cas

CAS，當oldVal與expected相同時，將oldVal更新爲newVal，原子操做

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.

爲了解決ABA問題，DCAS，Double-width CAS等都被相應提出

Exceptional backoff

 

 

reader-writer problem：