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

[Serguei Spitsyn]

On Tue, 7 Nov 2023 14:44:52 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   review: get rid of the VM_HandshakeUnmountedVirtualThread
>
> src/hotspot/share/prims/jvmtiEnvBase.cpp line 1974:
> 
>> 1972: 
>> 1973:   if (java_lang_VirtualThread::is_instance(target_h())) { // virtual thread
>> 1974:     if (!JvmtiEnvBase::is_vthread_alive(target_h())) {
> 
> There is only one issue I see in how this check is implemented and the removal of the VM_op for unmounted vthreads. The change of state to TERMINATED happens after notifyJvmtiUnmount(), i.e we can see that this vthread is alive here but a check later can return is not. This might hit the assert in JvmtiEnvBase::get_vthread_jvf() (maybe this the issue you saw on your first prototype). We can either change that order at the Java level, or maybe better change this function to read the state and add a case where if the state is RUNNING check whether the continuation is done or not (jdk_internal_vm_Continuation::done(cont)).

Thank you for the suggestion. Will check it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16460#discussion_r1386875290