RFR: 8351444: Shenandoah: Class Unloading may encounter recycled oops

[William Kemper]

On Fri, 7 Mar 2025 21:47:31 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Unloading classes may require a walk of unreachable oops. For this reason, it is not safe to recycle memory before class unloading is complete. This complements existing code to prevent mutators from recycling trash regions while weak roots is in progress.

It isn't just about trash regions. This path through the barrier will also allow access to objects in the collection set (without evacuating them). We also choose the collection set during the final mark safepoint. Alternatively, we could soften the constraint for `ShenandoahHeap::is_in` to allow access to trash regions if concurrent weak roots is in progress.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23951#issuecomment-2711330793