Every year, tens of millions of China's 246 million migrants return home to celebrate the Chinese New Year. It's the world's biggest annual migration, and it typically goes off smoothly. This year, however, something's amiss.

Although the holiday doesn't start until Feb. 8, millions of workers — especially in the construction and electrical-appliance industries — have already returned home due to the country's slowing economy. For local governments across China, this is raising a tough question: What happens if these laborers don't go back to work after the holiday?

The concern isn't a new one. In early 2009, 20 million unemployed migrants returned home for the holidays in the wake of the global financial crisis, raising fears of social unrest. Labor riots did, in fact, take place. But most of the unemployed appear to have gotten back to work when China's monster stimulus kicked in later that year.