I guess I don't understand why we see this in focal. The two events in
Colin's trace always happen on any Ubuntu kernel. We should see if we
can get another udev trace on bionic that captures both CHANGE events,
one will be from the bcache driver itself, and one is from the block
layer. THe order and content of the change event matter.
Another thing I don't understand is why does udev drop the a symlink created by
another rule? This seems like the core issue.
Looking at systemd/udev source code; udev will do a
FOREACH_DEVICE_DEVLINK and check if the name is in the database file for
the device. Its not clear to me yet how or when the database file get's
written.
The other question I have is: if we reversed the order of the focal
CHANGE events, wouldn't we see just the opposite happen (the
driver=bcache change event would not have all of the devlinks from a
blkid probe) and all of the /dev/disk/by-{id, uuid, ...} get removed
when running
I think the patch you're proposing should work; but I don't think we've
root caused why the link gets removed in the first place. Once we
understand the root cause, I think we can better understand what a fix
should look like.
Lastly, I think we might also test this out on mainline kernel; I wonder if our
SAUCE patch for emitting the CACHED_UUID needs an update (or could be dropped).
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https://bugs.launchpad.net/bugs/1861941
Title:
bcache by-uuid links disappear after mounting bcache0
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