RFR: 8338737: Shenandoah: Reset marking bitmaps after the cycle [v2]

[Xiaolong Peng]

On Thu, 17 Oct 2024 21:36:36 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

> > I have some concerns about the correctness here. Consider this scenario:
> > 
> > 1. At the end of a young _or mixed_ collection, `op_reset_after_collect` will reset young regions and unset the `needs reset` flag for the young generation.
> > 2. An old region (A) from the mixed collection could be repurposed as a young region (it's bitmap will not be reset when it is recycled).
> > 3. The next young/mixed collection will not reset the bitmap for region A because the `needs reset` flag is clear.
> > 4. The collector will see stale mark bits for region A and will probably crash.
> 
> My understanding is: op_reset_after_collect is called at the end of a successful collection after the old region (A) is repurposed as a young region, if so it is not an issue, I'll double confirm.

op_reset_after_collect also reset the regions are not affiliated, after a mixed/young collection, the old region should have been recycled before resetting bitmaps, so even the recycled old region is repurposed as young region later, it should be ok. 

If the young/mixed_ collection is set to bootstrap old GC, the reset of bitmaps is delayed to old GC to avoid dirty bitmaps

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PR Comment: https://git.openjdk.org/shenandoah/pull/516#issuecomment-2420740518