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

["Liya Fan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-12620?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16848865#comment-16848865 ] 

Liya Fan commented on FLINK-12620:
----------------------------------

[~mikekap] Thank you for finding this problem.

Would you please give more details about the problem, like the example code, or your patch?

> Deadlock in task deserialization
> --------------------------------
>
>                 Key: FLINK-12620
>                 URL: https://issues.apache.org/jira/browse/FLINK-12620
>             Project: Flink
>          Issue Type: Bug
>    Affects Versions: 1.8.0
>            Reporter: Mike Kaplinskiy
>            Priority: Major
>
> 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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