RFR: 8333105: Shenandoah: Results of concurrent mark may be lost for degenerated cycle [v2]

[William Kemper]

> If Shenandoah does not detect a cancellation after completing concurrent mark, but before completing final mark it will fall through the concurrent cycle without collecting anything. This will shortly lead to a degenerated cycle which will _not_ use the results from the nearly complete concurrent mark. This will result in an unnecessarily longer degenerated cycle.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19434/files
  - new: https://git.openjdk.org/jdk/pull/19434/files/165b3f76..05e8850d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19434&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19434&range=00-01

  Stats: 15 lines in 2 files changed: 1 ins; 10 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19434.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19434/head:pull/19434

PR: https://git.openjdk.org/jdk/pull/19434