在ReentrantReadWriteLock#tryLock失败时被jcstress测试困惑

Question

我正试图掌握JCStress.为了确保我理解它,我决定为我知道必须正确的东西写一些简单的测试:java.util.concurrent.locks.ReentrantReadWriteLock.

我写了一些非常简单的测试来检查锁模式兼容性.不幸的是,有两项压力测试失败了:

1. X_S:

   true, true        32,768     FORBIDDEN  No default case provided, assume FORBIDDEN
   

2. X_X:

   true, true        32,767     FORBIDDEN  No default case provided, assume FORBIDDEN
   

在我看来,一个线程不应该能够保持读锁定,而另一个线程也保持写锁定.同样,两个线程不可能同时保持写锁定.

我意识到这个问题很可能没有ReentrantReadWriteLock.我想我可能在jcstress测试中犯了一些关于JMM和读取锁状态的愚蠢错误.

不幸的是,我无法发现问题.有人可以帮我理解我所犯的(愚蠢？)错误吗？

import org.openjdk.jcstress.annotations.*;
import org.openjdk.jcstress.infra.results.ZZ_Result;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/*
 * |-----------------|
 * |  COMPATIBILITY  |
 * |-----------------|
 * |     | S   | X   |
 * |-----------------|
 * | S   | YES | NO  |
 * | X   | NO  | NO  |
 * |-----------------|
 */
public class ReentrantReadWriteLockBooleanCompatibilityTest {

    @State
    public static class S {
        public final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

        public boolean shared() {
            return lock.readLock().tryLock();
        }

        public boolean exclusive() {
            return lock.writeLock().tryLock();
        }
    }

    @JCStressTest
    @Outcome(id = "true, true", expect = Expect.ACCEPTABLE, desc = "T1 and T2 are both acquired S")
    public static class S_S {
        @Actor
        public void actor1(S s, ZZ_Result r) { r.r1 = s.shared(); }
        @Actor
        public void actor2(S s, ZZ_Result r) { r.r2 = s.shared(); }
    }

    @JCStressTest
    @Outcome(id = "true, false", expect = Expect.ACCEPTABLE, desc = "T1 acquired S, and T2 could not acquire X")
    @Outcome(id = "false, true", expect = Expect.ACCEPTABLE, desc = "T2 acquired X, and T1 could not acquire S")
    public static class S_X {
        @Actor
        public void actor1(S s, ZZ_Result r) { r.r1 = s.shared(); }
        @Actor
        public void actor2(S s, ZZ_Result r) { r.r2 = s.exclusive(); }
    }

    @JCStressTest
    @Outcome(id = "true, false", expect = Expect.ACCEPTABLE, desc = "T1 acquired X, and T2 could not acquire S")
    @Outcome(id = "false, true", expect = Expect.ACCEPTABLE, desc = "T2 acquired S and T1 could not acquire X")
    public static class X_S {
        @Actor
        public void actor1(S s, ZZ_Result r) { r.r1 = s.exclusive(); }
        @Actor
        public void actor2(S s, ZZ_Result r) { r.r2 = s.shared(); }
    }

    @JCStressTest
    @Outcome(id = "true, false", expect = Expect.ACCEPTABLE, desc = "T1 acquired X, and T2 could not acquire X")
    @Outcome(id = "false, true", expect = Expect.ACCEPTABLE, desc = "T2 acquired X and T1 could not acquire X")
    public static class X_X {
        @Actor
        public void actor1(S s, ZZ_Result r) { r.r1 = s.exclusive(); }
        @Actor
        public void actor2(S s, ZZ_Result r) { r.r2 = s.exclusive(); }
    }
}

我确实尝试过这个问题,jcstress-dev但从未收到过回复 - http://mail.openjdk.java.net/pipermail/jcstress-dev/2018-August/000346.html.为交叉发布道歉,但我需要帮助,因此我将重新发布到StackOverflow,以期获得更多受众的关注.

Answer 1

在针对jcstress 0.3运行时,您的测试通过.在版本0.4中,行为已更改为包括在启动时运行的健全性检查的结果(请参阅针对错误的提交jcstress忽略在健全性检查期间收集的样本).

一些健全性检查在一个线程中运行,并且您的测试不处理两个actor都被同一个线程调用的情况; 您正在测试可重入锁,因此如果已经保持写锁,则读锁将通过.

这可以说是jcstress中的一个错误,因为关于@Actor不变量的文档是:

• 每个方法仅由一个特定线程调用.
• 每个State实例只调用一次.

虽然文档的措辞并不明确,但生成的源代码清楚地表明,目的是在每个线程中运行每个actor.

解决它的一种方法是允许单线程案例通过:

@State
public static class S {
    public final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public boolean shared() {
        return lock.readLock().tryLock();
    }

    public boolean exclusive() {
        return lock.writeLock().tryLock();
    }

    public boolean locked() {
        return lock.isWriteLockedByCurrentThread();
    }
}

@JCStressTest
@Outcome(id = "true, false, false", expect = Expect.ACCEPTABLE, desc = "T1 acquired X, and T2 could not acquire S")
@Outcome(id = "false, false, true", expect = Expect.ACCEPTABLE, desc = "T2 acquired S and T1 could not acquire X")
@Outcome(id = "true, true, true", expect = Expect.ACCEPTABLE, desc = "T1 acquired X and then acquired S")
public static class X_S {
    @Actor
    public void actor1(S s, ZZZ_Result r) {
        r.r1 = s.exclusive();
    }
    @Actor
    public void actor2(S s, ZZZ_Result r) {
        r.r2 = s.locked();
        r.r3 = s.shared();
    }
}

或者检查单线程案例并将其标记为"有趣"而不是接受:

@State
public static class S {
    public final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    public AtomicReference<Thread> firstThread = new AtomicReference<>();

    public boolean shared() {
        firstThread.compareAndSet(null, Thread.currentThread());
        return lock.readLock().tryLock();
    }

    public boolean exclusive() {
        firstThread.compareAndSet(null, Thread.currentThread());
        return lock.writeLock().tryLock();
    }

    public boolean sameThread() {
        return Thread.currentThread().equals(firstThread.get());
    }

    public boolean locked() {
        return lock.isWriteLockedByCurrentThread();
    }
}

@JCStressTest
@Outcome(id = "false, true, false, false", expect = Expect.ACCEPTABLE, desc = "T1 acquired X, and T2 could not acquire X")
@Outcome(id = "false, false, false, true", expect = Expect.ACCEPTABLE, desc = "T2 acquired X and T1 could not acquire X")
@Outcome(id = "false, true, true, true", expect = Expect.ACCEPTABLE_INTERESTING, desc = "Both actors ran in the same thread!")
@Outcome(id = "true, true, false, true", expect = Expect.ACCEPTABLE_INTERESTING, desc = "Both actors ran in the same thread!")
public static class X_X {
    @Actor
    public void actor1(S s, ZZZZ_Result r) {
        r.r1 = s.sameThread();
        r.r2 = s.exclusive();
    }
    @Actor
    public void actor2(S s, ZZZZ_Result r) {
        r.r3 = s.sameThread();
        r.r4 = s.exclusive();
    }
}

正如您在评论中指出的那样,@Outcome上述测试中的最终结果从未发生过.这是因为单线程健全性检查在运行之前不会对actor进行混洗(请参阅sanityCheck_Footprints生成的测试类中的方法).