Doug Lea writes:
----------------------------------------------------------------
Contrary to expectations, a recursive writelock attempt
may block if there are other waiting threads. This is both a flaw
in the spec (which was not clear about this case) and code.

Test case:

import java.util.concurrent.locks.*;
import java.util.concurrent.*;

public class T720 {
     static ReentrantReadWriteLock rl = new ReentrantReadWriteLock(true);
     static Thread t1 = new Thread() {
             public void run() {
                 try {
                     if (!rl.readLock().tryLock(2000, 
TimeUnit.MILLISECONDS))
                         throw new Error("RRWL ordering error");
                     Thread.sleep(2000);
                     if (!rl.readLock().tryLock(2000, 
TimeUnit.MILLISECONDS))
                         throw new Error("RRWL ordering error");
                     rl.readLock().unlock();
                     rl.readLock().unlock();
                 } catch(InterruptedException ignore) {}

             }
         };


     static Thread t2 = new Thread() {
             public void run() {
                 try {
                     Thread.sleep(1000);
                     if (!rl.writeLock().tryLock(2000, 
TimeUnit.MILLISECONDS))
                         throw new Error("RRWL ordering error");
                     Thread.sleep(1000);
                     if (!rl.writeLock().tryLock(2000, 
TimeUnit.MILLISECONDS))
                         throw new Error("RRWL ordering error");
                 } catch(InterruptedException ignore) {}
             }
         };

     public static void main(String[] args) {
         t1.start();
         t2.start();
     }
}