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From "Cameron McKenzie (JIRA)" <>
Subject [jira] [Commented] (CURATOR-172) Deadlock when performing background operation
Date Wed, 17 Dec 2014 21:47:13 GMT


Cameron McKenzie commented on CURATOR-172:

Had a bit more of a look, and I don't think that this is the issue the we encountered, as
it seems that this can only happen when there's connection loss to ZK and we don't seem to
be experiencing this.

I did step through the process with a debugger though and had a look at the ZK code, and I'm
not quite sure how this situation can arise. The only way it seems possible is if the packet
never finishes, but the timeout and connection loss handling seems to handle these cases.
The second curator thread is being blocked because the checkTimeouts() method is synchronized,
but the root of it is that the packet.finished state is not being reached for some reason.
I don't know why this is though. It doesn't seem to me to be related to locking issues.

> Deadlock when performing background operation
> ---------------------------------------------
>                 Key: CURATOR-172
>                 URL:
>             Project: Apache Curator
>          Issue Type: Bug
>          Components: Client
>    Affects Versions: 2.4.2
>         Environment: Linux HOSTNAME-REMOVED 2.6.32-279.19.1.el6.x86_64 #1 SMP Tue Dec
18 15:04:44 PST 2012 x86_64 x86_64 x86_64 GNU/Linux
> java version "1.7.0_60"
> Java(TM) SE Runtime Environment (build 1.7.0_60-b19)
> Java HotSpot(TM) 64-Bit Server VM (build 24.60-b09, mixed mode)
>            Reporter: Tom Byrne
> Had a box get into a state where our ZK connections were all deadlocked, waiting on an
object monitor. jstack shows that our background thread that was creating a node was waiting
on a lock that was held by the CuratorFramework thread, who was waiting on an object monitor
that looks like it couldn't be completed until our other write was finished (packet.finish
would never return true.) 
> We have seen this happen twice, but don't notice it until afterwards, and don't have
enough logging to know what's triggering it (possible ZK connections going away?) 
> Rest of the box is fine, network connections are not flapping, main IO threads continue
to accept and process connections, until we get backed up waiting for ZK. 
> Here are the two stack traces:
> "ZooChangeWatcher-BackgroundReader--2-1-SendThread()" daemon prio=10 tid=0x00007fcf64108000
nid=0x88d waiting for monitor entry [0x00007fcbf5d16000]
>    java.lang.Thread.State: BLOCKED (on object monitor)
> 	at org.apache.curator.ConnectionState.checkTimeouts(
> 	- waiting to lock <0x00000000d526bcc8> (a org.apache.curator.ConnectionState)
> 	at org.apache.curator.ConnectionState.getZooKeeper(
> 	at org.apache.curator.CuratorZookeeperClient.getZooKeeper(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.performBackgroundOperation(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.processBackgroundOperation(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl.pathInBackground(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl.forPath(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl.forPath(
> 	at
> 	at$000(
> 	at$4.processResult(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.sendToBackgroundCallback(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.checkBackgroundRetry(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.processBackgroundOperation(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl.sendBackgroundResponse(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl.access$600(
> 	at org.apache.curator.framework.imps.CreateBuilderImpl$6.processResult(
> 	at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(
> 	at org.apache.zookeeper.ClientCnxn$EventThread.queuePacket(
> 	- locked <0x00000000fa8e16f8> (a java.util.concurrent.LinkedBlockingQueue)
> 	at org.apache.zookeeper.ClientCnxn.finishPacket(
> 	at org.apache.zookeeper.ClientCnxn.conLossPacket(
> 	at org.apache.zookeeper.ClientCnxn.access$2400(
> 	at org.apache.zookeeper.ClientCnxn$SendThread.cleanup(
> 	- locked <0x00000000fa8e1380> (a java.util.LinkedList)
> 	at org.apache.zookeeper.ClientCnxn$
> "CuratorFramework-0" daemon prio=10 tid=0x00007fd02cb57800 nid=0x4425 in Object.wait()
>    java.lang.Thread.State: WAITING (on object monitor)
> 	at java.lang.Object.wait(Native Method)
> 	at java.lang.Object.wait(
> 	at org.apache.zookeeper.ClientCnxn.submitRequest(
> 	- locked <0x00000000fa8e6750> (a org.apache.zookeeper.ClientCnxn$Packet)
> 	at org.apache.zookeeper.ClientCnxn.close(
> 	at org.apache.zookeeper.ZooKeeper.close(
> 	- locked <0x00000000fa8e0948> (a org.apache.zookeeper.ZooKeeper)
> 	at org.apache.curator.HandleHolder.internalClose(
> 	at org.apache.curator.HandleHolder.closeAndReset(
> 	at org.apache.curator.ConnectionState.reset(
> 	- locked <0x00000000d526bcc8> (a org.apache.curator.ConnectionState)
> 	at org.apache.curator.ConnectionState.checkTimeouts(
> 	- locked <0x00000000d526bcc8> (a org.apache.curator.ConnectionState)
> 	at org.apache.curator.ConnectionState.getZooKeeper(
> 	at org.apache.curator.CuratorZookeeperClient.getZooKeeper(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.performBackgroundOperation(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.backgroundOperationsLoop(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl.access$300(
> 	at org.apache.curator.framework.imps.CuratorFrameworkImpl$
> 	at java.util.concurrent.FutureTask$Sync.innerRun(
> 	at
> 	at java.util.concurrent.ThreadPoolExecutor.runWorker(
> 	at java.util.concurrent.ThreadPoolExecutor$
> 	at
> Help me Obi-Wan Kenobi, you're my only hope. 

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