[jdk17] RFR: 8269897: Shenandoah: Treat UNKNOWN refs access as strong [v4]

[Zhengyu Gu]

On Thu, 8 Jul 2021 12:47:15 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> We've observed test failures in jcstress, see:
>> https://bugs.openjdk.java.net/browse/JDK-8269897
>> 
>> We used to treat UNKNOWN reference accesses like weak accesses. UNKNOWN is used for Unsafe, reflection and JNI accesses, where it cannot be determined at compilation-time if we are accessing a regular field or a Reflection.referent field. The rationale for treating UNKNOWN as weak was that if the reference is a regular reference, then the value would be strongly reachable anyway, and only if it is a referent field would reachability matter. However, it turns out that this assumption is wrong: the test shows that a reference that is only weakly reachable can be legitimately written into a field, thus resurrecting the reference, and when that weakly reachable reference is loaded, it would be (wrongly) filtered as NULL.
>> 
>> A fix is to treat UNKNOWN accesses as strong. Accessing Reference.referent via reflection, JNI or Unsafe is Bad Idea anyway.
>> This test shows the problem with CAS, but I believe it affects all accesses via reflection, JNI, etc.
>> 
>> Testing:
>>  - [x] the provided jcstress test
>>  - [x] hotspot_gc_shenandoah
>>  - [x] tier1
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix assert

src/hotspot/share/gc/shenandoah/c2/shenandoahBarrierSetC2.cpp line 643:

> 641:     }
> 642: #endif
> 643:     load_store = kit->gvn().transform(new ShenandoahLoadReferenceBarrierNode(NULL, load_store, access.decorators() & ~ON_UNKNOWN_OOP_REF));

This load should go through slow path when GC is in progress, right? Should let slow path to resolve the strength? sounds there is possibility to resurrect dead oop.

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PR: https://git.openjdk.java.net/jdk17/pull/219