使用 InterlockedCompareExchange 无锁

发布于 2024-11-05 00:11:24 字数 361 浏览 0 评论 0原文

我正在尝试使用互锁操作使以下代码片段无锁，知道如何翻译吗？

if (m_Ref == 0xFFFF)
    m_Ref = 1;
else
{
    if (++m_Ref == 1)
        CallSomething(); //

}

我在想

if (InterlockedCompareExchange(&m_Ref, 1, 0xFFFF) != 0xFFFF))
{
    if (InterlockedIncrement(&m_Ref) == 1)
         CallSomething();
}

这里面有什么问题/种族吗？

原文

I am trying to make following snip of code lockless using interlocked operations, Any idea how to translate this?

if (m_Ref == 0xFFFF)
    m_Ref = 1;
else
{
    if (++m_Ref == 1)
        CallSomething(); //

}

I was thinking something like

if (InterlockedCompareExchange(&m_Ref, 1, 0xFFFF) != 0xFFFF))
{
    if (InterlockedIncrement(&m_Ref) == 1)
         CallSomething();
}

Is there any issues/race in this?

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土豪我们做朋友吧 2024-11-12 00:11:24

从表面上看，这看起来是正确的，但每次连续使用两个互锁操作时，您都会将自己暴露在 ABA 问题。在这种情况下，一个线程无法将其从 0xFFFF 更改为 1（ICX 返回 !=0xFFFF），因此它会继续执行 if 分支并递增它。在运行 InterlockedIncrement 之前，另一个线程将 m_ref 更改回 0xFFFF，并且原始线程递增 0xFFFF。根据 m_ref 的类型/语义，效果会很谨慎，但肯定会很糟糕。

您应该对 0xFFF 到 1 以及 X 到 X+1 执行一次 ICX 操作，并且如果丢失 ICX，请务必重试：

volatile <type> m_ref;

<type> ref, newRef, icxref;
do
{
   ref = m_ref;
   newRef = (0xFFFF == ref) ? 1 : ++ref;
   icxref = InterlockedCompareExchange (&m_ref, newRef, ref);
} while (icxref != ref);
if (newRef == 1 && ref != 0xFFFF)
{
   DoSomething ();
}

This looks correct at a superficial glance, but every time you use two interlocked operations in a row you are exposing yourself to the ABA problem. In this case one thread fail to change it from 0xFFFF to 1 (the ICX returns !=0xFFFF) so it goes ahead and takes the if branch and increments it. Before it runs the InterlockedIncrement another threads changes the m_ref back to 0xFFFF and the original thread increments 0xFFFF. Depending on the type/semantics of m_ref the effect will wary, but will surely be bad.

You should do one single ICX operation, for both the 0xFFF to 1 and X to X+1, and always retry if you lost the ICX:

volatile <type> m_ref;

<type> ref, newRef, icxref;
do
{
   ref = m_ref;
   newRef = (0xFFFF == ref) ? 1 : ++ref;
   icxref = InterlockedCompareExchange (&m_ref, newRef, ref);
} while (icxref != ref);
if (newRef == 1 && ref != 0xFFFF)
{
   DoSomething ();
}

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