多线程的偏执狂 [英] Multithreaded paranoia

问题描述

这是一个复杂的问题，请在回答之前仔细考虑。

This is a complex question, please consider carefully before answering.

考虑这种情况。两个线程（读者和作家）访问一个单一的全球 INT 。这安全吗？通常情况下，我会不假思索回应，是的！

Consider this situation. Two threads (a reader and a writer) access a single global int. Is this safe? Normally, I would respond without thought, yes!

不过，在我看来，香草萨特并不这么认为。在他的有效的并发性的文章，他讨论了有缺陷的无锁队列和的修正版本。

However, it seems to me that Herb Sutter doesn't think so. In his articles on effective concurrency he discusses a flawed lock-free queue and the corrected version.

在第一篇文章中，他讨论的变量很少考虑特质第二的开头结尾，写序。诠释的是原子的，不错，但整数不一定订购了可以摧毁任何无锁的算法，包括我上面的场景。我完全同意，唯一的办法的保证的正确的多线程在所有平台上present和未来的行为是使用原子能公司（又名记忆障碍）或互斥。

In the end of the first article and the beginning of the second he discusses a rarely considered trait of variables, write ordering. Int's are atomic, good, but ints aren't necessarily ordered which could destroy any lock-free algorithm, including my above scenario. I fully agree that the only way to guarantee correct multithreaded behavior on all platforms present and future is to use atomics(AKA memory barriers) or mutexes.

我的问题;是写重odering曾经在真实的硬件出了问题？或者是多线程的偏执只是被迂腐？结果
什么经典的单处理器系统？结果
关于简单的RISC处理器就像一个嵌入式电源，PC是什么？

My question; is write re-odering ever a problem on real hardware? Or is the multithreaded paranoia just being pedantic?
What about classic uniprocessor systems?
What about simpler RISC processors like an embedded power-pc?

的澄清的：我更感兴趣的是萨特先生说一下硬件（处理器/缓存）重新排序变量写道。我可以从破code编译器与交换机或装配后编译的手检查停止优化。不过，我想知道如果硬件尚可乱了code在实践中。

Clarification: I'm more interested in what Mr. Sutter said about the hardware (processor/cache) reordering variable writes. I can stop the optimizer from breaking code with compiler switches or hand inspection of the assembly post-compilation. However, I'd like to know if the hardware can still mess up the code in practice.

推荐答案

您检查装配的想法是不够的;重排序可以在硬件级别发生

Your idea of inspecting the assembly is not good enough; the reordering can happen at the hardware level.

要回答你的问题，是这个不断在读取硬件出现了问题：是其实我碰到这个问题我自己

To answer your question "is this ever a problem on read hardware:" Yes! In fact I've run into that problem myself.

是否确定裙子单处理器系统或其他特殊情况的情况下，这个问题？我要说不，因为从现在起5年，你可能需要在多核毕竟运行，然后找到所有这些地点将是棘手的（不可能的？）。

Is it OK to skirt the issue with uniprocessor systems or other special-case situations? I would argue "no" because five years from now you might need to run on multi-core after all, and then finding all these locations will be tricky (impossible?).

一个例外：软件设计的嵌入式硬件应用中确实已经完全超过了硬件控制。其实我已经在例如这些情况下，骗像这样ARM处理器。

One exception: Software designed for embedded hardware applications where indeed you have completely control over the hardware. In fact I have "cheated" like this in those situations on e.g. an ARM processor.

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