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

[Thomas Schatzl]

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

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Commit messages:
 - 8372684

Changes: https://git.openjdk.org/jdk/pull/28543/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28543&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372684
  Stats: 50 lines in 3 files changed: 6 ins; 3 del; 41 mod
  Patch: https://git.openjdk.org/jdk/pull/28543.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28543/head:pull/28543

PR: https://git.openjdk.org/jdk/pull/28543