概念：CPU Cache flush & invalidate

Flush does write back the contents of cache to main memory, and invalidate does mark cache lines as invalid so that future reads go to main memory.

invalidate处理悖论：如果需要invalidate的cache/cache line里面包含无用的数据，需要丢弃；又包含其他进程需要的有用的数据，需要写回内存。