RFR: 8281615: Deadlock caused by jdwp agent [v4]

[Zhengyu Gu]

On Thu, 17 Feb 2022 06:57:07 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Simplify classTrack_reset and revert bagSize check
>
> Actually a bit of a correction to my last statement above. processUnloads is only called if there has been a GC since the last event came in. This also reduces the need to optimize processUnloads for the case where deletedSignatures is NULL or is empty.

> Thanks for the additional explanation @plummercj. I agree with your suggestion to move this forward and revisit.
> 
> I suspect the issue with directly issuing the CLASS_UNLOAD event is due to when, and on what thread, the OBJECT_FREE event is issued.

I believe OBJECT_FREE event is always issued from VMThread, at a GC safepoint prior to JDK16 or a dedicated safepoint post.

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PR: https://git.openjdk.java.net/jdk/pull/7461