RFR: Update card table when cycle degenerates during root scan

[Kelvin Nilsen]

On Thu, 27 Jan 2022 18:32:33 GMT, William Kemper <wkemper at openjdk.org> wrote:

> If the concurrent cycle degenerates during the root scan (after the card table has been swapped), we need to update the _read_ table (used during the STW mark) with any cards written during the transition to the degenerated cycle. Without this change, the remembered set scan on the STW mark will skip old to young pointers created during the transition.

Thanks for this fix.

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Marked as reviewed by kdnilsen (Committer).

PR: https://git.openjdk.java.net/shenandoah/pull/106