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

Roman Kennke rkennke at openjdk.java.net
Thu Jul 8 20:02:56 UTC 2021


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

Fixing this in jdk18 first, should backport to jdk17 later, after some testing in 18. See: https://github.com/openjdk/jdk/pull/4697

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


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