src/share/classes/java/lang/ref/ReferenceQueue.java

*** 53,65 ****
      private Lock lock = new Lock();
      private volatile Reference<? extends T> head = null;
      private long queueLength = 0;
  
      boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
-         synchronized (r) {
-             if (r.queue == ENQUEUED) return false;
              synchronized (lock) {
                  r.queue = ENQUEUED;
                  r.next = (head == null) ? r : head;
                  head = r;
                  queueLength++;
                  if (r instanceof FinalReference) {
--- 53,70 ----
      private Lock lock = new Lock();
      private volatile Reference<? extends T> head = null;
      private long queueLength = 0;
  
      boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
          synchronized (lock) {
+             // Check that since getting the lock this reference hasn't already been
+             // enqueued (and even then removed)
+             ReferenceQueue queue = r.queue;
+             if ((queue == NULL) || (queue == ENQUEUED)) {
+                 return false;
+             }
+             assert queue == this;
              r.queue = ENQUEUED;
              r.next = (head == null) ? r : head;
              head = r;
              queueLength++;
              if (r instanceof FinalReference) {
*** 67,81 ****
                  }
                  lock.notifyAll();
                  return true;
              }
          }
-     }
  
      private Reference<? extends T> reallyPoll() {       /* Must hold lock */
-         if (head != null) {
              Reference<? extends T> r = head;
              head = (r.next == r) ? null : r.next;
              r.queue = NULL;
              r.next = r;
              queueLength--;
              if (r instanceof FinalReference) {
--- 72,85 ----
              }
              lock.notifyAll();
              return true;
          }
      }
  
      private Reference<? extends T> reallyPoll() {       /* Must hold lock */
          Reference<? extends T> r = head;
+         if (r != null) {
              head = (r.next == r) ? null : r.next;
              r.queue = NULL;
              r.next = r;
              queueLength--;
              if (r instanceof FinalReference) {