c++ - std::atomic 加载和存储都需要吗？

Question

根据this article :

Any time two threads operate on a shared variable concurrently, and one of those operations performs a write, both threads must use atomic operations.

但是，如果较低优先级的线程是写入者，而较高优先级的线程是读取者，那么较低优先级的线程是否需要强制执行原子存储？在我看来，只有较高优先级的线程需要强制执行原子加载:

#include <atomic>

std::atomic<T*> ptr; // assume initialized to some non-null value

void highPriThreadFcn(void)
{
    T* local_ptr = ptr.load(); // need atomic load here in case lowPriThread write/store was interrupted
}

void lowPriThreadFcn(T* some_ptr)
{
    ptr = some_ptr; // do I need an atomic store here? I'd think not, as ptr is not written to by highPriThread
}

类似的问题适用于“反向”情况(在高优先级线程中写入，从低优先级线程中读取):

void highPriThreadFcn(T* some_ptr)
{
    ptr = some_ptr; // do I need an atomic store here? I'd think not, as write to ptr cannot be interrupted
}

void lowPriThreadFcn(void)
{
    T* local_ptr = ptr.load(); // need atomic load here in case read/load was interrupted
}

Answer 1

您无法对原子变量执行非原子存储或加载。 API 确保没有办法尝试它。您所谓的“非原子商店”，

ptr = some_ptr;

实际上是一个具有连续一致排序的原子存储。请参阅atomic::operator= on cppreference .

顺便说一句，如果您正在考虑将原子变量更改为仅以原子方式执行某些操作的非原子变量:请不要这样做。每当对同一内存位置的加载和存储“可能并发”时，该标准就要求它们两者都是原子的。否则，行为是未定义的。这意味着编译器可以以破坏代码的方式进行“优化”。一个线程的优先级是否比另一个线程“更高”并不影响这一点。