This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

4004. The load and store operation in §[atomics.order] p1 is ambiguous

Section: 33.5.4 [atomics.order] Status: New Submitter: jim x Opened: 2023-10-30 Last modified: 2023-11-04

Priority: Not Prioritized

View other active issues in [atomics.order].

View all other issues in [atomics.order].

View all issues with New status.


33.5.4 [atomics.order] p1 says:

  1. (1.2) — memory_order::release, memory_order::acq_rel, and memory_order::seq_cst: a store operation performs a release operation on the affected memory location.

  2. (1.3) — memory_order::consume: a load operation performs a consume operation on the affected memory location. […]

  3. (1.4) — memory_order::acquire, memory_order::acq_rel, and memory_order::seq_cst: a load operation performs an acquire operation on the affected memory location.

What do the store and load operations intend to mean in this context? If there is no extra specification, it is easy to consider them as the operations performed by the non-static member functions "store" and "load" defined in the atomic class (template).

33.5.4 [atomics.order] p2 says

An atomic operation A that performs a release operation on an atomic object M synchronizes with an atomic operation B that performs an acquire operation on M and takes its value from any side effect in the release sequence headed by A.

According to the above interpretation, A is an operation performed by the non-static member function store, however, I think the following example can establish the synchronization relationship

std::atomic<int> x{0};
Thread 1:
int expected = 0;
x.compare_exchange_strong(expected, 1, memory_order::release, memory_order::relaxed ); //#1

Thread 2:
int expected = 1;
while(x.compare_exchange_strong( expected, 2, memory_order::acquire, memory_order::relaxed )){} // #2

Assuming the RMW operations are successful in the two threads, I think #1 intends to perform a release operation while #2 performs an acquire operation, and hence they can establish the synchronization relationship, however, they both are RMW operations.

It should be clearly defined which are store operations and which are load operations.

Proposed resolution: