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From "Michael K. Edwards (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2778) Potential server deadlock between follower sync with leader and follower receiving external connection requests.
Date Tue, 20 Nov 2018 13:27:01 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16693231#comment-16693231
] 

Michael K. Edwards commented on ZOOKEEPER-2778:
-----------------------------------------------

May I suggest a different approach?  There are three fragments of data here (myQuorumAddr,
myClientAddr, and myElectionAddr) that should be 1) updated atomically as a group, and 2)
aggressively made visible to concurrent threads on other CPUs.  There isn't really a need
to lock out access to them while other code that holds QV_LOCK runs.  Seems like an ideal
candidate for an AtomicReference to an immutable POJO that holds the three addresses.  Suggested
patch attached.

> 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: maoling
>            Priority: Blocker
>              Labels: pull-request-available
>             Fix For: 3.6.0, 3.5.5
>
>          Time Spent: 50m
>  Remaining Estimate: 0h
>
> 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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