RFR: 8323630: GenShen: Control thread may (still) ignore requests to start concurrent GC

[William Kemper]

Testing if bots can help us with shenandoah-21u backports

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Commit messages:
 - Backport a40ce85907ca7eb918af0e3563c76f43a203cf71

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/5/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=5&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323630
  Stats: 81 lines in 2 files changed: 22 ins; 27 del; 32 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/5.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/5/head:pull/5

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/5