[Bug 227404] UP FreeBSD VM always hangs on reboot since 20180329-r331740

[Bruce Evans]

On Tue, 10 Apr 2018 a bug that doesn't want replies at freebsd.org wrote:

(The bug didn't even Cc freebsd-bugs for this followup.)

> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227404

> --- Comment #4 from Dexuan Cui <decui at microsoft.com> ---
> I think the first bad patch is this one:
> https://github.com/freebsd/freebsd/commit/63a483ed5f4eaadb8979992c7a5de24c7a471c61
> (Fix a bug introduced in r329612 that slowly invalidates all clean bufs.):
>
> Today's
> https://github.com/freebsd/freebsd/commit/66e8725e8d24141506bc4f458ec7d1a51e86304c
> is broken, but if I revert 63a483ed5f4eaadb8979992c7a5de24c7a471c61, the bug
> can not reproduce.
>
> Cc bde & jeff.

I think I saw this a few months before that.

My only history of this is that I built a UP kernel on 17 Dec 2017 to see
if UP kernels had the bug.  So SMP kernels probably had the bug then.

Bruce