<para><filename>systemd-halt.service</filename> is a
system service that is pulled in by
<filename>halt.target</filename> and is responsible
- for the actual system halt. Similar,
+ for the actual system halt. Similarly,
<filename>systemd-poweroff.service</filename> is
pulled in by <filename>poweroff.target</filename>,
<filename>systemd-reboot.service</filename> by
<filename>kexec.target</filename> to execute the
respective actions.</para>
- <para>When these services are run they ensure that PID
+ <para>When these services are run, they ensure that PID
1 is replaced by the
<filename>/usr/lib/systemd/systemd-shutdown</filename>
tool which is then responsible for the actual
- shutdown. Before shutting down this binary will try to
+ shutdown. Before shutting down, this binary will try to
unmount all remaining file systems, disable all
remaining swap devices, detach all remaining storage
devices and kill all remaining processes.</para>
+ <para>It is necessary to have this code in a separate binary
+ because otherwise rebooting after an upgrade might be broken —
+ the running PID 1 could still depend on libraries which are not
+ available any more, thus keeping the file system busy, which
+ then cannot be re-mounted read-only.</para>
+
<para>Immediately before executing the actual system
halt/poweroff/reboot/kexec
<filename>systemd-shutdown</filename> will run all
~ianmdlvl / elogind.git / blobdiff