how to detect upside down face?
You could use the same library to detect upside down faces. If the library is unable to detect the face initially, transform it 180° and check again. If it is recognized in this condition, you know it was an upside down face.
I would like to detect upright and upside-down faces, however faces weren't recognized in upside-down images.
I used the dlib library in Python with shape_predictor_68_face_landmarks.dat. 
Is there a library that can recognize upright and upside-down faces?