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Date: Sat, 6 May 2017 05:09:04 +0000 (UTC)
From: "Michael Han (JIRA)" <jira@apache.org>
To: dev@zookeeper.apache.org
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Subject: [jira] [Created] (ZOOKEEPER-2778) Potential server deadlock between
 follower sync with leader and follower receiving external connection
 requests.
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archived-at: Sat, 06 May 2017 05:09:10 -0000

Michael Han created ZOOKEEPER-2778:
--------------------------------------

             Summary: 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. . 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.


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