为什么 volatile 不能解决比较期间的数据竞争

Question

我正在尝试使用多线程进行试验并遵循此处的示例：https://docs.oracle.com/javase/specs/jls/se10/html/jls-8.html#jls-8.3.1.4

我已经在下面发布了我的代码。你能帮我理解为什么数据竞争发生在“if (x < y) {”而不是“if (y > x) {”吗？

我正在使用 openjdk-14.0.1:

Linux void-MS-7678 5.4.0-29-generic #33-Ubuntu SMP Wed Apr 29 14:32:27 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

代码：

public class Main {
    public static void main(String[] args) {
        DataRace dr = new DataRace();
        Thread t1 = new Thread(()-> {
            for (int i = 0; i < 100_000; i++) {
                dr.increment();
            }
        });

        Thread t2 = new Thread(()-> {
            for (int i = 0; i < 100_000; i++) {
                dr.check();
            }
        });

        t1.start();
        t2.start();
    }

    private static class DataRace {
        private volatile int x = 0, y = 0;

        public void increment() {
            x++;
            y++;
        }

        public void check() {
            // System.out.println("x=" + x + " y="+ y); // - NO ISSUES
            // if (y > x) { - NO ISSUES
            // if (x < y) { - ISSUES
            if (x < y) {
                System.out.println("DataRace detected: x < y");
            }
        }
    }
}

输出：

/home/void/.jdks/openjdk-14.0.1/bin/java -javaagent:/home/void/Development/idea-IC-183.4588.61/lib/idea_rt.jar=46411:/home/void/Development/idea-IC-183.4588.61/bin -Dfile.encoding=UTF-8 -classpath /home/void/Development/multithreading/out/production/classes Main
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y
DataRace detected: x < y

Process finished with exit code 0

Answer 1

比较 if (x < y) { 不是原子的。

t2 加载 x 进行比较
t2 停止工作
t1 递增 x 和 y
t1 停止
t2 开始
t2 加载 y 进行比较
因为 x 是旧值，y 是新值，递增，x < y 是 true。

下面是一个如何用 synchronized 解决这个问题的例子：

class Main {
    public static void main(String[] args) {
        DataRace dr = new DataRace();
        Thread t1 = new Thread(()-> {
            for (int i = 0; i < 100_000; i++) {
                dr.increment();
            }
        });

        Thread t2 = new Thread(()-> {
            for (int i = 0; i < 100_000; i++) {
                dr.check();
            }
        });

        t1.start();
        t2.start();
    }

    private static class DataRace {
        private volatile int x = 0, y = 0;

        public synchronized void increment() {
            x++;
            y++;
        }

        public void check() {
            // System.out.println("x=" + x + " y="+ y); // - NO ISSUES
            // if (y > x) { - NO ISSUES
            // if (x < y) { - ISSUES
            boolean xSmallerY = false;
            synchronized (this) {
                xSmallerY = x < y;
            }
            if (xSmallerY) {
                System.out.println("DataRace detected: x < y");
            }
        }
    }
}

Answer 2

akuzminykh 已经为什么 if (x < y) 可以是真的。您还问过为什么在 if (y > x).

时从未看到相同的现象

原因是在 java 中表达式总是从左到右求值，当你这样做 y > x 时，y 总是首先从内存中加载所以 x 在 y 之前已经递增，如果要从后续迭代中读取 x，它也将大于 y。

当您执行 y > x 时，您仍然可以看到 "DataRace detected" 正在打印，但当且仅当 x 接近 Integer.MAX_VALUE 并且它溢出并变为在从内存中读取 y 之后的后续迭代中为负，然后才从内存中读取 x。

public class Check {

    public static void main(String[] args) {
        DataRace dr = new DataRace();
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 100_000; i++) {
                dr.increment();
            }
        });
        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 100_000; i++) {
                dr.check();
            }
        });
        t1.start();
        t2.start();
    }

    private static class DataRace {

        private volatile int x,y;

        public void increment() {
            // to make sure the race condition is caused by the ++ and not by the assignment
            synchronized (this) {
                x = Integer.MAX_VALUE;
                y = Integer.MAX_VALUE;
            }
            x++;
            y++;
        }

        public synchronized void check() {
             if (y > x) {
                 System.out.println("DataRace detected: y > x");
            }
        }
    }
}

为什么 volatile 不能解决比较期间的数据竞争

Why volatile does not solve data race during comparison

java

multithreading

volatile