modernescpp Acquire-Release Semantic

Synchronisation and ordering constraints

The acquire-release semantic is based on one key idea. A release operation synchronises with an acquire operation on the same atomic and establishes, in addition, an ordering constraint. So, all read and write operations can not be moved before an acquire operation, all read and write operations can not be move behind a release operation. But what is an acquire or release operation? The reading of an atomic variable with load or test_and_set is an acquire operation. But in addition the acquiring of a lock. Of course, the opposite is also true. The releasing of a lock is a release operation. Accordingly, a store or clear operation on an atomic variable.

NOTE: 先acquire，然后release

read-acquire

write-release

It's worth to reason once more on the few last sentences from a different perspective. The lock of a mutex is an acquire operation, the unlock of a mutex a release operation. Figuratively speaking that implies, that an operation on a variable can not moved outside of a critical section. That hold for both directions. On the other side, a variable can be moved inside of a criticial section. Because the variable moves from the not protected to the protected area. Now with a little delay the picture.