RFR: 8318053: GenShen: Control thread may ignore requests to start concurrent GC

[William Kemper]

If a request to interrupt an old cycle is accepted, but there is an allocation failure before the requested cycle begins, then the control thread may become stuck in a state where it believes it should ignore further concurrent cycle requests.

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Commit messages:
 - Clear preemption requested flag when old gc is cancelled for allocation failure
 - Log when control thread does _not_ start a cycle

Changes: https://git.openjdk.org/shenandoah/pull/339/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=339&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318053
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/339.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/339/head:pull/339

PR: https://git.openjdk.org/shenandoah/pull/339