Question

我正在练习readers and writers problem，并提出以下解决方案。但是，该程序在打印出以下结果后会自行阻止。

$ Read content: planets

我的想法是first solution的修改版本。使用2个信号量，一个称为 readMutex 的信号量确保一次只能读取一个线程来更新 numOfReaders ，而另一个信号量的 accessToResource 确保该时间点是读者正在阅读内容，作者应该等待。这是我的代码。

import java.util.concurrent.Semaphore;

public class ReadersAndWriters {
    public static final Semaphore accessToResource = new Semaphore(1, true);
    public static final Semaphore readMutex = new Semaphore(1, true);
    public static String content = "planets";
    public static int numOfReaders;

    static class Reader extends Thread {
        void read() {
            try {
                readMutex.acquire();
                numOfReaders++;
                if (numOfReaders == 1) {
                    accessToResource.acquire();
                }
                readMutex.release();

                // read content, not a critical section
                System.out.println("Read content:\t" + content);

                readMutex.acquire();
                numOfReaders--;
                if (numOfReaders <= 0) {
                    accessToResource.release();
                }
                readMutex.release();
            } 
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void run() {
            read();
        }
    }

static class Writer extends Thread {
    String text;

    Writer(String text) {
        this.text = text;
    }

    void write() {
        try {
            readMutex.acquire();
            accessToResource.acquire();

            // critical section
            content = text;
            System.out.println("Content changed:\t" + content);
            // end of critical section

            accessToResource.release();
            readMutex.release();
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        write();
    }
}

public static void main(String[] args) {
    Reader t1 = new Reader();
    Reader t2 = new Reader();
    Writer t3 = new Writer("stars");
    Reader t4 = new Reader();
    Writer t5 = new Writer("restaurant at the end of universe");
    Reader t6 = new Reader();
    Reader t7 = new Reader();
    t1.start();
    t2.start();
    t3.start();
    t4.start();
    t5.start();
    t6.start();
    t7.start();
}
}

Answer 1

当一个Reader获得了accessToResource并尝试获得readMutex时：

if (numOfReaders == 1) {
    accessToResource.acquire();
}
readMutex.release();

System.out.println("Read content:\t" + content);

readMutex.acquire();  // -------> here

另一个Writter已获得readMutex并尝试获得accessToResource：

readMutex.acquire();
accessToResource.acquire(); // ------> here

发生死锁。

由于accessToResource和readMutex的许可是1，因此Reader和Writter都无法前进。

为什么给许多读者和作家造成僵局

1 个答案: