RFR: 8264279: Shenandoah: Missing handshake after JDK-8263427

[Aleksey Shipilev]

On Mon, 29 Mar 2021 18:03:37 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> JDK-8263427 mistakenly removed handshake after concurrent weak roots, which is needed to flush out stale oops before we can safely recycle trash regions. Otherwise, those stale oops are suddenly resurrected, because they are above TAMS after regions are recycled.
> 
> Test:
> - [x] hotspot_gc_shenandoah

D'oh. Looks good.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3248