RFR: 8315560: GenShen: assert failed: Object klass pointer must go to metaspace

[William Kemper]

We have seen crashes caused when the interned string table is cleaned concurrently by mutator actions. Strings which have become unreachable may be inserted in the SATB buffer during concurrent mark of old. If these pointers reside in young regions which are later promoted in place to old regions, the referenced object (which is young and unmarked) will be filled.  When the old marking tries to follow through the pointer, it may crash.

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Commit messages:
 - Do not promote regions in place during marking of the old generation

Changes: https://git.openjdk.org/shenandoah/pull/348/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315560
  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/348.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/348/head:pull/348

PR: https://git.openjdk.org/shenandoah/pull/348