RFR: 8373367: interp-only mechanism fails to work for carrier threads in a corner case

[Leonid Mesnik]

On Tue, 27 Jan 2026 01:48:03 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> The `interp-only` mechanism is based on the `JavaThread` objects. Carrier and virtual threads can temporary share the same `JavaThread`. The `java_thread->jvmti_thread_state()` is re-linked to a virtual thread at `mount` and to the carrier thread at `unmount`. The `JvmtiThreadState` has a back link to the `JavaThread` which is also set for virtual thread at a `mount` and carrier thread at an `unmount`. Just one of these two links at the same time is set to the `JavaThread`, the other one has to be set to `nullptr`. The `interp-only` mechanism needs this invariant.
> However, there is a corner case when this invariant is broken. It happens when the `JvmtiThreadState` for carrier thread has just been created. In such case, the link to `JavaThread` is always `non-nullptr` even though a virtual thread is currently mounted on a carrier thread. This simple update fixes the issue in the  `JvmtiThreadState` ctor.
> 
> Testing:
>  - TBD: Mach5 tiers 1-6

src/hotspot/share/prims/jvmtiThreadState.cpp line 127:

> 125:     thread->set_interp_only_mode(false);
> 126:   }
> 127:   if (JvmtiEnvBase::is_thread_carrying_vthread(thread, thread_oop)) {

wouldn't be better to move this into the beginning of ctor:
  if(!JvmtiEnvBase::is_thread_carrying_vthread(thread, thread_oop)) {   
    _thread = thread;
    _thread_saved = nullptr;
  } else { 
    _thread_saved = thread;
    _thread = nullptr;
  }
and might be add comments with explanation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29436#discussion_r2733381607