RFR: 8314972: GenShen: promote_in_place needs to prepare remembered set before it enables old allocations within region

[William Kemper]

On Thu, 24 Aug 2023 21:44:29 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> An error in the original implementation left us vulnerable to a race between a worker thread which is promoting a region in place and another worker or mutator thread who is allocating memory for old objects within the same region.
> 
> This PR prevents this race by requiring the worker thread that promotes a region in place to finish the preparation of the region's remembered set before it makes the region available to other threads for allocation of old-gen memory.

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/310#pullrequestreview-1596229319