RFR: 8352185: Shenandoah: Invalid logic for remembered set verification [v13]

[Kelvin Nilsen]

On Thu, 20 Mar 2025 22:48:24 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. 
>> 
>> 4. After concurrent young cycle evacuates objects from a young region, it update refs using marking bitmaps from marking context, therefore it won't update references of dead old objects(is_marked(obj) is false: obj is not marking strong/weak and it is below tams). In this case, if the next cycle if global concurrent GC, remembered set can't be verified before init-mark because of the dead pointers. 
>> 
>> ### Solution
>> * After a full GC, always set marking completeness flag to false after reseting the marking bitmaps.
>> * Because there could be dead pointers in old gen were not updated to point to new address after evacuation and refs update, we should disable rem-set validation before init-mark&update-refs if old marking context is incomplete. 
>> 
>> ### Test
>> - [x] `make test TEST=hotspot_gc_shenandoah`
>> - [x] GHA
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   tide up

Thanks for the refinements.  LGTM.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/jdk/pull/24092#pullrequestreview-2715403856