void G1CollectedHeap::wait_while_free_regions_coming() {

  {- -------------------------------------------
  (1) (参考:
       G1CollectedHeap::free_regions_coming() は
       ConcurrentMarkThread の Cleanup 処理で free region が発見されると true になり, 
       ConcurrentMarkThread がそれらの free region を処理し終わると false に戻る.
       See: ConcurrentMark::cleanup(), ConcurrentMarkThread::run())
      ---------------------------------------- -}

  {- -------------------------------------------
  (1) もし未処理の free region がなければ, 何もすることはないので, ここでリターン.
      ---------------------------------------- -}

      // Most of the time we won't have to wait, so let's do a quick test
      // first before we take the lock.
      if (!free_regions_coming()) {
        return;
      }

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      if (G1ConcRegionFreeingVerbose) {
        gclog_or_tty->print_cr("G1ConcRegionFreeing [other] : "
                               "waiting for free regions");
      }

  {- -------------------------------------------
  (1) G1CollectedHeap::free_regions_coming() が false になるまで, ここで待機.
      (待機処理は SecondaryFreeList_lock に対して Monitor::wait() を呼ぶことで行う)
      ---------------------------------------- -}

      {
        MutexLockerEx x(SecondaryFreeList_lock, Mutex::_no_safepoint_check_flag);
        while (free_regions_coming()) {
          SecondaryFreeList_lock->wait(Mutex::_no_safepoint_check_flag);
        }
      }

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      if (G1ConcRegionFreeingVerbose) {
        gclog_or_tty->print_cr("G1ConcRegionFreeing [other] : "
                               "done waiting for free regions");
      }
    }