[
https://issues.apache.org/jira/browse/ZOOKEEPER-4504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17514033#comment-17514033
]
Mohammad Arshad commented on ZOOKEEPER-4504:
--------------------------------------------
yes, I am sure problem is not because of low value of rateLimit. If it was
because of low concurrency, call would have taken bit longer but not stuck
forever. Also the call was not deleting a lot of znode, so there was no need to
submit multiple batches, one batch was sufficient.
> 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
> Priority: Critical
> 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.
--
This message was sent by Atlassian Jira
(v8.20.1#820001)
