我需要使用内存屏障来保护共享资源吗？ [英] Do I need to use memory barriers to protect a shared resource?

问题描述

在多生产者，多消费者的情况下。如果生产者写入 int a ，并且消费者正在从 int a 中读取，我需要在 int a ？

In a multi-producer, multi-consumer situation. If producers are writing into int a, and consumers are reading from int a, do I need memory barriers around int a?

我们都知道：共享资源应该总是受到保护，标准不保证正确的行为。

We all learned that: Shared resources should always be protected and the standard does not guarantee a proper behavior otherwise.

然而，在缓存一致性架构上，可自动确保8,16,32和64位变量的原子性 MOV 操作。

However on cache-coherent architectures visibility is ensured automatically and atomicity of 8, 16, 32 and 64 bit variables MOV operation is guaranteed.

因此，为什么要保护 int a

？

推荐答案

至少在C ++ 11（或更高版本）中，您不需要（显式地）用互斥或内存屏障保护您的变量。

At least in C++11 (or later), you don't need to (explicitly) protect your variable with a mutex or memory barriers.

您可以使用 std :: atomic 创建一个原子变量。对该变量的更改将确保在线程间传播。

You can use std::atomic to create an atomic variable. Changes to that variable are guaranteed to propagate across threads.

std::atomic<int> a;

// thread 1:
a = 1;

// thread 2 (later):
std::cout << a;    // shows `a` has the value 1.

当然，例如，不能保证 std :: cout 以原子方式工作，因此您可能必须保护它（如果您尝试从多个线程写入，无论如何）。

Of course, there's a little more to it than that--for example, there's no guarantee that std::cout works atomically, so you probably will have to protect that (if you try to write from more than one thread, anyway).

然后由编译器/标准库找出处理原子性要求的最佳方法。在确保高速缓存一致性的典型体系结构上，它可能意味着不在寄存器中分配此变量。它可以施加内存障碍，但只有在一个真正需要它们的系统上才可能这样做。

It's then up to the compiler/standard library to figure out the best way to handle the atomicity requirements. On a typical architecture that ensures cache coherence, it may mean nothing more than "don't allocate this variable in a register". It could impose memory barriers, but is only likely to do so on a system that really requires them.

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