kern/58923: Official "update NetBSD from source" instructions disrupt service depending on kernel modules

[als%thangorodrim.ch@localhost]

>Number:         58923
>Category:       kern
>Synopsis:       Official "update NetBSD from source" instructions disrupt service depending on kernel modules
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Dec 20 21:30:00 +0000 2024
>Originator:     Alexander Schreiber
>Release:        NetBSD 10.1
>Organization:
not much
>Environment:
NetBSD [hostname censored] 10.1 NetBSD 10.1 (GENERIC) #0: Thu Dec 19 22:08:33 CET 2024  root@[hostname censored]:/usr/obj/sys/arch/amd64/compile/GENERIC amd64

>Description:
If one follows the official instructions for "Updating an existing system from
sources" at https://www.netbsd.org/docs/guide/en/chap-updating.html then this
will result in a service interruption for any services depending on loading
specific kernel modules. This was discovered by (temporarily) losing ZFS
support during a 10.0 -> 10.1 upgrade.

This happens because first the kernel and distribution targets are built,
then the new kernel is placed at /netbsd and the machine rebooted. At this
point, the new kernel modules under /stand have not yet been installed and
thus the freshly booted kernel can't load them. In my case, I noticed that
ZFS had "disappeared" and saw the "can't start ZFS" complaint on the console.
The the distribution is installed and the post-install steps completed,
followed by another reboot. Now the new kernel modules are installed, the
kernel can load them and service (e.g. ZFS) is restored.
>How-To-Repeat:
 - set up a system with a service that requires loading of (a) kernel
   module(s), e.g. ZFS
 - follow https://www.netbsd.org/docs/guide/en/chap-updating.html
 - specifically
   - build kernel & distribution
   - "35.1.3. Installing the kernel and userland"
     - copy the kernel image into place as /netbsd
     - reboot as instructed
     - observe ZFS not being available
     - install distribution and follow cleanup instructions
     - reboot as instructed
     - observe ZFS being available again
>Fix: