Is it possible for ConcurrentHashMap to "deadlock"?

Question

We have come across a strange issue with ConcurrentHashMap, where two threads appears to be calling put(), and then waiting forever inside the method Unsafe.park(). From the outside, it looks like a deadlock inside ConcurrentHashMap.

We have only seen this happen once so far.

Can anyone think of anything that could cause these symptoms?

EDIT: The thread dump for the relevant threads is here:

  "[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

I don't think this is what's happening in your case, but it is possible to write a deadlock with a single instance of ConcurrentHashMap, and it only needs one thread! Kept me stuck for quite a while.

Let's say you're using a ConcurrentHashMap<String, Integer> to calculate a histogram. You might do something like this:

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

Which works just fine.

But let's say you decide instead to write it like this:

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

You will now get a 1-thread deadlock with a stack like this:

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   

In the example above, it's easy to see that we're attempting to modify the map inside of an atomic modification, which seems like a bad idea. However, if there are a hundred stack frames of event-callbacks between the call to map.compute and map.putIfAbsent, then it can be quite difficult to track down.