RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v2]

[Patricio Chilano Mateo]

On Thu, 7 Nov 2024 00:37:17 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> Thank you for the comment! I'm okay with your modified suggestion in general if there are no road blocks.
>> 
>>> but does the specs say the event has to be posted in the context of the vthread?
>> 
>> As Alan said below we do not have an official spec for this but still the events need to be posted in vthread context.
>> 
>>> For pop_frame/early_ret checks ...
>> 
>> The pop_frame/early_ret conditions are installed in handshakes with a context of `JvmtiVTMSTransitionDisabler`. As you noted the `JVMTI_ERROR_OPAQUE_FRAME` might be also returned by the JVMTI `FramePop` and `ForceEarlyReturn*` for some specific cases. So, it feels like it should not be a problem. I'm thinking if adding an assert at the VTMS transition end would help.
>> 
>>> Maybe we could go with this simplified code now and work on it later...
>> 
>> Whatever works better for you. An alternate approach could be to file an enhancement to simplify/refactor this.
>> It would be nice to fix a couple of nits though:
>>  - the call to `java_lang_Thread::set_is_in_VTMS_transition()`is not needed in `JvmtiUnmountBeginMark`
>>  - the function `is_vthread_safe_to_preempt()` does not need the `vthread` parameter
>
> Great, I applied the suggested simplification. I had to update test `VThreadEventTest.java` to check the stack during the mount/unmount events to only count the real cases. This is because now we are getting a variable number of spurious mount/unmount events (freeze failed) generated during the initialization of some class (`VirtualThreadEndEvent`) after the task is finished.

> So, it feels like it should not be a problem. I'm thinking if adding an assert at the VTMS transition end would help.
>
The problem here is that for monitorenter the top frame will not be a native method, so the bail out will not happen as it would when unmounting from Java.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1831898439