The script call is quite expensive (it does multiple things, including
checking device-mapper, qubesdb etc). Don't call it for devices we (or
else) already excluded earlier.
This is the most relevant for dom0, where udev "change" event is
triggered quite often, for multiple LVM volumes - all excluded, because
being VM's disks.
1. Do not detach device forcefully when it's removed. This breaks
libvirt (which thinks the device is still there). After this change, it
is possible to detach device using libvirt, even if it was already
removed physically from backend domain (unless it is dom0 - in which
case it is still broken). So, this is partial fix for
QubesOS/qubes-issues#1082.
2. Do not trigger "change" udev event when only QubesDB state needs to
be updated - this leads to massive udev events queue, and heavy I/O
usage - for example scanning all LVM many times. In some cases it even
caused infinite event queue.
3. Do not use QUBES_EXPOSED udev property - it was needed a while back
before QubesDB, because concurrent xenstore accesses are expensive
(because of transactions). It isn't the problem on QubesDB.
4. Cache information about device-mapper, so it is possible to
reconstruct it at device remove - when the actual device cannot be
queried anymore. This is specifically about list of lower layer devices
used.
5. Allow excluding loop devices pointing at a file in directory marked
with ".qubes-exclude-block-devices" file. This is more generic than
hardcoding /var/lib/qubes.
QubesOS/qubes-issues#3084FixesQubesOS/qubes-issues#3073QubesOS/qubes-issues#1082
For example do not exclude (from exposing to qvm-block) loop devices
with this flag set, otherwise `qvm-block -l` would not list just
attached disk image files.
FixesQubesOS/qubes-issues#2453
There are already some other rules to ignore not interesting devices.
This includes device-mapper assembled in initramfs manually. 'dmroot'
isn't properly detected as mounted because /dev/mapper/dmroot isn't a
symlink to /dev/dm-0 and /proc/mounts contains the former name, while
udev event the later.
FixesQubesOS/qubes-issues#1586
Since migration to QubesDB, it isn't needed anymore (QubesDB have no
problem with concurrent writes, as transactions are not supported).
This should speedup system startup.
It can happen during device reconfiguration - do not decide to expose
the device until its known what device it will be.
This fixes bug where root.img was visible in qvm-block as normal device
and could be detached.
xenstored does handle concurrent writes very harshly - it aborts the
whole transaction if any other write happened the same time. For udev
(which process all the events almost at once) it means hundreds of
retries and in some times even exceed udev timeout (60s or so).
To prevent this problem, add locking to allow only one such event being
processed at once. It looks like it should slow down the system startup,
but actually it does otherwise.
