[jira] [Commented] (FLINK-12620) Deadlock in task deserialization

["Mike Kaplinskiy (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-12620?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16859581#comment-16859581 ] 

Mike Kaplinskiy commented on FLINK-12620:
-----------------------------------------

Unfortunately my use-case is a bit strange - I'm running Flink via Beam - and via a Clojure API on top of Beam at that.

Attached [^BatchJob.java] is my attempt to recreate the issue - via Flink's Java API. I triggered it by making a diamond - but I'm pretty sure you can trigger this issue easier via the stream api - where there is always more than 1 subtask running. Also attached is the jstack output [^jstack_repro.txt] from trying to run the job. Let me know if you need anything else.

> Deadlock in task deserialization
> --------------------------------
>
>                 Key: FLINK-12620
>                 URL: https://issues.apache.org/jira/browse/FLINK-12620
>             Project: Flink
>          Issue Type: Bug
>          Components: Runtime / Task
>    Affects Versions: 1.8.0
>            Reporter: Mike Kaplinskiy
>            Priority: Major
>         Attachments: BatchJob.java, jstack_repro.txt, jstack_snippet.txt
>
>
> When running a batch job, I ran into an issue where task deserialization caused a deadlock. Specifically, if you have a static initialization dependency graph that looks like this (these are all classes):
> {code:java}
> Task1 depends on A
>     A depends on B
>     B depends on C
>     C depends on B [cycle]
> Task2 depends on C{code}
> What seems to happen is a deadlock. Specifically, threads are started on the task managers that simultaneously call BatchTask.instantiateUserCode on both Task1 and Task2. This starts deserializing the classes and initializing them. Here's the deadlock scenario, as a stack:
> {code:java}
> Time---->
> T1: [deserialize] -> Task1<clinit> -> A<clinit> -> B<clinit> -> (wait for C<clinit>)
> T2: [deserialize] -> Task2<clinit>              -> C<clinit> -> (wait for B<clinit>){code}
>  
> A similar scenario from the web: [https://www.farside.org.uk/201510/deadlocks_in_java_class_initialisation] .
>  
> For my specific problem, I'm running into this within Clojure - {{clojure.lang.RT}} has a dep on {{clojure.lang.Util}} which has a dep with {{clojure.lang.Numbers}} which depends on {{clojure.lang.RT}} again. Deserializing different clojure functions calls one or the other first which deadlocks task managers.
>  
> I built a version of flink-core that had {{org.apache.flink.util.InstantiationUtil.readObjectFromConfig}} synchronized, but I'm not sure that it's the proper fix. I'm happy to submit that as a patch, but I'm not familiar enough with the codebase to say that it's the correct solution - ideally all Java class loading is synchronized, but I'm not sure how to do that.



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