[jira] [Created] (ZOOKEEPER-4504) ZKUtil#deleteRecursive causing deadlock in HDFS HA functionality

["Mohammad Arshad (Jira)" <ji...@apache.org>]

Mohammad Arshad created ZOOKEEPER-4504:
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             Summary: ZKUtil#deleteRecursive causing deadlock in HDFS HA functionality
                 Key: ZOOKEEPER-4504
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-4504
             Project: ZooKeeper
          Issue Type: Bug
            Reporter: Mohammad Arshad
            Assignee: Mohammad Arshad
             Fix For: 3.7.1, 3.6.4, 3.9.0, 3.8.1


*Problem and Analysis:*
After integrating ZooKeeper 3.6.3 we observed deadlock in HDFS HA functionality as shown in below thread dumps.
{code:java}
"main-EventThread" #33 daemon prio=5 os_prio=0 tid=0x00007f9c017f1000 nid=0x101b waiting for monitor entry [0x00007f9bda8a6000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at org.apache.hadoop.ha.ActiveStandbyElector.processWatchEvent(ActiveStandbyElector.java:603)
	- waiting to lock <0x00000000c17986c0> (a org.apache.hadoop.ha.ActiveStandbyElector)
	at org.apache.hadoop.ha.ActiveStandbyElector$WatcherWithClientRef.process(ActiveStandbyElector.java:1193)
	at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:626)
	at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:582)
{code}
{code:java}
"main" #1 prio=5 os_prio=0 tid=0x00007f9c00060000 nid=0xea3 waiting on condition [0x00007f9c06404000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x00000000c1b383c8> (a java.util.concurrent.Semaphore$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:838)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:999)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1306)
	at java.util.concurrent.Semaphore.acquire(Semaphore.java:467)
	at org.apache.zookeeper.ZKUtil.deleteInBatch(ZKUtil.java:122)
	at org.apache.zookeeper.ZKUtil.deleteRecursive(ZKUtil.java:64)
	at org.apache.zookeeper.ZKUtil.deleteRecursive(ZKUtil.java:76)
	at org.apache.hadoop.ha.ActiveStandbyElector$1.run(ActiveStandbyElector.java:386)
	at org.apache.hadoop.ha.ActiveStandbyElector$1.run(ActiveStandbyElector.java:383)
	at org.apache.hadoop.ha.ActiveStandbyElector.zkDoWithRetries(ActiveStandbyElector.java:1103)
	at org.apache.hadoop.ha.ActiveStandbyElector.zkDoWithRetries(ActiveStandbyElector.java:1095)
	at org.apache.hadoop.ha.ActiveStandbyElector.clearParentZNode(ActiveStandbyElector.java:383)
	- locked <0x00000000c17986c0> (a org.apache.hadoop.ha.ActiveStandbyElector)
	at org.apache.hadoop.ha.ZKFailoverController.formatZK(ZKFailoverController.java:290)
	at org.apache.hadoop.ha.ZKFailoverController.doRun(ZKFailoverController.java:227)
	at org.apache.hadoop.ha.ZKFailoverController.access$000(ZKFailoverController.java:66)
	at org.apache.hadoop.ha.ZKFailoverController$1.run(ZKFailoverController.java:186)
	at org.apache.hadoop.ha.ZKFailoverController$1.run(ZKFailoverController.java:182)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:360)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1741)
	at org.apache.hadoop.security.SecurityUtil.doAsLoginUserOrFatal(SecurityUtil.java:498)
	at org.apache.hadoop.ha.ZKFailoverController.run(ZKFailoverController.java:182)
	at org.apache.hadoop.hdfs.tools.DFSZKFailoverController.main(DFSZKFailoverController.java:220)
{code}
org.apache.hadoop.ha.ActiveStandbyElector#clearParentZNode is instance synchronized and calls ZKUtil.deleteRecursive(zk, pathRoot)

ZKUtil.deleteRecursive is async API call and in callback it is invoking ActiveStandbyElector#processWatchEvent which is synchronized on ActiveStandbyElector instance.

So there is deadlock, clearParentZNode() is waiting processWatchEvent() to complete and processWatchEvent() is waiting clearParentZNode to complete

 

*Why this problem was not happening with earlier versions (3.5.x)?*

In earlier zk versions, ZKUtil.deleteRecursive was using sync zk API intnernally. So there was no callback (processWatchEvent) coming into the scenario.


*Proposed Fix:*
There are two approaches to fix this problem. 
1. We can fix the problem in HDFS, modify the HDFS code to avoid the deadlock. But we may get similar bugs in other projects.
2. Fix the problem in ZK. Make the API behavior same as the old behavior(use sync API to delete the ZK node) and provide new overloaded API with new behavior(use async API to delete the ZK node)

I propose to fix the problem with 2nd approach.



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