Java 中的虚假唤醒真的发生了吗? [英] Do spurious wakeups in Java actually happen?

问题描述

看到各种与锁定相关的问题，并且(几乎)总是找到由于虚假唤醒而导致的循环"术语¹我想知道，有没有人经历过这种唤醒(假设硬件/软件环境不错)例如)?

Seeing various locking related question and (almost) always finding the 'loop because of spurious wakeups' terms¹ I wonder, has anyone experienced such kind of a wakeup (assuming a decent hardware/software environment for example)?

我知道术语虚假"的意思是没有明显的原因，但这种事件的原因是什么?

I know the term 'spurious' means no apparent reason but what can be the reasons for such kind of an event?

(¹ 注意:我不是在质疑循环实践.)

(¹ Note: I'm not questioning the looping practice.)

一个帮助问题(对于那些喜欢代码示例的人):

A helper question (for those who like code samples):

如果我有以下程序，并运行它:

If I have the following program, and I run it:

public class Spurious {
    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition cond = lock.newCondition();
        lock.lock();
        try {
            try {
                cond.await();
                System.out.println("Spurious wakeup!");
            } catch (InterruptedException ex) {
                System.out.println("Just a regular interrupt.");
            }
        } finally {
            lock.unlock();
        }
    }
}

我能做些什么来虚假地唤醒这个 await 而不永远等待一个随机事件?

What can I do to wake this await up spuriously without waiting forever for a random event?

推荐答案

维基百科关于虚假唤醒的文章 有这个花絮:

The Wikipedia article on spurious wakeups has this tidbit:

Linux 中的pthread_cond_wait() 函数是使用futex 系统调用实现的.当进程接收到信号时，Linux 上的每个阻塞系统调用都会突然返回 EINTR.... pthread_cond_wait() 无法重新开始等待，因为它可能会在 futex 系统调用之外的一小段时间内错过真正的唤醒.这种竞争条件只能通过调用者检查不变量来避免.因此，POSIX 信号会产生虚假唤醒.

The pthread_cond_wait() function in Linux is implemented using the futex system call. Each blocking system call on Linux returns abruptly with EINTR when the process receives a signal. ... pthread_cond_wait() can't restart the waiting because it may miss a real wakeup in the little time it was outside the futex system call. This race condition can only be avoided by the caller checking for an invariant. A POSIX signal will therefore generate a spurious wakeup.

总结:如果 Linux 进程收到信号，它的等待线程将各自享受良好的虚假唤醒.

Summary: If a Linux process is signaled its waiting threads will each enjoy a nice, hot spurious wakeup.

我买了.与通常给出的通常含糊的这是为了性能"的原因相比，这更容易吞咽.

I buy it. That's an easier pill to swallow than the typically vague "it's for performance" reason often given.

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