对 ReentrantReadWriteLock#tryLock 失败的 jcstress 测试感到困惑

Question

我正在努力掌握 JCStress。为了确保我理解它，我决定为我知道必须正确的东西编写一些简单的测试：java.util.concurrent.locks.ReentrantReadWriteLock.

我写了一些非常简单的测试来检查锁定模式的兼容性。不幸的是，有两项压力测试失败了：

X_S:

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

X_X:

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

在我看来，一个线程不能持有读锁，而另一个线程也持有写锁。同样，应该不可能有两个线程同时持有写锁。

我意识到问题可能与 ReentrantReadWriteLock 无关。我想我可能在关于 JMM 和读取锁状态的 jcstress 测试中犯了一些愚蠢的错误。

不幸的是，我无法发现问题所在。有人可以帮我理解我犯的（愚蠢的？）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(); }
    }
}

我曾尝试在 jcstress-dev 上询问过这个问题，但从未收到回复 - http://mail.openjdk.java.net/pipermail/jcstress-dev/2018-August/000346.html。很抱歉交叉发布，但我需要帮助，所以我重新发布到 Whosebug，希望得到更多观众的关注。

Answer 1

当运行针对 jcstress 0.3 时，您的测试通过。在 0.4 版中，行为更改为包括启动时运行的健全性检查结果（参见 this commit against the bug jcstress omits samples gathered during sanity checks）。

一些健全性检查运行在单个线程中进行，而您的测试无法处理两个参与者都被同一线程调用的情况；您正在测试 可重入 锁，因此如果写锁已被持有，则读锁将通过。

这可以说是 jcstress 中的一个错误，因为 @Actor 上的文档说不变量是：

Each method is called only by one particular thread.

Each method is called exactly once per State instance.

虽然文档措辞不明确，但生成的源代码清楚地表明其目的是运行每个参与者都在自己的线程中。

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();
    }
}

或者检查单线程情况并将其标记为 "interesting" 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，上述测试中的最终 @Outcome 从未发生过。这是因为单线程健全性检查不会在运行之前对 actor 进行洗牌（请参阅生成的测试 class 上的方法 sanityCheck_Footprints）。

对 ReentrantReadWriteLock#tryLock 失败的 jcstress 测试感到困惑

Confused by jcstress test on ReentrantReadWriteLock#tryLock failing

java

multithreading

concurrency

java.util.concurrent

jcstress