RFR: 8343132: Remove temporary transitions from Virtual thread implementation [v2]

[David Holmes]

On Thu, 31 Oct 2024 07:19:10 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> This is an update to the Virtual thread implementation that we'd like to integrate in advance of JEP 491.
>> 
>> The update removes the use of  "temporary transitions", basically cases where the thread identity switches to the carrier thread to do something in the context of the carrier while a virtual thread is mounted. These cases create complexity for JVMTI and observability tools. It has also attracted attention in the review of the JEP 491 implementation as the object monitor changes have to deal with the possibility of entering monitors while in this state. There are 3 usages changes:
>> 
>> 1. In submitRunContinuation the submit to the scheduler is changed so that it executes in the context of a virtual thread for cases where one virtual thread unparks another. This requires pinning to prevent preemption during this sensitive operation. ForkJoinPool.poolSubmit is changed so that it uses the identity of the carrier. This change has no impact on the uses of lazySubmit or externalSubmit.
>> 2. Timed-park. The current implementation schedules/cancels the timer task with the virtual thread mounted. This runs in the context of the carrier as any contention would infer with thread state, park blocker and the parking permit. The implementation is changed to schedule the timeout after unmounting, and to cancel before re-mounting. The downside of this is that it will scheduled later (maybe 200us later than before). We could capture the time and adjust but it doesn't seem worth it.
>> 3. jdk.tracePinnedThreads. This is a diagnostic option for finding usages of thread locals in code executed by virtual threads. This is changed so use a thread local to detect reentrancy.
>> 
>> The changes means that notifyJvmtiHideFrames, its intrinsic, and the JVMTI "tmp VTMS_transition" bit go away.
>
> Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Fix typo in comment
>  - Merge branch 'master' into JDK-8343132
>  - Merge branch 'master' into JDK-8343132
>  - Initial commit

Marked as reviewed by dholmes (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/21735#pullrequestreview-2407378053