RFR: 8351444: Shenandoah: Class Unloading may encounter recycled oops [v2]

[William Kemper]

> Unloading classes may require a walk of unreachable oops. For this reason, it is not safe to recycle memory before class unloading is complete. This complements existing code to prevent mutators from recycling trash regions while weak roots is in progress.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Consider trash regions to be in the heap during concurrent weak roots
 - Better comment for LRB when accessing unreachable oops

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23951/files
  - new: https://git.openjdk.org/jdk/pull/23951/files/b231d4fe..a5db7360

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23951&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23951&range=00-01

  Stats: 27 lines in 2 files changed: 16 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/23951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23951/head:pull/23951

PR: https://git.openjdk.org/jdk/pull/23951