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https://issues.apache.org/jira/browse/ZOOKEEPER-2778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15999307#comment-15999307
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Hadoop QA commented on ZOOKEEPER-2778:
--------------------------------------
+1 overall. GitHub Pull Request Build
+1 @author. The patch does not contain any @author tags.
+0 tests included. The patch appears to be a documentation patch that
doesn't require tests.
+1 javadoc. The javadoc tool did not generate any warning messages.
+1 javac. The applied patch does not increase the total number of javac
compiler warnings.
+1 findbugs. The patch does not introduce any new Findbugs (version 3.0.1)
warnings.
+1 release audit. The applied patch does not increase the total number of
release audit warnings.
+1 core tests. The patch passed core unit tests.
+1 contrib tests. The patch passed contrib unit tests.
Test results:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/667//testReport/
Findbugs warnings:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/667//artifact/trunk/build/test/findbugs/newPatchFindbugsWarnings.html
Console output:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/667//console
This message is automatically generated.
> Potential server deadlock between follower sync with leader and follower
> receiving external connection requests.
> ----------------------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-2778
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2778
> Project: ZooKeeper
> Issue Type: Bug
> Components: quorum
> Affects Versions: 3.5.3
> Reporter: Michael Han
> Assignee: Michael Han
> Priority: Critical
>
> It's possible to have a deadlock during recovery phase.
> Found this issue by analyzing thread dumps of "flaky" ReconfigRecoveryTest
> [1]. . Here is a sample thread dump that illustrates the state of the
> execution:
> {noformat}
> [junit] java.lang.Thread.State: BLOCKED
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumPeer.getElectionAddress(QuorumPeer.java:686)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumCnxManager.initiateConnection(QuorumCnxManager.java:265)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumCnxManager.connectOne(QuorumCnxManager.java:445)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumCnxManager.receiveConnection(QuorumCnxManager.java:369)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumCnxManager$Listener.run(QuorumCnxManager.java:642)
> [junit]
> [junit] java.lang.Thread.State: BLOCKED
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumCnxManager.connectOne(QuorumCnxManager.java:472)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumPeer.connectNewPeers(QuorumPeer.java:1438)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumPeer.setLastSeenQuorumVerifier(QuorumPeer.java:1471)
> [junit] at
> org.apache.zookeeper.server.quorum.Learner.syncWithLeader(Learner.java:520)
> [junit] at
> org.apache.zookeeper.server.quorum.Follower.followLeader(Follower.java:88)
> [junit] at
> org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1133)
> {noformat}
> The dead lock happens between the quorum peer thread which running the
> follower that doing sync with leader work, and the listener of the qcm of the
> same quorum peer that doing the receiving connection work. Basically to
> finish sync with leader, the follower needs to synchronize on both QV_LOCK
> and the qmc object it owns; while in the receiver thread to finish setup an
> incoming connection the thread needs to synchronize on both the qcm object
> the quorum peer owns, and the same QV_LOCK. It's easy to see the problem here
> is the order of acquiring two locks are different, thus depends on timing /
> actual execution order, two threads might end up acquiring one lock while
> holding another.
> [1]
> org.apache.zookeeper.server.quorum.ReconfigRecoveryTest.testCurrentServersAreObserversInNextConfig
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