For an atomic object, modification order is the single order of all its modifications that remains consistent with each participating thread’s sequenced-before order. Every thread dealing with that object perceives its changes consistently with this order. TAKEAWAY: modification order is local to one atomic object; it is not automatically one global order for every atomic object. section 19.1 section 19.3
One atomic object, six modifications
Consider atomic x with an initial value of 11, two increments, and three assignments. These six modifications form x’s modification order. Threads A and B must each perceive those changes in an order consistent with it, even when they perform the operations concurrently. An atomic read that observes a value written by another thread can establish synchronization between the threads. section 19.1 section 19.1
Keep these distinctions in view:
- Modification order applies to modifications of the same atomic object and must respect each thread’s sequenced-before relation for those modifications. section 19.1
- Sequential consistency adds a single global modification order for sequentially consistent operations, including operations on different atomic objects. section 19.3
- Sequential consistency also enforces acquire-release semantics, but its stronger total ordering can constrain parallel execution. section 19.3
