c++ - 这里的顺序一致性有什么问题？

Question

我正在研究 C 和 C++ 中的无锁算法，最近偶然发现了一个我不太理解的行为。如果你有下面的代码，运行它会给你类似的东西

reader startedwriter startediters=79895047, less=401131, eq=48996928, more=30496988

Aren't std::atomics are expected to be sequentially-consistent? If so, why does the reader sometimes see b being updated before a? I also tried to do various tricks involving memory fences with no success. The full compilable code can be seen at https://github.com/akamaus/fence_test

What's wrong with the example?

std::atomic<uint> a(0);
std::atomic<uint> b(0);

volatile bool stop = false;

void *reader(void *p) {
    uint64_t iter_counter = 0;
    uint cnt_less = 0,
         cnt_eq = 0,
         cnt_more = 0;

    uint aa, bb;


    printf("reader started\n");

    while(!stop) {
        iter_counter++;
        aa = a.load(std::memory_order_seq_cst);
        bb = b.load(std::memory_order_seq_cst);
        if (aa < bb) {
            cnt_less++;
        } else if (aa > bb) {
            cnt_more++;
        } else {
            cnt_eq++;
        }
    }
        printf("iters=%lu, less=%u, eq=%u, more=%u\n", iter_counter, cnt_less, cnt_eq, cnt_more);

    return NULL;
}

void *writer(void *p) {
    printf("writer started\n");
    uint counter = 0;
    while(!stop) {
        a.store(counter, std::memory_order_seq_cst);
        b.store(counter, std::memory_order_seq_cst);
        counter++;
    }
}

Answer 1

顺序一致的内存排序意味着所有线程观察到的(使用 seq cst 操作的原子对象的)修改顺序是一致的。该程序的行为就好像所有这些操作都交织在一个单一的总顺序中。考虑以下情况:

---

Writer Reader       a == 0a = 1b = 1       b == 1

结果:aa < bb .

---

Writer Readera = 1       a == 1       b == 0b = 1

结果:aa > bb

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带锁，例如一个mutex ，您可以确保操作不会交错。