Mike Kaplinskiy created FLINK-12620:
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Summary: 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
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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