System.out.println(threadName+ " unlock Lock1");
        lock1.unlock();
        System.out.println(threadName+ " unlock Lock2");
        lock2.unlock();

    }
}


public class Runnable2 implements Runnable{

    private Lock lock1 = null;
    private Lock lock2 = null;

    public Runnable2(Lock lock1, Lock lock2) {
        this.lock1 = lock1;
        this.lock2 = lock2;
    }

    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName+ " attempt to lock Lock2");
        lock2.lock();
        System.out.println(threadName+ " lock Lock2");


        try {
sleep(3000);
        } catch (InterruptedException e) {
        }
        System.out.println(threadName+ " attempt to lock Lock1");
        lock1.lock();
        System.out.println(threadName+ " lock Lock1");

        //unlock
        System.out.println(threadName+ " unlock Lock1");
        lock1.unlock();
        System.out.println(threadName+ " unlock Lock2");
        lock2.unlock();
    }
}

실행결과 - unlock 안함 (그림의 상황)


Thread-1 attempt to lock Lock2
Thread-0 attempt to lock Lock1
Thread-0 lock Lock1
Thread-1 lock Lock2
Thread-0 attempt to lock Lock2
Thread-1 attempt to lock Lock1

Deadlock code example using two synchronized blocks in Java


public class DeadLockSyncExample {

    public static void main(String[] args) {

        Lock lock1 = new ReentrantLock();
        Lock lock2 = new ReentrantLock();

        Thread thread1 = new Thread(new RunnableSync1(lock1, lock2));
        Thread thread2 = new Thread(new RunnableSync2(lock1, lock2));

        thread1.start();
        thread2.start();
    }
}


public class RunnableSync1 implements Runnable{

    private Object lock1 = null;
    private Object lock2 = null;

    public RunnableSync1(Lock lock1, Lock lock2) {
        this.lock1 = lock1;
        this.lock2 = lock2;
    }

    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName+ " attempt to lock Lock1");
        synchronized (lock1) {
            System.out.println(threadName + " lock Lock1");

            try {
sleep(3000);
            } catch (InterruptedException e) {
            }
            System.out.println(threadName + " attempt to lock Lock2");
            synchronized (lock2){
                System.out.println(threadName + " lock Lock2");

                //do the work - 두 lock 모두 lock함

                System.out.println(threadName + " unlock Lock2");
            }
            System.out.println(threadName + " unlock Lock1");
        }
    }
}


public class RunnableSync2 implements Runnable{

    private Object lock1 = null;
    private Object lock2 = null;

    public RunnableSync2(Lock lock1, Lock lock2) {
        this.lock1 = lock1;
        this.lock2 = lock2;
    }

    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName+ " attempt to lock Lock2");
        synchronized (lock2) {
            System.out.println(threadName + " lock Lock2");

            try {
sleep(3000);
            } catch (InterruptedException e) {
            }
            System.out.println(threadName + " attempt to lock Lock1");
            synchronized (lock1){
                System.out.println(threadName + " lock Lock1");

                //do the work - 두 lock 모두 lock함

                System.out.println(threadName + " unlock Lock1");
            }
            System.out.println(threadName + " unlock Lock2");
        }
    }
}

여기도 데드락 걸림

Deadlock can occur with any mutually exclusive access / lockable resource

데드락은 여러 쓰레드가 접근해서 락을 거는 어떠한 리소스에도 터질 수 있다.

Deadlock fix via lock ordering

lock 접근 순서를 둘다 lock1, lock2로 함 → 데드락 안걸림

More complex deadlock scenario

e.g. DB 테이블 락, 파일 락 등등

A deadlock will block all threads trying to lock a resource involved in the deadlock

데드락에 접근하는 새로운 쓰레드들은 block된다.

The four conditions required for a deadlock to occur

Mutual Exclusion

No Preemption

Hold and Wait

Circular Wait

Deadlock in Java