RFR: 8271215: Fix data races in G1PeriodicGCTask

[Aleksey Shipilev]

On Sat, 24 Jul 2021 09:02:24 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this small change to G1PeriodicGCTask, eliminating unlikely
> but possible data races with a concurrently running GC pause.  An STS-joiner
> is used to ensure there isn't a GC pause while doing the checks to decide
> whether to perform a periodic GC.
> 
> Testing:
> mach5 tier1

Looks fine.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4899