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https://issues.apache.org/jira/browse/ZOOKEEPER-2380?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15180222#comment-15180222
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Rakesh R commented on ZOOKEEPER-2380:
-------------------------------------
Good catch, [~arshad.mohammad]. Probably we could try moving the
{{this.shutdown}} call outside synchronization block.
*Deadlock sequence is:*
Quorum Leader Thread:
1=> {{leader.lead()}}
2=> Say, leader losts quorum and call {{this.shutdown()}}. This shutdown call
is made under {{synchronized (this)}}
[Leader.java#L554|https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/Leader.java#L554]
3=> Shutdown will trigger {{firstProcessor.shutdown();}} and reaches
{{SyncRequestProcessor#shutdown()}} call.
4=> Now, SyncRequestProcessor will add {{requestOfDeath}} and wait to
{{this.join()}};
SyncThread:
1=> SyncRequestProcessor forwards the request to its next processor
{{nextProcessor.processRequest(si);}}, which is AckRequestProcessor.
2=> AckRequestProcessor, forwards the request the leader {{leader.processAck}}
3=> Since {{leader.processAck}} is synchronized at method level and requires
{{leader.this}} lock
[Leader.java#L783|https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/Leader.java#L783].
Since {{leader.this}} is already acquired by
[Leader.java#L554|https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/Leader.java#L554],
thus results in deadlock.
> Deadlock while shutting down the zookeeper
> ------------------------------------------
>
> Key: ZOOKEEPER-2380
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2380
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.5.0
> Reporter: Arshad Mohammad
> Assignee: Arshad Mohammad
> Priority: Blocker
>
> Zookeeper enters into deadlock while shutting down itself, thus making
> zookeeper service unavailable as deadlocked server is a leader. Here is the
> thread dump:
> {code}
> "QuorumPeer[myid=1](plain=/0:0:0:0:0:0:0:0:2181)(secure=disabled)" #25 prio=5
> os_prio=0 tid=0x00007fbc502a6800 nid=0x834 in Object.wait()
> [0x00007fbc4d9a8000] java.lang.Thread.State: WAITING (on object monitor)
> at java.lang.Object.wait(Native Method) at
> java.lang.Thread.join(Thread.java:1245) - locked <
> 0x00000000feb78000> (a org.apache.zookeeper.server.SyncRequestProcessor)
> at java.lang.Thread.join(Thread.java:1319) at
> org.apache.zookeeper.server.SyncRequestProcessor.shutdown(SyncRequestProcessor.java:196)
> at
> org.apache.zookeeper.server.quorum.ProposalRequestProcessor.shutdown(ProposalRequestProcessor.java:90)
> at
> org.apache.zookeeper.server.PrepRequestProcessor.shutdown(PrepRequestProcessor.java:1016)
> at
> org.apache.zookeeper.server.quorum.LeaderRequestProcessor.shutdown(LeaderRequestProcessor.java:78)
> at
> org.apache.zookeeper.server.ZooKeeperServer.shutdown(ZooKeeperServer.java:561)
> - locked <
> 0x00000000feb61e20> (a
> org.apache.zookeeper.server.quorum.LeaderZooKeeperServer) at
> org.apache.zookeeper.server.quorum.QuorumZooKeeperServer.shutdown(QuorumZooKeeperServer.java:169)
> - locked <
> 0x00000000feb61e20> (a
> org.apache.zookeeper.server.quorum.LeaderZooKeeperServer) at
> org.apache.zookeeper.server.quorum.LeaderZooKeeperServer.shutdown(LeaderZooKeeperServer.java:102)
> - locked <
> 0x00000000feb61e20> (a
> org.apache.zookeeper.server.quorum.LeaderZooKeeperServer) at
> org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:637) at
> org.apache.zookeeper.server.quorum.Leader.lead(Leader.java:590) - locked
> <
> 0x00000000feb781a0> (a org.apache.zookeeper.server.quorum.Leader) at
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1108)
> "SyncThread:1" #46 prio=5 os_prio=0 tid=0x00007fbc5848f000 nid=0x867 waiting
> for monitor entry [0x00007fbc4ca90000] java.lang.Thread.State: BLOCKED
> (on object monitor) at
> org.apache.zookeeper.server.quorum.Leader.processAck(Leader.java:784) -
> waiting to lock <0x00000000feb781a0> (a
> org.apache.zookeeper.server.quorum.Leader) at
> org.apache.zookeeper.server.quorum.AckRequestProcessor.processRequest(AckRequestProcessor.java:46)
> at
> org.apache.zookeeper.server.SyncRequestProcessor.flush(SyncRequestProcessor.java:183)
> at
> org.apache.zookeeper.server.SyncRequestProcessor.run(SyncRequestProcessor.java:113)
> {code}
> Leader.lead() calls shutdown() from the synchronized block, it acquired lock
> on Leader.java instance
> {code}
> while (true) {
> synchronized (this) {
> long start = Time.currentElapsedTime();
> .....
> {code}
> In the shutdown flow SyncThread is trying to acquire lock on the same
> Leader.java instance.
> Leader thread acquired lock and waiting for SyncThread shutdown. SyncThread
> waiting for the lock to complete its shutdown. This is how ZooKeeper entered
> into deadlock
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