>Just looking at your final example, I can't understand how you can say that is unsafe

Let me change the example a bit. Say we have two locks aL and bL, that we must always acquire in the order aL first and then bL.

Following the rule, say we write code like this:

  import java.util.concurrent.locks.ReentrantLock;
  class X {
    private static ReentrantLock aL = new ReentrantLock();
    private static ReentrantLock bL = new ReentrantLock();
    static int x = 0;
    static int c = 0;
    static public void main(String[] args) {
	for(aL.lock(); c < 100; c++) {
	  synchronized(bL) {
		x = x + 0x42;
	  }
	}
	aL.unlock();
    }
  }

If I understood it right, the blog post was asking a question whether JVM can transform this to:

  import java.util.concurrent.locks.ReentrantLock;
  class X {
    private static ReentrantLock aL = new ReentrantLock();
    private static ReentrantLock bL = new ReentrantLock();
    static int x = 0;
    static int c = 0;
    static public void main(String[] args) {
      synchronized(bL) {
        for(aL.lock(); c < 100; c++) {
            x = x + 0x42;
        }
        aL.unlock();
      } // end synnchronized
    }
  }

Since the locks are now acquired in a different order, does that not qualify as observable behavior?