Description of problem:

When setting ShutdownWatchdogSec=2min for example and using the softdog watchdog (or any other implementation), we can see through a systemtap kernel module that the setting is not taken into account but 600 is used instead (== 10 minutes, the default):
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
# cat /etc/systemd/system.conf
...
RuntimeWatchdogSec=5min
ShutdownWatchdogSec=2min
...

# mkdir -p /var/log/journal
# systemctl restart systemd-journald

# stap -g -m watch_systemd -p 4 -d /usr/lib/systemd/systemd --ldd -e 'probe process("/usr/lib/systemd/libsystemd-shared-239.so").function("watchdog_set_timeout") { printk(2, sprintf("%ld (%s) calling watchdog_set_timeout(%ld)", pid(), execname(), user_long($usec)))
# staprun -L ./watch_systemd.ko
# reboot
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Result:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
# journalctl -b -1 SYSLOG_IDENTIFIER=kernel -o short-precise --no-pager | grep watchdog_set_timeout
Sep 15 13:52:21.248313 vm-rhel8 kernel: 1 (systemd) calling watchdog_set_timeout(600000000)
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

The root cause for this is missing Upstream commit 7d9eea2bd3d4f83668c7a78754d201b226acbf1e.
Without this commit, *arg_shutdown_watchdog* variable is reset to the default before calling the shutdown code, as shown below, excerpt of systemd-239/src/core/main.c:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
 :
2617 finish:
2618         pager_close();
2619 
2620         if (m) {
2621                 arg_shutdown_watchdog = m->shutdown_watchdog;  <<<< VALUE RETRIEVED FROM RUNTIME SETTING
2622                 m = manager_free(m);
2623         }
2624 
2625         reset_arguments(); <<<< VALUE RESET TO DEFAULT (10 MINS)
 :
2654         if (shutdown_verb) {
2655                 r = become_shutdown(shutdown_verb, retval); <<<< SHUTDOWN CODE CALLED
 :
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------


Version-Release number of selected component (if applicable):

systemd-239

How reproducible:

Always, see above.