RFR: 8372684: G1: Missing load_acquire() in G1 allocation path

[Thomas Schatzl]

On Fri, 28 Nov 2025 09:13:32 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that changes some volatile atomics to the new `Atomic` construct, fixing some issue with missing load_acquire when allocating in `G1HeapRegion::par_allocate()`.
>> 
>> That `load_acquire` seems necessary as when setting a new allocation region, there needs to be synchronization about the contents (top/end) of the heap region. Otherwise the allocating thread might get old/previous top values sent from the corresponding storestore (I changed to a `release_store()` in `G1AllocRegion::update_alloc_region()`.
>> 
>> Similar issue exists in the handling of the retained regions.
>> 
>> I also made the `G1AllocatorRegion::_dummy_region` Atomic; synchronization is necessary for a single `release_store` at initialization. No further synchronization is necessary (just using relaxed loads) as its contents are never changed afterwards.
>> 
>> Performance testing, both manually on targeted benchmarks and a few general ones without differences.
>> 
>> Testing: gha, tier1-3
>> 
>> Thanks,
>>   Thomas
>
> Looks good.

Thanks @kimbarrett @kstefanj for your reviews

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PR Comment: https://git.openjdk.org/jdk/pull/28543#issuecomment-3595152537