RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning

[Serguei Spitsyn]

On Wed, 6 Nov 2024 09:24:03 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> So at some point I think we need to figure out how to make them go away  ...

Yes, the 2 extension events (`VirtualThreadMount` and `VirtualThreadUnmount`) were added for testing purposes. We wanted to get rid of them at some point but the Graal team was using them for some purposes.

> It's posted before. We post the mount event at the end of thaw only if we are able to acquire the monitor...

The two extension events were designed to be posted when the current thread identity is virtual, so this behavior needs to be considered as a bug. My understanding is that it is not easy to fix. We most likely, we have no tests to fail because of this though.

> That's the path a virtual thread will take if pinned. 

Got it, thanks. I realize it is because we do not thaw and freeze the VM frames. It is not easy to comprehend.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1831293112