问题描述

我使用 Windows 中的 Interlocked 函数制作了一个非常简单的自旋锁，并在双核 CPU(两个线程递增一个变量)上对其进行了测试；

I made a very simple spinlock using the Interlocked functions in Windows and tested it on a dual-core CPU (two threads that increment a variable);

该程序似乎工作正常(每次都给出相同的结果，在不使用同步时情况并非如此)，但 Intel Parallel Inspector 说在 value += j(见下面的代码).使用关键部分而不是我的 SpinLock 时，警告会消失.

The program seems to work OK (it gives the same result every time, which is not the case when no synchronization is used), but Intel Parallel Inspector says that there is a race condition at value += j (see the code below). The warning disappears when using Critical Sections instead of my SpinLock.

我的 SpinLock 实现是否正确?这真的很奇怪，因为所有使用的操作都是原子的，并且有适当的内存屏障，它不应该导致竞争条件.

Is my implementation of SpinLock correct or not ? It's really strange, because all the used operations are atomic and have the proper memory barriers and it shouldn't lead to race conditions.

class SpinLock
{
   int *lockValue;
   SpinLock(int *value) : lockValue(value) { }

   void Lock() {
      while(InterlockedCompareExchange((volatile LONG*)lockValue, 1, 0) != 0) {
          WaitABit();
      }
   }

   void Unlock() { InterlockedExchange((volatile LONG*)lockValue, 0); }
};

测试程序:

static const int THREADS = 2;
HANDLE completedEvents[THREADS];
int value = 0;
int lock = 0; // Global.

DWORD WINAPI TestThread(void *param) {
    HANDLE completed = (HANDLE)param;
    SpinLock testLock(&lock);

    for(int i = 0;i < 1000*20; i++) {
        for(int j = 0;j < 10*10; j++) {
            // Add something to the variable.
            testLock.Lock();
            value += j;
            testLock.Unlock();
        }
    }
    SetEvent(completed);
}

int main() {
   for(int i = 0; i < THREADS; i++) {
        completedEvents[i] = CreateEvent(NULL, true, false, NULL);
   }
   for(int i = 0; i < THREADS; i++) {
        DWORD id;
        CreateThread(NULL, 0, TestThread, completedEvents[i], 0, &id);
   }

   WaitForMultipleObjects(THREADS, completedEvents, true, INFINITE);
   cout<<value;
}

推荐答案

Parallel Inspector 的文档 数据竞争 建议使用临界区或互斥体来修复 Windows 上的竞争.其中没有任何内容表明 Parallel Inspector 知道如何识别您可能发明的任何其他锁定机制.

Parallel Inspector's documentation for data race suggests using a critical section or a mutex to fix races on Windows. There's nothing in it which suggests that Parallel Inspector knows how to recognise any other locking mechanism you might invent.

用于分析新锁定机制的工具往往是静态工具，它们会查看代码中每条可能的路径，Parallel Inspector 的文档暗示它只执行一次代码.

Tools for analysis of novel locking mechanisms tend to be static tools which look at every possible path through the code, Parallel Inspector's documentation implies that it executes the code once.

如果你想尝试新的锁定机制，我在学术文献中看到的最常用的工具是 自旋模型检查器.还有 ESP，可能会减少状态空间，但我没有知道它是否已应用于并发问题，以及 mobility workbench 如果你能用 pi-calculus 解决你的问题，它会给出分析.英特尔 Parallel Inspector 看起来并不像这些工具那么复杂，而是旨在使用启发式方法检查常见问题.

If you want to experiment with novel locking mechanisms, the most common tool I've seen used in academic literature is the Spin model checker. There's also ESP, which might reduce the state space, but I don't know if it's been applied to concurrent problems, and also the mobility workbench which would give an analysis if you can couch your problem in pi-calculus. Intel Parallel Inspector doesn't seem anything like as complicated as these tools, but rather designed to check for commonly occurring issues using heuristics.

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