if (accessQueue.contains(e)) {
          accessQueue.add(e);
        }
      }
    }

    // expiration

    /** Cleanup expired entries when the lock is available. */
    void tryExpireEntries(long now) {
      if (tryLock()) {
        try {
          expireEntries(now);
        } finally {
          unlock();
          // don't call postWriteCleanup as we're in a read
        }