How do memory_order_seq_cst and memory_order_acq_rel differ?

Question

Stores are release operations and loads are acquire operations for both. I know that memory_order_seq_cst is meant to impose an additional total ordering for al

Answer 1

Still use the definition and example from memory_order. But replace memory_order_seq_cst with memory_order_release in store and memory_order_acquire in load.

Release-Acquire ordering guarantees everything that happened-before a store in one thread becomes a visible side effect in the thread that did a load. But in our example, nothing happens before store in both thread0 and thread1.

x.store(true, std::memory_order_release); // thread0

y.store(true, std::memory_order_release); // thread1

Further more, without memory_order_seq_cst, the sequential ordering of thread2 and thread3 are not guaranteed. You can imagine they becomes:

if (y.load(std::memory_order_acquire)) { ++z; } // thread2, load y first
while (!x.load(std::memory_order_acquire)); // and then, load x

if (x.load(std::memory_order_acquire)) { ++z; } // thread3, load x first
while (!y.load(std::memory_order_acquire)); // and then, load y

So, if thread2 and thread3 are executed before thread0 and thread1, that means both x and y stay false, thus, ++z is never touched, z stay 0 and the assert fires.

However, if memory_order_seq_cst enters the picture, it establishes a single total modification order of all atomic operations that are so tagged. Thus, in thread2, x.load then y.load; in thread3, y.load then x.load are sure things.