CircleBuffer code thread security

[Mxiaoyu]

public boolean writeToCharBuffer(char c) {
    boolean result = false;
    // if we can write to the buffer
    if (_readable_data.get() < _buffer_size) {
      // write to buffer
      _buffer[getTrueIndex(_write_index)] = c;
      _readable_data.incrementAndGet();
      _write_index++;
      result = true;
    }
    return result;
  }

thread 1 and thread 2 execute "_buffer[getTrueIndex(_write_index)] = c;" At the same time，the result is not definite.
How about this issue? Or I missed this code.

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