RFR: 8361912: ThreadsListHandle::cv_internal_thread_to_JavaThread does not deal with a virtual thread's carrier thread

[Alex Menkov]

On Mon, 14 Jul 2025 05:34:57 GMT, David Holmes <dholmes at openjdk.org> wrote:

> The `cv_internal_thread_to_JavaThread` method will now check if the thread is a mounted virtual thread, and if so protect the carrier thread. User's of the API that need to deal with virtual threads must still check the carrier themselves as it could change asynchronously, but it is now guaranteed that the carrier JavaThread returned via this method cannot terminate whilst being used (the same as regular platform JavaThreads).
> 
> It was noticed whilst updating the documentation that the `JvmtiExport` variant `cv_oop_to_JavaThread` is unused and so can be removed.
> 
> Testing:
> - tiers 1-4
> 
> Thanks

src/hotspot/share/runtime/threadSMR.hpp line 78:

> 76: // longer protected by a ThreadsListHandle.
> 77: //
> 78: // Note that for virtual threads, we obtain a reference to the carrier JavaThread

This is not correct for `JvmtiExport::cv_external_thread_to_JavaThread` (it does not return carrier's JavaThread).
JVMTI has `JvmtiEnvBase::get_threadOop_and_JavaThread` and `JvmtiEnvBase::get_JavaThread_or_null` to get carrier thread

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PR Review Comment: https://git.openjdk.org/jdk/pull/26287#discussion_r2219977613