Comment # 12 on bug 929564 from Michal Koutny

Restarting dbus-daemon is problematic (just picket at random [1]).

I'm not sure how much dbus-daemon restart should be supported. The issue here
is that if it's restarted, systemd-logind loses its connection to it, however,
the daemon itself keeps running. When a request comes to restarted dbus-daemon
to some methods of org.freedesktop.login1 service, it attempts to start
systemd-logind (dbus activation), which fails as systemd-logind is already
running and it isn't able to reconnect to dbus-daemon under these
circumstances.

Possible workaround might be to add Requires=dbus.service (instead of
dbus.socket), so that dbus.service restart would trigger systemd-logind.service
restart as well.

[1] https://github.com/systemd/systemd/issues/2925#issuecomment-204330401