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

[Zhengyu Gu]

On Wed, 4 Dec 2024 15:16:30 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> 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.
>
> Yes, it's needed.  The purpose of this release/acquire pair is to ensure there
> is a happens-before chain between the use of a State and it's cleanup/reuse.
> Transfers through the taskqueue don't help with that.
> 
> In other places, we have operations on one side of the taskqueue that need to
> be ordered wrto operations on the other side of the taskqueue.  We don't have
> that here.
> 
> Consider two threads which have both obtained access to a State.  (At least
> one of them must have obtained it via stealing from another thread.)  Assume
> these are the last two references to the State (it's refcount == 2), and no
> further tasks for it will be needed.  These two threads will use the State
> (getting source/destination, claiming a chunk), and then release the State,
> decrementing its refcount.  So one of them will decrement to 0.  We need to
> ensure the cleanup that follows that can't corrupt the use by the accesses
> made by the other thread, by ensuring those accesses happen-before the
> cleanup.  There is no intervening taskqueue manipulation in this scenario.
> The operations we need to order are all on the same side of the taskqueue.

I see.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22287#discussion_r1870196252