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

[Dean Long]

On Mon, 28 Oct 2024 21:13:33 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> If preemption was cancelled, we skip over the cleanup.  The native frames haven't been unwound yet.  So when we call thaw, does it cleanup the native frames first, or does it copy the frames back on top of the existing frames (overwrite)?  It seems like we could avoid redundant copying if we could somehow throw out the freeze data and use the native frames still on the stack, which would probably involve not patching in this stub until we know that the preemption wasn't canceled.  Some some finalize actions would be delated, like a two-stage commit.
>
>> If preemption was cancelled, we skip over the cleanup.
>>
> We only skip the cleanup for the enterSpecial frame since we are going to call thaw again, all other frames are removed: https://github.com/openjdk/jdk/pull/21565/files#diff-b938ab8a7bd9f57eb02271e2dd24a305bca30f06e9f8b028e18a139c4908ec92R3791

OK got it.  I guess it's too early to know if it's worth it to further optimize this case, which is hopefully rare.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819865539