RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v5]

[Alan Bateman]

On Tue, 22 Oct 2024 19:02:50 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> Just to say that we hope to eventually remove these "temporary transitions". This PR brings in a change that we've had in the loom repo to not need this when calling out to the scheduler. The only significant remaining use is timed-park. Once we address that then we will remove the need to switch the thread identity and remove some complexity, esp. for JVMTI and serviceability.
>> 
>> In the mean-time, yes, the JavaThread.lock_id will temporarily switch to the carrier so a thread-dump/safepoint at just the right time looks like it print will be tid of the carrier rather than the mounted virtual thread. So we should fix that. (The original code in main line skipped this case so was lossy when taking a thread dump when hitting this case, David might remember the discussion on that issue).
>
> The problem is that within that window we don't have access to the virtual thread's tid. The current thread has already been changed and we haven't yet set the lock id back. Since this will be a rare corner case maybe we can just print tid unavailable if we hit it. We could also add a boolean to setCurrentThread to indicate we don't want to change the lock_id, but not sure it's worth it.

It should be rare and once we make further progress on timers then the use of temporary transitions will mostly disappear. I think the main thing for the thread dump is not to print a confusing "Carrying virtual thread" with the tid of the carrier. This came up in [pull/19482](https://github.com/openjdk/jdk/pull/19482) when the thread was extended.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1812377091