Re: [systemd-devel] Trying to turn off a UPS with home-made service unit

2013-09-15 Thread Roger Price

On Wed Sep 11 10:45:48 PDT 2013 Lennart Poettering wrote:

So, it's a really bad idea to turn off UPS from userspace, simply 
because you will race against the kernel's fs sync() code, and you never 
know what will finish first: your UPS shutdown timeout or your fs 
sync(). Doing this from userspace is hence racy.


If you want to use a hack like this then at least do it via the 
/usr/lib/systemd/system-shutdown/ drop-in directory which is called very 
late during shutdown. It will shorten the race window, but not erase it, 
since the kernel will flush various buffers only after the reboot() 
system call is invoked.


Here is the effect on a simple openSUSE 12.3 test rig of replacing my 
home-made service unit /lib/systemd/system/ups-delayed-shutdown.service 
with a script in the /usr/lib/systemd/system-shutdown/ drop-in directory. 
I used the default UPS offdelay of 20 secs. The X's indicate the race 
window. (Sorry for the ascii art.)


1) My bad idea ups-delayed-shutdown.service

systemctl   system
poweroff halt
   | ||
   0   2 | 4   6   8  10  12  14  16  18  20  22  24  26  28  30  32 secs
 |   |
  upsdrvctl UPS
  shutdownshutdown

2) Replaced by script in /usr/lib/systemd/system-shutdown/

systemctl   system
poweroff halt
   | ||
   0   2   4   6   8 |10  12  14  16  18  20  22  24  26  28  30  32 secs
 |   |
 upsdrvctl  UPS
 shutdown shutdown

The script solution has the advantage of reducing the race window from 9 
to 3 secs.  Note that the same security can also be achieved by the 
service unit by setting offdelay = 26 in ups.conf.  My script is


   #! /bin/bash
   # Delayed turn off for the UPS unit.
   # Needed for automatic system restart when wall power returns.
   UPSDRVCTL_BIN=/usr/lib/ups/driver/upsdrvctl
   $UPSDRVCTL_BIN shutdown

I tried adding

   echo `date -I` `date +%T` $0 calls $UPSDRVCTL_BIN shutdown  
/var/log/UPS.log

to get a trace of the activity, but nothing is written in 
/var/log/UPS.log.


Roger
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Re: [systemd-devel] Trying to turn off a UPS with home-made service unit

2013-09-11 Thread Lennart Poettering
On Sat, 10.08.13 17:02, Roger Price (ro...@rogerprice.org) wrote:

 I am new to systemd and I am trying to power off a UPS unit using a
 home-made service unit
 /lib/systemd/system/ups-delayed-shutdown.service running on openSUSE
 12.3.

So, it's a really bad idea to turn off UPS from userspace, simply
because you will race against the kernel's fs sync() code, and you
never know what will finish first: your UPS shutdown timeout or your fs
sync(). Doing this from userspace is hence racy.

If you want to use a hack like this then at least do it via the
/usr/lib/systemd/system-shutdown/ drop-in directory which is called very
late during shutdown. It will shorten the race window, but not erase it,
since the kernel will flush various buffers only after the reboot()
system call is invoked.

See systemd-shutdown(8) for details.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.
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Re: [systemd-devel] Trying to turn off a UPS with home-made service unit

2013-08-10 Thread Roger Price

On Sat, 10 Aug 2013, Andrey Borzenkov wrote:


Home made service should normally go into /etc/systemd.


I moved my service to /etc/systemd/system/


[Unit]

You need to add DefaultDependencies=no. Otherwise your service will
conflict with shutdown.target.


I added DefaultDependencies=no and my service works!


systemd --system --test --unit poweroff.target --log-level debug
can be used to see what would be started and why.


Thanks for your help!
Roger

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