[CentOS] NFSroot over wifi in CentOS 7 - graceful shutdown problem

Mon Aug 3 13:50:12 UTC 2020
Simon Matter <simon.matter at invoca.ch>

> Hi,
> I've got a task to have a small number of laptops netboot Linux over
> WiFi. The kernel is loaded off the USB stick of cource, it's off topic
> for now.
> The WPA-supplicant daemon is started early by dracut off initrd. It
> works. Mostly.
> The problem is that upon shutdown systemd terminates all the processes
> FIRST and unmounts filesystems NEXT.
> Guess what? Upon termination, wpa-supplicant brings the wireless
> interface down and the system hangs being unable to unmount now-defunct
> NFSroot.
> There were some discussions regarding similar matter and there's even
> the RH Errata:
> https://access.redhat.com/errata/RHBA-2018:2447
> But I'm woking on a fully updated CentOS Linux release 7.8.2003 and
> nevertheless I've got the problem.
> I don't have rights to see the BZ.
> https://bugzilla.redhat.com/show_bug.cgi?id=1593649
> I'm deciding to use a quick and dirty hack to do a totally ungraceful
> shutdown/reboot:
> https://unix.stackexchange.com/questions/533307/systemd-fails-to-umount-manually-mounted-nfs-shares-in-initramfs
> Any better ideas?

Short answer:

Looks like this should be fixed in systemd or its configuration.

Long answer is the one I already gave you on centos-devel. I post it here
because someone may have deeper knowledge of what exactly goes on:

What I'm wondering here is, do you see the hangs because of wpa_supplicant
was terminated and therefore also the wireless interface is down, or do
you see the hangs because networking is already completely down.

If it is because wpa_supplicant gets terminated, then maybe it would be
possible to terminate wpa_supplicant in a way that it lets the wireless
interface still up and functional at least for a certain amount of time.

I don't know wpa_supplicant good enough but I think maybe it is not always
needed to be alive for the wireless interface to work.

Otherwise, I guess it's more a question for the systemd developers than
for CentOS or Linux in general.