多线程程序卡在优化模式但在 -O0 中正常运行

2021-12-06 00:00:00 multithreading thread-safety c++ undefined-behavior data-race

我写了一个简单的多线程程序如下:

I wrote a simple multithreading programs as follows:

static bool finished = false;

int func()
{
    size_t i = 0;
    while (!finished)
        ++i;
    return i;
}

int main()
{
    auto result=std::async(std::launch::async, func);
    std::this_thread::sleep_for(std::chrono::seconds(1));
    finished=true;
    std::cout<<"result ="<<result.get();
    std::cout<<"
main thread id="<<std::this_thread::get_id()<<std::endl;
}

它在Visual Studio或-O0中在gcc中的调试模式下正常运行，并在1<后打印结果/code> 秒.但是在发布模式或-O1 -O2 -O3下它卡住了并且不打印任何东西.

It behaves normally in debug mode in Visual studio or -O0 in gcc and print out the result after 1 seconds. But it stuck and does not print anything in Release mode or -O1 -O2 -O3.

推荐答案

两个线程，访问一个非原子的、非保护的变量是 UB 这涉及finished.您可以制作 std::atomic 类型的 finished 来解决这个问题.

Two threads, accessing a non-atomic, non-guarded variable are U.B. This concerns finished. You could make finished of type std::atomic<bool> to fix this.

我的修复:

#include <iostream>
#include <future>
#include <atomic>

static std::atomic<bool> finished = false;

int func()
{
    size_t i = 0;
    while (!finished)
        ++i;
    return i;
}

int main()
{
    auto result=std::async(std::launch::async, func);
    std::this_thread::sleep_for(std::chrono::seconds(1));
    finished=true;
    std::cout<<"result ="<<result.get();
    std::cout<<"
main thread id="<<std::this_thread::get_id()<<std::endl;
}

输出:

result =1023045342
main thread id=140147660588864

coliru 现场演示

有人可能会认为'这是一个 bool –大概有一点.这怎么可能是非原子的?(当我自己开始使用多线程时，我就这样做了.)

Somebody may think 'It's a bool – probably one bit. How can this be non-atomic?' (I did when I started with multi-threading myself.)

但请注意，std::atomic 提供给您的不只是缺乏撕裂.它还使来自多个线程的并发读+写访问得到明确定义，阻止编译器假设重新读取变量将始终看到相同的值.

But note that lack-of-tearing is not the only thing that std::atomic gives you. It also makes concurrent read+write access from multiple threads well-defined, stopping the compiler from assuming that re-reading the variable will always see the same value.

使 bool 不受保护、非原子会导致其他问题:

Making a bool unguarded, non-atomic can cause additional issues:

  • 编译器可能会决定将变量优化为一个寄存器，甚至将 CSE 多次访问优化为一个，并从循环中提升负载.
  • 可能会为 CPU 内核缓存该变量.(在现实生活中，CPU 具有一致的缓存.这不是一个真正的问题，但 C++ 标准足够宽松，可以涵盖非连贯共享内存上的假设 C++ 实现，其中 atomic 和 memory_order_relaxed 存储/加载将工作，但 volatile 不会.为此使用 volatile 将是 UB，即使它在实际 C++ 实现中实际工作.)
  • The compiler might decide to optimize variable into a register or even CSE multiple accesses into one and hoist a load out of a loop.
  • The variable might be cached for a CPU core. (In real life, CPUs have coherent caches. This is not a real problem, but the C++ standard is loose enough to cover hypothetical C++ implementations on non-coherent shared memory where atomic<bool> with memory_order_relaxed store/load would work, but where volatile wouldn't. Using volatile for this would be UB, even though it works in practice on real C++ implementations.)

为了防止这种情况发生，必须明确告知编译器不要这样做.

To prevent this to happen, the compiler must be told explicitly not to do.

关于 volatile 与这个问题的潜在关系的不断发展的讨论让我有点惊讶.因此，我想花掉我的两分钱:

I'm a little bit surprised about the evolving discussion concerning the potential relation of volatile to this issue. Thus, I'd like to spent my two cents:

  • volatile 对线程有用
  • 谁害怕糟糕的优化编译器?.

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