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

[William Kemper]

The SATB barrier which is on during concurrent marking of old may cause pointers into the collection set to be held by the SATB buffer. When a concurrent cycle degenerates during or before the root scan, these buffers are purged. However, they must also be purged at the end of a degenerated cycle. The buffers cannot be purged unconditionally at the beginning of a degenerated cycle because it could lose pointers necessary to complete the concurrent mark of the young generation.

-------------

Commit messages:
 - Fix typo in comment
 - Update comment
 - Merge remote-tracking branch 'shenandoah/master' into purge-old-satb-after-degen-update-refs
 - Purge old SATB at end of degen cycle (in case we didn't at the start)

Changes: https://git.openjdk.org/shenandoah/pull/326/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=326&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315560
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/326.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/326/head:pull/326

PR: https://git.openjdk.org/shenandoah/pull/326