java - 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，其中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，一个线程不应该持有读锁，而另一个线程也持有写锁。同样，两个线程也不可能同时持有写锁。

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，所以我将重新发布到 StackOverflow，希望引起更多观众的注意。

Answer 1

当针对 jcstress 0.3 运行时，您的测试通过了。在 0.4 版中，行为更改为包括在启动时运行的健全性检查的结果(参见 this commit 针对错误 jcstress omits samples gathered during sanity checks)。

一些健全性检查在单个线程中运行，并且您的测试无法处理两个参与者都被同一个线程调用的情况；您正在测试一个可重入 锁，因此如果写锁已被持有，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在自己的线程中运行每个 actor。

解决它的一种方法是允许单线程情况通过:

@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“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，上述测试中的最终 @Outcome 永远不会发生。这是因为单线程健全性检查不会在运行 actor 之前对其进行洗牌(请参阅生成的测试类中的方法 sanityCheck_Footprints)。