/dev is a separate filesystem and is never read-only. 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.
On Wed, Aug 9, 2017, 12:03 Marek Floriańczyk <marek.florianc...@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.florianc...@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-cubieboard#/ > > > 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 > > > _______________________________________________ > systemd-devel mailing list > systemd-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/systemd-devel > -- Mantas Mikulėnas <graw...@gmail.com> Sent from my phone
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