[systemd-devel] question about system reboot and shutdown

[Lennart Poettering]

On Di, 08.08.17 16:03, Marek Floriańczyk (marek.florianczyk at gmail.com) wrote:

> Hi all,
> 
> I have a small device MicroUPS which helps me to shutdown my system on 
> embedded devices, it is controlled by script /etc/init.d/microups and in this 
> script I need to know whether system is going down for reboot or for halt, 
> because in case of halt I need to send small data over RS232 to MicroUPS 
> device to cut the power off.

Note that this is necessarily racy: you can't really know how long the
system will actually take to shut down, hence if you trigger your
hardware for shutdown at an early phase of the shutdown process you
now start racing the remaining shutdown phase against the hardware turning
off power...

If you want to fix this properly, and remove this race entirely the
only fully correct way out I see is to use an initrd for booting, and
doing the RS232 thing from that. Note that if you use a properly
prepared initrd, systemd will actually transition back to it at
shutdown, and while doing so it will permit unmounting the root file
system properly at shutdown. And only if you start the RS232 thing
after the point where the root fs is unmounted you can fully remove
the race in the generic case, since only at that point everything is
fully synced to disk, all complex storage is disassembled and so on.

Now, adding this to the initrd is not the easiest thing in the world,
and in particular in embedded devices avoiding an initrd might be a
good thing. As long as you have no complex storage (i.e. no DM, no
LVM, no LUKS, no RAID, no iscsi, yaddayadda) you can instead cut a
corner and just drop in a shutdown script to
/usr/lib/systemd/system-shutdown/. All executable files in that
directory are run at very late boot, at a point where all file systems
that can be unmounted have been unmounted and the rest have been
remounted read-only (i.e. the root fs will be r/o and everything else
is gone). All services are gone at that point too, hence you live in a
very minimal, very reduced environment. If you issue your RS232
commands from that environment you should be mostly good. (but again,
if you do complex storage all of this falls apart, and you have to do
the initrd thing instead).

Note that /usr/lib/systemd/system-shutdown/ is outside of usual
service management. It's run at a point where service management is
already turned off. As such, you really just drop executable scripts
or binaries there, and nothing long-running, no daemons or such, just
simple programs that run and exit.

For details about this facility see the systemd-halt.service(8) man
page. The scripts executed that way will get one parameter, which
tells it what operation is being executed. And if its "poweroff", then
you know that the system is going down for powering off rather than
reboot.

Lennart

-- 
Lennart Poettering, Red Hat