8381382: Shenandoah: assert(capacity > 0) failed: free regions must have allocation capacity

[pengxiaolong]

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The improvement I made in ticket https://bugs.openjdk.org/browse/JDK-8345423 makes trash region be recycled by GC workers w/o holding heap lock, this may cause assert from mutator memory allocation path in which mutator calls ShenandoahFreeSet::assert_bounds() perform verifications, given GC workers recycle trashed region w/o heap lock, it is possible that one mutator can see inconsistent heap stats while a GC worker thread is recycling the same trashed region. It is not possible to happen for mem allocation except the verification, because before allocating inside a region, mutator always calls ShenandoahHeapRegion::try_recycle_under_lock() which ensure the region is ready for use, but ShenandoahFreeSet::assert_bounds scans all regions rather than the region mutator tries to allocate in.

I feel the assert used to rarely happen, because ShenandoahFreeSet::assert_bounds is called only when the region is retired due to insufficient memory, but this behavior was changed in recently commit which unifies the heap usage accounting, with it, mutator calls assert_bounds for every single allocation from heap, this make the assert to happen more often.

Test

• 300 times of test/hotspot/jtreg/applications/jcstress/volatiles.java
• hotspot_gc_shenandoah
• GHA

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Issue

• JDK-8381382: Shenandoah: assert(capacity > 0) failed: free regions must have allocation capacity (Bug - P3)

Reviewers

• Kelvin Nilsen (@kdnilsen - Reviewer) 🔄 Re-review required (review applies to 0d55413a)

Reviewing

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[bridgekeeper]

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[openjdk]

@pengxiaolong This change is no longer ready for integration - check the PR body for details.

[openjdk]

@pengxiaolong The following labels will be automatically applied to this pull request:

• hotspot-gc
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[mlbridge]

Webrevs

• 03: Full - Incremental (ffa450c0)
• 02: Full - Incremental (2c524d61)
• 01: Full - Incremental (aaf85937)
• 00: Full (0d55413a)

[kdnilsen]

Thanks for fixing this.

[pengxiaolong]

Converting it back to draft, I am working and testing on lightweight solution instead of creating ShenandoahLock instance for each region(it works, but it is bit overkill, I have been running the stress test for over 3 days, not crash so far).

[pengxiaolong]

Closing this one, we have lightweight solution to fix this.

#30723