RFR: 8352185: Shenandoah: Invalid logic for remembered set verification

[Kelvin Nilsen]

On Tue, 18 Mar 2025 00:19:35 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

> 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`

Can we confirm that this addresses JBS issue with further testing before integration?

src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp line 660:

> 658: 
> 659:     // Verify before mark is done before swapping card tables,
> 660:     // therefore the write card table will be verified before being taken snapshot.

Not a big deal, but this is two sentences.  "... swapping card tables.  Therefore, the write card table is verified before we swap read and write card tables."

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/jdk/pull/24092#pullrequestreview-2696536061
PR Review Comment: https://git.openjdk.org/jdk/pull/24092#discussion_r2002170767