RFR: 8373367: interp-only mechanism fails to work for carrier threads in a corner case [v8]

[Leonid Mesnik]

On Sat, 21 Feb 2026 08:01:48 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.
>> 
>> Also, this update the includes the `interp_only` implementation simplifications and more asserts are added to relevant places. One of the simplification is a removal of the field `JvmtiThreadState::_thread_saved`.
>> 
>> Testing:
>>  - TBD: Mach5 tiers 1-6
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review: restore assert, will remove it as a separate bug fix

thanks for addressing feedback

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Marked as reviewed by lmesnik (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29436#pullrequestreview-3835361564