RFR: 8320331: G1 Full GC Heap verification relies on metadata not reset before verification

[Ivan Walulya]

On Mon, 20 Nov 2023 11:24:12 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this fix to full gc to properly set `parsable_bottom` for the heap regions marked through so that "during" verification (`-XX:+VerifyDuringGC`) does not access metadata already unlinked but not yet completely purged.
> 
> The test case reproduces crashes fairly well without the patch; the new test case is run with debug VMs only purely to save testing time.
> 
> Testing: gha, test case
> 
> Thanks,
>   Thomas

LGTM!

Minor:
- update the comment associated with `_parsable_bottom`
- Add a comment to `prepare_for_full_gc` to hint on why this is required.

-------------

Marked as reviewed by iwalulya (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16733#pullrequestreview-1744030751