CAS，当oldVal与expected相同时，将oldVal更新为newVal，原子操作

ABA problem：

when a location is read twice, has the same value for both reads, and "value is same“ is used to indicate "nothing has changed". however, another thread can execute between the two reads and change the value, do other work, then change the value back.

为了解决ABA问题，DCAS，Double-width CAS等都被相应提出

Exceptional backoff

reader-writer problem：