code: static int index() {
      int probe = UnsafeAccess.UNSAFE.getInt(Thread.currentThread(), PROBE);
      if (probe == 0) {
        ThreadLocalRandom.current(); // force initialization
        probe = UnsafeAccess.UNSAFE.getInt(Thread.currentThread(), PROBE);
      }
      return (probe & ARENA_MASK);
    }
docs: Returns the arena index for the current thread.