C ++ 11使用< atomic> [英] C++11 Implementation of Spinlock using <atomic>

问题描述

我实现了SpinLock类，如下

I implemented SpinLock class, as followed

struct Node {
    int number;
    std::atomic_bool latch;

    void add() {
        lock();
        number++;
        unlock();
    }
    void lock() {
        bool unlatched = false;
        while(!latch.compare_exchange_weak(unlatched, true, std::memory_order_acquire));
    }
    void unlock() {
        latch.store(false , std::memory_order_release);
    }
};

我在类上面实现了两个线程，它们调用Node类的同一个实例的add（）方法每线程1000万次。

I implemented above class and made two threads which call add() method of a same instance of Node class 10 million times per thread.

结果是，不幸的是，不是2000万。
我在这里缺少什么？

the result is , unfortunately, not 20 million. What am I missing here?

推荐答案

问题是 compare_exchange_weak 更新 unlatched 变量失败。从 compare_exchange_weak 的文档：

The problem is that compare_exchange_weak updates the unlatched variable once it fails. From the documentation of compare_exchange_weak:

比较原子对象的包含值with
expected：
- 如果为true，它用val（like store）替换包含的值。
- 如果为false，则用期望值替换包含的值。

首先失败的 compare_exchange_weak ，未锁定将更新为 true 因此下一个循环迭代将尝试 compare_exchange_weak true 与 true 。

I.e., after the first failing compare_exchange_weak, unlatched will be updated to true, so the next loop iteration will try to compare_exchange_weak true with true. This succeeds and you just took a lock that was held by another thread.

解决方案：
确保设置未锁定返回之前 compare_exchange_weak 之前的 false ，例如：

Solution: Make sure to set unlatched back to false before each compare_exchange_weak, e.g.:

while(!latch.compare_exchange_weak(unlatched, true, std::memory_order_acquire)) {
    unlatched = false;
}

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