RFR: Start young collect instead of old if mixed evacuations are pending

[Kelvin Nilsen]

On Wed, 18 May 2022 16:24:49 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This change addresses a race condition in which the heuristic could start an old gc while there are pending mixed evacuations. This condition could lead to a crash because the nascent old gen collect will clear the mark bitmap, but the remembered set scan of the bootstrap collection will use the mark bitmap to find objects if it believes mixed evacuations are in progress.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/141