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如果我在子函数中而不是在使用数据的函数中放置内存栅栏，线程安全性是否存在差异。底部示例包括两个版本。我想知道是否有我不知道的差异。功能A_function和B_function同样线程安全吗？

#include<atomic>

using std::atomic;
using std::atomic_thread_fence;
using std::memory_order_acquire;
using std::memory_order_release;

typedef struct 
{
    atomic<int> lock;
    int counter;
}Data;

void A_acquire(atomic<int> * lock);
void A_release(atomic<int> * lock);
void A_function(Data * data);
void B_acquire(atomic<int> * lock);
void B_release(atomic<int> * lock);
void B_function(Data * data);

void A_acquire(atomic<int> * lock)
{
    int ticket = lock->fetch_add(1);
    while (0 != ticket)
    {
        lock->fetch_sub(1);
        ticket = lock->fetch_add(1);
    }
    //DIFFERENCE HERE
}

void A_release(atomic<int> * lock)
{
    //DIFFERENCE HERE
    lock->fetch_sub(1);
}

void A_function(Data * data)
{
    A_acquire(&data->lock);
    atomic_thread_fence(std::memory_order_acquire); //DIFFERENCE HERE
    data->counter += 1;
    atomic_thread_fence(std::memory_order_release); //DIFFERENCE HERE
    A_release(&data->lock);
}

void B_acquire(atomic<int> * lock)
{
    int ticket = lock->fetch_add(1);
    while (0 != ticket)
    {
        lock->fetch_sub(1);
        ticket = lock->fetch_add(1);
    }
    atomic_thread_fence(std::memory_order_acquire); //DIFFERENCE HERE
}

void B_release(atomic<int> * lock)
{
    atomic_thread_fence(std::memory_order_release); //DIFFERENCE HERE
    lock->fetch_sub(1);
}

void B_function(Data * data)
{
    B_acquire(&data->lock);
    //DIFFERENCE HERE
    data->counter += 1;
    //DIFFERENCE HERE
    B_release(&data->lock);
}

int main(void)
{
    Data dat = { 0, 0 };
    A_function(&dat);
    B_function(&dat);
    return 0;
}
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1 回答 1

2

A_function和之间在语义上没有区别B_function。内存栅栏效果不受函数体的限制。

此外，正如 Phantom 所指出的，您的示例中的内存围栏是不必要的：两者都fetch_sub()已经fetch_add()具有获取 + 释放语义。

但是通过下面的修改，release fetch 变得至关重要：

void A_acquire(atomic<int> * lock)
{
    int ticket = lock->exchange(1);
    while (0 != ticket)
    {
        ticket = lock->exchange(1);
    }
    //DIFFERENCE HERE
}

void A_release(atomic<int> * lock)
{
    //DIFFERENCE HERE
    lock->store(0, memory_order_relaxed);
}
于 2015-04-30T22:32:12.573 回答