[ZOOKEEPER-2778] Potential server deadlock between follower sync with leader and follower receiving external connection requests.

• Type: Bug
• Status: Closed
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: 3.5.3
• Fix Version/s: 3.6.0, 3.5.5
• Component/s: quorum
• Labels: pull-request-available
• Assignee: Michael K. Edwards
• Reporter: Michael Han
• Created: 06/May/17 05:08
• Updated: 04/Oct/19 14:55
• Resolved: 07/Dec/18 12:12

Description

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:

    [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)

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

Issue Links

• is broken by: ZOOKEEPER-2080 Fix deadlock in dynamic reconfiguration
• is related to: ZOOKEEPER-2080 Fix deadlock in dynamic reconfiguration
• links to: GitHub Pull Request #247
• links to: GitHub Pull Request #247
• links to: GitHub Pull Request #707
• links to: GitHub Pull Request #715
• links to: GitHub Pull Request #719