RFR: 8319244: implement JVMTI handshakes support for virtual threads [v4]

[Serguei Spitsyn]

On Fri, 17 Nov 2023 02:10:50 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> If it is a part of the `JvmtiEnvBase::is_vthread_alive()` then it is racy for mounted virtual threads.
>> It is not racy for unmounted virtual threads.
>
> So we should only see that a continuation is done for an unmounted vthread. The last place where we could see a mounted vthread is at notifyJvmtiEnd(), blocked in start_VTMS_transition(), but the continuation is not marked done yet. 
> Also I realize the window for the problematic case I mentioned starts even earlier at notifyJvmtiEnd(), not notifyJvmtiUnmount(), because blocking due to JvmtiVTMSTransitionDisabler happens in start_VTMS_transition() not finish_VTMS_transition(). Once the vthread executed notifyJvmtiEnd() any JvmtiVTMSTransitionDisabler that happens afterwards will fall into this case. So maybe the first sentence of the comment should instead be: "The vthread could have already executed the last unmount but might not have changed state to TERMINATED yet."

Thank you. Now I see that `done` is set to `true` during an unmount transition.
So, I'm convinced to move the check for `jdk_internal_vm_Continuation::done(cont)` to `JvmtiEnvBase::is_vthread_alive()`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16460#discussion_r1396695782