ConcurrentHashMap是否可能"死锁"？

Question

我们遇到了一个奇怪的问题ConcurrentHashMap,其中两个线程似乎在调用put(),然后在方法内永远等待Unsafe.park().从外面看,它看起来像是一个僵局ConcurrentHashMap.

到目前为止,我们只看到过这种情况.

谁能想到任何可能导致这些症状的事情？

编辑:相关线程的线程转储在这里:

"[redacted] Thread 2" prio=10 tid=0x000000005bbbc800 nid=0x921 waiting on condition [0x0000000040e93000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00002aaaf1207b40> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:747)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:778)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1114)
    at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:186)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
    at java.util.concurrent.ConcurrentHashMap$Segment.put(ConcurrentHashMap.java:417)
    at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:883)
    at [redacted]


"[redacted] Thread 0" prio=10 tid=0x000000005bf38000 nid=0x91f waiting on condition [0x000000004151d000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00002aaaf1207b40> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:747)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:778)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1114)
    at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:186)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
    at java.util.concurrent.ConcurrentHashMap$Segment.put(ConcurrentHashMap.java:417)
    at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:883)
    at [redacted]

Answer 1

我不认为这是你的情况下发生的事情,但是可以用单个实例写一个死锁,ConcurrentHashMap它只需要一个线程!让我坚持了很长一段时间.

假设您使用a ConcurrentHashMap<String, Integer>来计算直方图.你可能会这样做:

int count = map.compute(key, (k, oldValue) -> {
    return oldValue == null ? 1 : oldValue + 1;
});

哪个工作得很好.

但是让我们说你决定改写它:

int count = map.compute(key, (k, oldValue) -> {
    return map.putIfAbsent(k, 0) + 1;
});

你现在将得到一个带有这样的堆栈的1线程死锁:

Thread [main] (Suspended)   
    owns: ConcurrentHashMap$ReservationNode<K,V>  (id=25)   
    ConcurrentHashMap<K,V>.putVal(K, V, boolean) line: not available    
    ConcurrentHashMap<K,V>.putIfAbsent(K, V) line: not available    
    ConcurrentHashMapDeadlock.lambda$0(ConcurrentHashMap, String, Integer) line: 32 
    1613255205.apply(Object, Object) line: not available    
    ConcurrentHashMap<K,V>.compute(K, BiFunction<? super K,? super V,? extends V>) line: not available  

在上面的例子中,我们很容易看到我们试图在原子修改中修改地图,这看起来是个坏主意.但是,如果在调用map.compute和之间有一百个事件回调堆栈帧map.putIfAbsent,则跟踪起来可能非常困难.

Answer 2

也许不是您想要的答案，但这可能是一个 JVM 错误。请参阅JDK 6865591

Test6471091.java 在 Solaris-i586 上挂起