RFR: 8369611: Remove safepoint synchronization from ParallelScavengeHeap and SerialHeap

[Thomas Schatzl]

On Fri, 10 Oct 2025 22:38:14 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

> In the discussion of [JDK-8369392](https://bugs.openjdk.org/browse/JDK-8369392), @kimbarrett pointed out that StringDedup thread is now derived from JavaThread, but ParallelScavengeHeap and SerialHeap still have conditional STS synchronization at safepoint, which is not needed, as a result of removing the conditional STS synchronization, we could remove the remove safepoint synchronization from ParallelScavengeHeap and SerialHeap.
> 
> Test:
>   - [x] tier1

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27755#pullrequestreview-3330999015