code: public void lockRootEdge(LockMode mode) {
      Preconditions.checkState(mEntries.isEmpty());
  
      mEntries.add(new EdgeEntry(mInodeLockManager.lockEdge(ROOT_EDGE, mode), ROOT_EDGE));
      mLockMode = mode;
    }
docs: Locks the root edge in the specified mode.
  
  @param mode the mode to lock in