RFR: 8352185: Shenandoah: Invalid logic for remembered set verification [v12]

[Xiaolong Peng]

> There are some scenarios in which GenShen may have improper remembered set verification logic:
> 
> 1. Concurrent young cycles following a Full GC:
> 
> In the end of ShenandoahFullGC, it resets bitmaps for the entire heap w/o resetting marking context to be incomplete, but ShenandoahVerifier has code like below to get a complete old marking context for remembered set verification
> 
> 
> ShenandoahVerifier  
> ShenandoahMarkingContext* ShenandoahVerifier::get_marking_context_for_old() {
>   shenandoah_assert_generations_reconciled();
>   if (_heap->old_generation()->is_mark_complete() || _heap->gc_generation()->is_global()) {
>     return _heap->complete_marking_context();
>   }
>   return nullptr;
> }
> 
> 
> For the concurrent young GC cycles after a full GC, the old marking context used for remembered set verification is stale, and may cause unexpected result. 
> 
> 2. For the impl of `ShenandoahVerifier::get_marking_context_for_old` mentioned above, it always return a marking context for global GC, but marking bitmaps is already reset before before init-mark, `ShenandoahVerifier::help_verify_region_rem_set` always skip verification in this case. 
> 
> 3. ShenandoahConcurrentGC always clean remembered set read table, but only swap read/write table when gc generation is young, this issue causes remembered set verification before init-mark to use a completely clean remembered set, but it is covered by issue 2. 
> 
> 
> ### Test
> - [x] `make test TEST=hotspot_gc_shenandoah`

Xiaolong Peng has updated the pull request incrementally with two additional commits since the last revision:

 - Not validate remembered set w/o complete old marking
 - Use decode_raw_not_null instead of decode_raw

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24092/files
  - new: https://git.openjdk.org/jdk/pull/24092/files/39707d15..73e95e8a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24092&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24092&range=10-11

  Stats: 25 lines in 2 files changed: 1 ins; 14 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/24092.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24092/head:pull/24092

PR: https://git.openjdk.org/jdk/pull/24092