Integrated: 8373116: Genshen: arraycopy_work should be always done for arrays in old gen during young concurrent marking

[Xiaolong Peng]

On Thu, 4 Dec 2025 22:14:50 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

This pull request has now been integrated.

Changeset: c9ab330b
Author:    Xiaolong Peng <xpeng at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/c9ab330b7bdd3cc2410ffdb336a63aa0ac7256a3
Stats:     10 lines in 2 files changed: 5 ins; 0 del; 5 mod

8373116: Genshen: arraycopy_work should be always done for arrays in old gen during young concurrent marking
8372498: [genshen] gc/TestAllocHumongousFragment.java#generational causes intermittent SIGSEGV crashes

Reviewed-by: wkemper, kdnilsen

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PR: https://git.openjdk.org/jdk/pull/28669