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

[Xiaolong Peng]

On Tue, 18 Mar 2025 17:24:54 GMT, William Kemper <wkemper 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`
>
> src/hotspot/share/gc/shenandoah/shenandoahVerifier.cpp line 1389:
> 
>> 1387:   shenandoah_assert_safepoint();
>> 1388:   shenandoah_assert_generational();
>> 1389:   ShenandoahMarkingContext* ctx = get_marking_context_for_old();
> 
> This should always be `nullptr` after a full GC, right? The marking context is no longer valid after compaction.

Yes, get_marking_context_for_old always return `nullptr` after a full GC, the marking completeness has been set to false when we reset marking bitmaps after full GC. 

I think the method get_marking_context_for_old and the ctx arg of the helper function  can be removed, I'll do that in next update.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24092#discussion_r2001815702