RFR: 8344665: Refactor PartialArrayState allocation for reuse [v4]

[Zhengyu Gu]

On Tue, 3 Dec 2024 15:51:19 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> src/hotspot/share/gc/shared/partialArrayState.cpp line 96:
>> 
>>> 94: 
>>> 95: void PartialArrayStateAllocator::release(PartialArrayState* state) {
>>> 96:   size_t refcount = Atomic::sub(&state->_refcount, size_t(1), memory_order_release);
>> 
>> Could you explain why `release` order is needed here?
>
> This is part of the usual reference counting dance. Except, where did the
> acquire disappear to? There should be an acquire on the refcount == 0 branch!
> Looks like I accidentally deleted it. Sigh. Not too surprisingly, lots of
> tests were run without noticing that.

Make sense. My next question is that, if `PartialArrayState` is ever crossed thread boundaries, it is through job stealing via task queues. Can we depend on barriers of task queue to ensure the memory safety? because we don't need any additional barriers for objects popped/stole from task queues in other places.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22287#discussion_r1869520669