*
 * <ul>
 *   <li>for an unnested {@code lock()} and {@code unlock()}, a cycle detecting lock takes 38ns as
 *       opposed to the 24ns taken by a plain lock.
 *   <li>for nested locking, the cost increases with the depth of the nesting:
 *       <ul>
 *         <li>2 levels: average of 64ns per lock()/unlock()
 *         <li>3 levels: average of 77ns per lock()/unlock()
 *         <li>4 levels: average of 99ns per lock()/unlock()

          expireEntries(now); // calls drainRecencyQueue
          readCount.set(0);
        } finally {
          unlock();
        }
      }
    }

    void runUnlockedCleanup() {
      // locked cleanup may generate notifications we can send unlocked
      if (!isHeldByCurrentThread()) {
        map.processPendingNotifications();
      }
    }
  }

          expireEntries(now); // calls drainRecencyQueue
          readCount.set(0);
        } finally {
          unlock();
        }
      }
    }

    void runUnlockedCleanup() {
      // locked cleanup may generate notifications we can send unlocked
      if (!isHeldByCurrentThread()) {
        map.processPendingNotifications();
      }
    }
  }

    /** Cleanup collected entries when the lock is available. */
    void tryDrainReferenceQueues() {
      if (tryLock()) {
        try {
          maybeDrainReferenceQueues();
        } finally {
          unlock();
        }
      }
    }

    @GuardedBy("this")
    void drainKeyReferenceQueue(ReferenceQueue<K> keyReferenceQueue) {
      Reference<? extends K> ref;
      int i = 0;

    /** Cleanup collected entries when the lock is available. */
    void tryDrainReferenceQueues() {
      if (tryLock()) {
        try {
          maybeDrainReferenceQueues();
        } finally {
          unlock();
        }
      }
    }

    @GuardedBy("this")
    void drainKeyReferenceQueue(ReferenceQueue<K> keyReferenceQueue) {
      Reference<? extends K> ref;
      int i = 0;