RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v12]

[David Holmes]

On Mon, 28 Oct 2024 23:09:58 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Not sure. We would have to return from wait0 and immediately clear the physical stack from the frames just copied without safepoint polls in the middle. Otherwise if someone walks the thread's stack it will find the frames appearing twice: in the physical stack and in the heap.
>
> It's conceivable that in the future we might have more native methods we want to preempt.  Instead of enumerating them all, we could set a flag on the method.
> 
> I was assuming that David was suggesting we have the Java caller do a yield() or something, instead of having the native code call freeze.

Yes. Instead of calling wait0 for a virtual thread we would call another method `needToBlockForWait` that enqueues the VT in the wait-set, releases the monitor and returns true so that caller can then "yield". It would return false if there was no longer a need to block.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1820337946