[Bug 1713536] [NEW] udev: boot script does not trigger subsystem coldplug

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The udev initramfs-tools boot script does not trigger subsystem "add"
uevents. As a result, udev rules that listen to subsystem "add" events
are never activated. This problem exists on at least Ubuntu 16.04 and
17.10.

On s390, this results in a boot failure if the kernel is configured to
start with an active device black list (kernel parameter
cio_ignore=all,!condev). An example for an affected udev rule looks like
this:

    ACTION=="add", SUBSYSTEM=="subsystem", KERNEL=="ccw",
RUN{program}+="/bin/sh -c 'echo free 0009,ec30,ec32,f5f0-f5f2 >
/proc/cio_ignore'"

A proposed fix would be:

Modify /usr/share/initramfs-tools/scripts/init-top/udev:

Replace line
        udevadm trigger --action=add
with
        udevadm trigger --type=subsystems --action=add
        udevadm trigger --type=devices --action=add

This would also be consistent with the steps that the systemd udev
coldplug unit file performs (see /lib/systemd/system/systemd-udev-
trigger.service).

** Affects: systemd (Ubuntu)
     Importance: Undecided
     Assignee: Skipper Bug Screeners (skipper-screen-team)
         Status: New


** Tags: architecture-s39064 bugnameltc-158070 severity-medium targetmilestone-inin1604
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udev: boot script does not trigger subsystem coldplug
https://bugs.launchpad.net/bugs/1713536
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