Happens-before (${\to }_{\mathrm{hb}}$)

An ordering binary relation used by a memory model to constrain observation of shared data. For the mutex example, T1's write precedes its release, that release synchronizes with T2's successful acquisition, and that acquisition precedes T2's read. Together these steps order the write before the read. Wall-clock timing alone does not establish happens-before.^[ref]

A release on an atomic object can synchronize with an acquire that reads the released value, establishing happens-before for surrounding accesses. A test-and-set spinlock needs suitable acquisition and release semantics for its guarded data; relaxed flag operations alone do not supply that relationship. This is a separate question from the flag's indivisibility.^[ref]