[systemd-devel] question about system reboot and shutdown

Wed Aug 9 09:42:20 UTC 2017

Dnia środa, 9 sierpnia 2017 09:17:44 CEST Mantas Mikulėnas pisze:
> /dev is a separate filesystem and is never read-only.

right ;)

> 
> Another approach would be to let microupsd exit normally, but then start a
> separate microupsd instance (e.g. microupsd-shutdown.service) which
> schedules the UPS poweroff.

Thanks for suggestion
I will stick to the approach suggested by Lennart it looks like a simple 
solution to me, and the binary is called at the very late stage of shutdown 
process after every databases and daemons are closed.

> 
> On Wed, Aug 9, 2017, 12:03 Marek Floriańczyk <marek.florianczyk at gmail.com>
> 
> wrote:
> > Dnia środa, 9 sierpnia 2017 10:29:37 CEST Lennart Poettering pisze:
> > > 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.
> > 
> > Hi,
> > 
> > This looks like a good way for me.
> > I cannot really use initrd, because my MicroUPS is intended to work not
> > only
> > for me but also for an average people, with some .deb package to install.
> > This is an early version of my device, now it looks different, has higher
> > power output and so on
> > 
> > https://www.indiegogo.com/projects/microups-for-raspberry-beaglebone-cubie
> > board#/
> > 
> > 
> > I'm aware of race condition but some microcomputers like Raspberry if they
> > shutdown the operating system, the power is still ON, and I have no way to
> > figure out when to turn power off. So what I'm doing is telling microups
> > device
> > to cut power off 30 seconds after shutdown has been initiated (parameter
> > is
> > configurable).
> > Microcomputers usually have some SD card, sometimes built-in NAND memory,
> > and
> > sometimes single SATA SSD disk, so complex storage is not an issue (I
> > think)
> > 
> > The question is, will my binary be able to open RS232 port eg.
> > /dev/ttyACM0
> > when filesystem is Read-Only ?
> > And if it is binary it will need to load some libs at least libc at the
> > start.
> > 
> > The man page you pointed to says the parameter is: "halt", "poweroff",
> > "reboot" or "kexec", so the first two should work for me.
> > 
> > Best Regards
> > Marek
> > 
> > > Lennart
> > 
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