RFR: 8269897: Shenandoah: Resolve UNKNOWN access strength, where possible
rkennke at openjdk.java.net
Thu Jul 8 18:36:10 UTC 2021
We've observed test failures in jcstress, see:
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.
- [x] the provided jcstress test
- [x] hotspot_gc_shenandoah
- [x] tier1
- Determine access strength in all runtime barriers, where possible
- Treat UNKNOWN as STRONG more selectively only on CAE
- 8269897: Shenandoah: Treat UNKNOWN refs access as strong
Stats: 111 lines in 4 files changed: 43 ins; 35 del; 33 mod
Fetch: git fetch https://git.openjdk.java.net/jdk pull/4697/head:pull/4697
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