8257817: Shenandoah: Don't race with conc-weak-in-progress flag in weak-LRB

[rkennke]

The weak-LRB code is currently subject to a race. Consider this sequence of events between a Java thread and GC threads:
During conc-weak-root-in-progress:

• Java: Load referent out of Reference, it is unreachable but not-yet-cleared
• GC: Clears referent
• GC: Concurrently turn off conc-weak-root-in-progress
• Java: Checks conc-weak-root-in-progress, sees that it's false, continues to use/evac it -> successfully resurrected unreachable object. This must not happen.

AFAICT, this also affects conc-class-unloading and weak-roots.

Proposed fix is to check for evac-in-progress instead. This should be acceptable because this is not a very common path and not very performance-sensitive.

• hotspot_gc_shenandoah

---

Progress

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Issue

• JDK-8257817: Shenandoah: Don't race with conc-weak-in-progress flag in weak-LRB

Download

$ git fetch https://git.openjdk.java.net/jdk pull/1662/head:pull/1662
$ git checkout pull/1662

[bridgekeeper]

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[openjdk]

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• hotspot-gc
• shenandoah

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[mlbridge]

Webrevs

• 00: Full (7c51777)

[zhengyu123]

I wonder if it is more general problem. Given concurrent weak root in mandatory phase, we should just remove is_concurrent_weak_root_in_progress flag.