The Wikipedia page on Russo-Sino foreign relations notes that:
"The two countries share a land border which was demarcated in 1991, and they signed a Treaty of Good-Neighborliness and Friendly Cooperation in 2001. On the eve of a 2013 state visit to Moscow by Chinese leader Xi Jinping, Russian President Vladimir Putin remarked that the two nations were forging a special relationship. The two countries have enjoyed close relations militarily, economically, and politically, while supporting each other on various global issues."
However, researchers for the Middle East Institute write:
"Although China and Russia have strengthened their relationship, there remain obstacles for close cooperation. For example, Russian commentators have increasingly raised concerns about China’s ambitions and influence in Central Asia, an area historically within the Russian sphere of influence. Russian leaders have expressed growing concerns regarding China’s investments in the energy-rich but sparsely-populated Russian Far East. The Middle East is a new theater for potential friction between the two powers."
Given these two states' large militaries and nuclear arsenals, it is important to gain a clearer sense of the probability of conflict between them.