RFR: Fix misuse of atomic flag

[Aleksey Shipilev]

On Wed, 1 Sep 2021 20:56:44 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This change fixes an assert that GC isn't cancelled in the middle of a safepoint. The assert was triggered during the tier2 test: `TestJNIGlobalRefs`. The test uses the _aggressive_ heuristic which runs GC's back to back. The atomic `_preemption_allowed` variable was being misused in a non-atomic way which could lead to the regulator thread cancelling GC after the control thread has gone to the final mark safepoint.

Marked as reviewed by shade (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/63