Acquire-Release

memory_order_acq_rel is used with atomic RMW operations that need to both load-acquire and store-release a value. A typical example involves thread-safe reference counting, like in C++’s shared_ptr:

atomic_int refCount;

void inc()
{
    refCount.fetch_add(1, memory_order_relaxed);
}

void dec()
{
    if (refCount.fetch_sub(1, memory_order_acq_rel) == 1) {
        // No more references, delete the data.
    }
}

Order does not matter when incrementing the reference count since no action is taken as a result. However, when we decrement, we must ensure that:

1. All access to the referenced object happens before the count reaches zero.
2. Deletion happens after the reference count reaches zero.¹

Curious readers might be wondering about the difference between acquire-release and sequentially consistent operations. To quote Hans Boehm, chair of the ISO C++ Concurrency Study Group,

The difference between acq_rel and seq_cst is generally whether the operation is required to participate in the single global order of sequentially consistent operations.

In other words, acquire-release provides order relative to the variable being load-acquired and store-released, whereas sequentially consistent operation provides some global order across the entire program. If the distinction still seems hazy, you are not alone. Boehm goes on to say,

This has subtle and unintuitive effects. The [barriers] in the current standard may be the most experts-only construct we have in the language.