RFR: 8315373: Change VirtualThread to unmount after freezing, re-mount before thawing

[Patricio Chilano Mateo]

On Thu, 31 Aug 2023 10:37:26 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> In the virtual thread implementation, thread identity switches to the carrier before freezing and switches back to the virtual thread after thawing. This was a forced move due to issues getting JVMTI to work with virtual threads. JVMTI can now hide events during transitions so we can invert the sequence back to mounting before running the continuation, unmounting after freezing, and re-mounting after thawing. This sequence is important for future changes that will initiate the freezing from the VM.
>> 
>> The change requires an update to the JFR thread sampler to skip sampling when it samples during a transition.
>> 
>> Testing: tier1-5
>
> src/hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp line 415:
> 
>> 413:   } else {
>> 414:     assert(NATIVE_SAMPLE == type, "invariant");
>> 415:     if (thread_state_in_native(thread) && !is_vthread_in_transition(thread)) {
> 
> Is this possible? I assume the thread is in _thread_in_Java during the transition?

There are some native methods that we execute during mount/unmount transitions. From what I see they all seem to be defined as `@IntrinsicCandidate`, but if for some reason we don't execute the intrinsic version (interp only mode for instance) then we would call a normal native method.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15492#discussion_r1311970316