C++ standard: can relaxed atomic stores be lifted above a mutex lock?

Question

Is there any wording in the standard that guarantees that relaxed stores to atomics won\'t be lifted above the locking of a mutex? If not, is there any wording that explicitly s

Answer 1

No memory operation inside a mutex protected region can 'escape' from that area. That applies to all memory operations, atomic and non-atomic.

In section 1.10.1:

a call that acquires a mutex will perform an acquire operation on the locations comprising the mutex Correspondingly, a call that releases the same mutex will perform a release operation on those same locations

Furthermore, in section 1.10.1.6:

All operations on a given mutex occur in a single total order. Each mutex acquisition “reads the value written” by the last mutex release.

And in 30.4.3.1

A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents

This means, acquiring (locking) a mutex sets a one-way barrier that prevents operations that are sequenced after the acquire (inside the protected area) from moving up across the mutex lock.

Releasing (unlocking) a mutex sets a one-way barrier that prevents operations that are sequenced before the release (inside the protected area) from moving down across the mutex unlock.

In addition, memory operations that are released by a mutex are synchronized (visible) with another thread that acquires the same mutex.

In your example, foo_has_been_set is checked in CheckFoo.. If it reads true you know that the value 1 has been assigned to foo by SetFoo, but it is not synchronized yet. The mutex lock that follows will acquire foo, synchronization is complete and the assert cannot fire.