RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v9]

[Patricio Chilano Mateo]

On Mon, 28 Oct 2024 00:35:11 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> This vthread was unmounted on the call to `Object.wait`. Now it is mounted and "running" again, and we need to check which case it is in: notified, interrupted or timed-out. "First time" means it is the first time it's running after the original unmount on `Object.wait`. This is because once we are on the monitor reentry phase, the virtual thread can be potentially unmounted and mounted many times until it successfully acquires the monitor. Not sure how to rewrite the comment to make it clearer.
>
> The first sentence is not a sentence. Is it supposed to be saying:
> 
> // The first time we run after being preempted on Object.wait()
> // we check if we were interrupted or the wait timed-out ...
> 
> ?

Yes, I fixed the wording.

>> Only when facing contention on this call. But once we have the monitor we don't.
>
> But if this is from JNI then we have at least one native frame on the stack making the JNI call, so we have to be pinned if we were to block on the monitor. ???

We will have the native wrapper frame at the top, but we still need to add some extra check to differentiate this `jni_enter()` case with respect to the case of facing contention on a synchronize native method, where we do allow to unmount (only when coming from the interpreter since the changes to support it where minimal). I used the NoPreemptMark here, but we could filter this case anywhere along the freeze path. Another option could be to check `thread->current_pending_monitor_is_from_java()` in the ObjectMonitor code before trying to preempt.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819907304
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819907921