RFR: 8364343: Virtual Thread transition management needs to be independent of JVM TI [v4]

[Coleen Phillimore]

On Tue, 25 Nov 2025 23:49:29 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/runtime/mountUnmountDisabler.cpp line 126:
>> 
>>> 124:          || global_start_transition_disable_count() > base_disable_count
>>> 125:          JVMTI_ONLY(|| (JvmtiVTSuspender::is_vthread_suspended(java_lang_Thread::thread_id(vthread)) || thread->is_suspended()));
>>> 126: }
>> 
>> I like this approach with the JVMTIStartTransition and JVMTIEndTransition helper classes. It is a nice way to decouple the JVMTI part of the protocol. Introducing the `is_start_transition_disabled()` function was also long desired. Also, I like the functions `start_transition()` and `end_transition()` became pretty simple and clean!
>
> This is the function that needs a comment why you're testing all these things (and why base_disable_count is one for JVMTI). It's nice as a function that tests all the different values.

Looks good.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2578310137