RFR: 8287788: reuse intermediate segments allocated during FFM stub invocations [v8]

[Matthias Ernst]

On Wed, 22 Jan 2025 11:50:10 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> I'm told that TerminatingThreadLocal runs the "terminate" action for an object T from the same thread T refers to. So, in principle, using a TerminatingThreadLocal + confined arena should be ok.
>> 
>> If that works, I'd suggest to consider maybe moving all this sharing logic inside BufferStack, so that users only need to do:
>> 
>> 
>> static final BufferStack LINKER_BUFFER = new BufferStack(256);
>> 
>> ...
>> 
>> try (Arena arena = BufferStack.reserve(size)) {
>>     ...
>> }
>> 
>> 
>> Which seems a more re-usable API. After all, the fact that we decide to lock or not in certain cases is heavily influenced by the fact that we expect a BufferStack to be used with a carrier local, so we might just as well fold the caching in there.
>
> Isn't the problem access? E.g. a virtual thread `A` running on our carrier thread might create the confined arena, binding it to _virtual_ thread `A`, and then another virtual thread `B` comes along one the same carrier thread, and wants to use the same `BufferStack`, but can't because it is confined to thread `A`.

I think _using_ it might even be ok, since all it does to the stack is slicing and reinterpreting, but the final close() would be on the wrong thread (allocator = virtual, closer=terminator likely a platform thread).

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1925236362