C++ atomic memory_order使用场景

Relaxed ordering

Typical use for relaxed memory ordering is incrementing counters, such as the reference counters of std::shared_ptr, since this only requires atomicity, but not ordering or synchronization (note that decrementing the shared_ptr counters requires acquire-release synchronization with the destructor)

#include <vector>
#include <iostream>
#include <thread>
#include <atomic>
 
std::atomic<int> cnt = {0};
 
void f()
{
    for (int n = 0; n < 1000; ++n) {
        cnt.fetch_add(1, std::memory_order_relaxed);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n) {
        v.emplace_back(f);
    }
    for (auto& t : v) {
        t.join();
    }
    std::cout << "Final counter value is " << cnt << '\n';
}

Output:

Final counter value is 10000

Release-Acquire ordering

If an atomic store in thread A is tagged memory_order_release, an atomic load in thread B from the same variable is tagged memory_order_acquire, and the load in thread B reads a value written by the store in thread A, then the store in thread A synchronizes-with the load in thread B.

All memory writes (non-atomic and relaxed atomic) that happened-before the atomic store from the point of view of thread A, become visible side-effects in thread B. That is, once the atomic load is completed, thread B is guaranteed to see everything thread A wrote to memory. This promise only holds if B actually returns the value that A stored

#include <thread>
#include <atomic>
#include <cassert>
#include <string>
 
std::atomic<std::string*> ptr;
int data;
 
void producer()
{
    std::string* p  = new std::string("Hello");
    data = 42;
    ptr.store(p, std::memory_order_release);
}
 
void consumer()
{
    std::string* p2;
    while (!(p2 = ptr.load(std::memory_order_acquire)))
        ;
    assert(*p2 == "Hello"); // never fires
    assert(data == 42); // never fires
}
 
int main()
{
    std::thread t1(producer);
    std::thread t2(consumer);
    t1.join(); t2.join();
}

The following example demonstrates transitive release-acquire ordering across three threads, using a release sequence

#include <thread>
#include <atomic>
#include <cassert>
#include <vector>
 
std::vector<int> data;
std::atomic<int> flag = {0};
 
void thread_1()
{
    data.push_back(42);
    flag.store(1, std::memory_order_release);
}
 
void thread_2()
{
    int expected=1;
    // memory_order_relaxed is okay because this is an RMW,
    // and RMWs (with any ordering) following a release form a release sequence
    while (!flag.compare_exchange_strong(expected, 2, std::memory_order_relaxed)) {
        expected = 1;
    }
}
 
void thread_3()
{
    while (flag.load(std::memory_order_acquire) < 2)
        ;
    // if we read the value 2 from the atomic flag, we see 42 in the vector
    assert(data.at(0) == 42); // will never fire
}
 
int main()
{
    std::thread a(thread_1);
    std::thread b(thread_2);
    std::thread c(thread_3);
    a.join(); b.join(); c.join();
}