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

[Y. Srinivas Ramakrishna]

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.

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp line 1040:

> 1038:       assert(fill_size >= ShenandoahHeap::min_fill_size(), "previously allocated objects known to be larger than min_size");
> 1039:       ShenandoahHeap::fill_with_object(obj_addr, fill_size);
> 1040:       heap->card_scan()->register_object_without_lock(obj_addr);

On second thoughts, do we really need to register the dead objects? Does it help at all, since the preceding and succeeding live objects will be registered anyway. Not wrong, but I do wonder if it helps at all or not. Probably not a big deal either way since dead objects are likely very few in regions promoted in place.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/310#discussion_r1305945666