Deadlock detection mechanism incorrectly assumes that thread cannot block itself

[Dmytro Sheyko]

Hi,

One more patch regarding deadlock detection.
https://bugs.openjdk.java.net/show_bug.cgi?id=100059

Deadlock detection mechanism assumes that thread cannot block itself. In 
general this is not right, especially in case of non-reentrant mutexes. One of 
such examples we can find here (FIFOMutex sample):
http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/LockSupport.html

To fix this, we just need to drop unnecessary code.

Attached updated patch and testcase

Thanks,
Dmytro
                                          

hotspot0.diff
Description: Binary data

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/*
 * @test
 * @bug     TBD
 * @summary Enter non-reentrant lock twice, expecting deadlock with a single 
thread cycle.
 *
 * @run main/othervm SelfDeadlockTest
 */

import java.lang.management.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.concurrent.locks.*;

public class SelfDeadlockTest implements Runnable {
    static FIFOMutex mutex = new FIFOMutex();

    public static void main(String... args) throws Exception {
        Thread thread = new Thread(new SelfDeadlockTest());
        thread.setDaemon(true);
        thread.start();
        Thread.sleep(1000);
        long[] expected = new long[] { thread.getId() };
        long[] actual = 
ManagementFactory.getThreadMXBean().findDeadlockedThreads();
        if (!Arrays.equals(expected, actual)) {
            throw new AssertionError("expected: " + Arrays.toString(expected) + 
"; actual: " + Arrays.toString(actual));
        }
        System.out.println("Test passed");
    }

    public void run() {
        mutex.lock();
        mutex.lock();
    }
}

class FIFOMutex extends AbstractOwnableSynchronizer {
    private final AtomicBoolean locked = new AtomicBoolean(false);
    private final Queue<Thread> waiters = new ConcurrentLinkedQueue<Thread>();

    public void lock() {
        boolean wasInterrupted = false;
        Thread current = Thread.currentThread();
        waiters.add(current);

        // Block while not first in queue or cannot acquire lock
        while (waiters.peek() != current ||
                !locked.compareAndSet(false, true)) {
            LockSupport.park(this);
            if (Thread.interrupted()) // ignore interrupts while waiting
                wasInterrupted = true;
        }

        setExclusiveOwnerThread(current);

        waiters.remove();
        if (wasInterrupted)          // reassert interrupt status on exit
            current.interrupt();
        }

    public void unlock() {
        setExclusiveOwnerThread(null);
        locked.set(false);
        LockSupport.unpark(waiters.peek());
    }
}