Using boolean to synchronize

Question

Is the following code thread-safe on concurrent access to List?
Does the volatile qualifies add any value here?

   class concurrentList{

        private AtomicBoolean locked = new AtomicBoolean(true);
        volatile List<Integer> list=new LinkedList<Integer>();
        long start = System.currentTimeMillis();
        long end = start + 60*100;


        public void push(int e){
            while(!locked.get());
            list.add(e);
            while(!locked.compareAndSet(true,false));
        }

        public int pop(){
            int elem;
            while(locked.get());
            elem=(Integer)list.remove(0);
            while(!locked.compareAndSet(false,true));
            return elem;
        }
....
}

Answer 1

No, it's not thread-safe. Two threads calling push() can perfectly both read the locked as true, then add concurrently to the linked list. Since a LinkedList is not thread-safe, your code is not thread-safe.

To lock, use a lock, not an AtomicBoolean.