How did the Communist China and communist USSR prevail for so long?

Discuss the totalitarian nature of the communist governments in the USSR and China. In what ways did both governments guarantee that their power would remain unchallenged for much of the 1900s? By the 1980s both countries began to relax their control of the information available to the public. Why? How? How did the citizens of these countries react to the small degrees of freedom they had been given? How did Soviet citizens voice their concerns? Did it result in change? Why or why not? How did Chinese citizens voice their concerns? Did it result in change? Why or why not?