On Tue, Dec 19, 2017 at 10:24:52 -0800,
  Shaohua Li <shli@kernel.org> wrote:
>
>Not sure if this is MD related, but could you please check if this debug patch
>changes anything?

The system still had cpu hangs. I've attached dmesg output saved by systemd 
and retrieved after booting with a pre-rc2 kernel.