RFR: 8312777: notifyJvmtiMount before notifyJvmtiUnmount

[Alan Bateman]

On Sat, 14 Oct 2023 18:07:45 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> JVMTI is notified after a virtual thread unmounts. Right not, this notification is done after the virtual thread has finished parking or yielding so it's possible for the virtual to continue, and be mounted on a different carrier, before the unmount notification has completed on the original carrier. If this happens it means the the JVMTI mount and unmount notifications will race and it's possible they could be unbalanced. The unmount notification needs to move to after the unmount and before the virtual thread state is changed.
> 
> While in the area, I've removed @ChangesCurrentThread from VirtualThread.run. This annotation was in place to workaround an issue with the notifyJvmtiXXX instrinsics, fixed recently by JDK-8316130.
> 
> Testing: tier1-6.

I tested tier1-tier6 with the JVMTI notifications moved to mount/unmount and don't see any failures. So if you are okay with that then it would be a bit nicer. The main thing is that the notification is done before the virtual thread state changes.

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PR Comment: https://git.openjdk.org/jdk/pull/16194#issuecomment-1770419322