RFR: 8373116: Genshen: arraycopy_work should be always done for arrays in old gen during young concurrent marking [v6]

[William Kemper]

On Sun, 7 Dec 2025 20:34:13 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Chasing the root cause of JDK-8372498, I have narrowed down root cause to the commit https://github.com/openjdk/jdk/commit/f8cf9ca69cfef286c80559bfe1d147b6303d10d2
>> 
>> It is caused by the behavior change from follow code:
>> 
>> Original:
>> 
>>   if (ShenandoahSATBBarrier) {
>>     T* array = dst;
>>     HeapWord* array_addr = reinterpret_cast<HeapWord*>(array);
>>     ShenandoahHeapRegion* r = _heap->heap_region_containing(array_addr);
>>     if (is_old_marking) {
>>       // Generational, old marking
>>       assert(_heap->mode()->is_generational(), "Invariant");
>>       if (r->is_old() && (array_addr < _heap->marking_context()->top_at_mark_start(r))) {
>>         arraycopy_work<T, false, false, true>(array, count);
>>       }
>>     } else if (_heap->mode()->is_generational()) {
>>       // Generational, young marking
>>       if (r->is_old() || (array_addr < _heap->marking_context()->top_at_mark_start(r))) {
>>         arraycopy_work<T, false, false, true>(array, count);
>>       }
>>     } else if (array_addr < _heap->marking_context()->top_at_mark_start(r)) {
>>       // Non-generational, marking
>>       arraycopy_work<T, false, false, true>(array, count);
>>     }
>>   }
>> 
>> New:
>> 
>>   if (ShenandoahSATBBarrier) {
>>     if (!_heap->marking_context()->allocated_after_mark_start(reinterpret_cast<HeapWord*>(dst))) {
>>       arraycopy_work<T, false, false, true>(dst, count);
>>     }
>>   }
>> 
>> 
>> 
>> With the new STAB barrier code for arraycopy_marking, if is it young GC and the array is in old region, but array is above TAMS(Old GC may not be started, TAMS of old region is not captured), arraycopy_work won't be applied anymore, so we may have missed some pointers in SATB in such case during concurrent young marking. 
>> 
>> ### Test
>> - [x] hotspot_gc_shenandoah
>> - [x] repeat gc/TestAllocHumongousFragment.java#generational and sure it won't crash with the fix
>> - [x] GHA
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   enqueue objects stored in old array at ShenandoahSATBBarrier when concurrent young marking is in progress

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.inline.hpp line 451:

> 449:   if (ShenandoahSATBBarrier) {
> 450:     if (!_heap->marking_context()->allocated_after_mark_start(reinterpret_cast<HeapWord*>(dst)) ||
> 451:         (_heap->is_concurrent_young_mark_in_progress() && _heap->heap_region_containing(dst)->is_old())) {

We could also check if Shenandoah is running in generational mode. Even in non-generational mode, we set `YOUNG_MARKING` in gc state.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28669#discussion_r2599168582