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

[Coleen Phillimore]

On Fri, 24 Jun 2022 12:24:27 GMT, Coleen Phillimore <coleenp 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/3187f126...559b4bf1
>
> src/hotspot/share/prims/jvmtiTagMap.cpp line 1200:
> 
>> 1198: };
>> 1199: 
>> 1200: // PostObjectFree can't be called by JavaThread, so call it from the VM thread.
> 
> I wish I'd written why here.  I think it's because we can't hold a Mutex to do the callback, but now I don't remember. I need to spend some time to understand this.  Thanks @plummercj  for adding Kim and I.

Ok, thanks I reread your description and see it now.  At this point, you are neither holding the Mutex and have already collected the objects, so that you can transition to native.

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