RFR: 8256811: Delayed/missed jdwp class unloading events [v3]

[Serguei Spitsyn]

On Fri, 24 Jun 2022 04:20:45 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 18 additional commits since the last revision:
>> 
>>  - Improve naming and cleanup
>>  - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event
>>  - v4
>>  - v3
>>  - v2
>>  - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event
>>  - Merge branch 'jdi_tmp' into JDK-8256811-jdi-missing-class-unloading-event
>>  - v0
>>  - v2
>>  - v1
>>  - ... and 8 more: https://git.openjdk.org/jdk/compare/fae7ca88...559b4bf1
>
> src/hotspot/share/prims/jvmtiExport.cpp line 1702:
> 
>> 1700:   } else {
>> 1701:     post_object_free_on_java_thread(env, objects);
>> 1702:   }
> 
> Can you explain why sometimes it is a VMThread and why sometimes it is a JavaThread?

I hope, it is okay if I explain it. :)
The `post_object_free` can be called in different contexts.
It is (recursively) called from `JvmtiTagMap::check_hashmaps_for_heapwalk()` which is used in VM_ops:
- VM_HeapIterateOperation
- VM_HeapWalkOperation

Also, the `post_object_free` is called from `JvmtiTagMap` functions:
- flush_object_free_events
- get_objects_with_tags
which can be called from the `ServiceThread` or the JVMTI functions like `SetEvenNotificationMode`, `SetEventCallBacks` or `GetObjectsWithTags`.

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PR: https://git.openjdk.org/jdk/pull/9168