How to properly wait for udev?

[Richard Weinberger]

Hello!

After upgrading my main test worker to a recent distribution, the UBI
test suite [0] fails at various places with -EBUSY.
The reason is that these tests create and remove UBI volumes rapidly.
A typical test sequence is as follows:
1. creation of /dev/ubi0_0
2. some exclusive operation, such as atomic update or volume resize on
/dev/ubi0_0
3. removal of /dev/ubi0_0

Both steps 2 and 3 can fail with -EBUSY because the udev worker still
holds a file descriptor to /dev/ubi0_0.

FWIW, the problem can also get triggered using UBI's shell utilities
if the system is fast enough, e.g.
# ubimkvol -N testv -S 50 -n 0 /dev/ubi0 && ubirmvol -n 0 /dev/ubi0
Volume ID 0, size 50 LEBs (793600 bytes, 775.0 KiB), LEB size 15872
bytes (15.5 KiB), dynamic, name "testv", alignment 1
ubirmvol: error!: cannot UBI remove volume
         error 16 (Device or resource busy)

Instead of adding a retry loop around -EBUSY, I believe the best
solution is to add code to wait for udev.
For example, having a udev barrier in ubi_mkvol() and ubi_rmvol() [1]
seems like a good idea to me.

What function from libsystemd do you suggest for waiting until udev is
done with rule processing?
My naive approach, using udev_queue_is_empty() and
sd_device_get_is_initialized(), does not resolve all failures so far.
Firstly, udev_queue_is_empty() doesn't seem to be exported by
libsystemd. I have open-coded it as:
static int udev_queue_is_empty(void) {
       return access("/run/udev/queue", F_OK) < 0 ?
               (errno == ENOENT ? true : -errno) : false;
}

Additionally, sd_device_get_is_initialized() seems to return sometimes
true even if the udev worker still has the volume open.
In short, which API do you recommend to ensure that the device my
thread has created is actually usable?

[0]: http://git.infradead.org/mtd-utils.git/tree/HEAD:/tests/ubi-tests
[1]: http://git.infradead.org/mtd-utils.git/blob/HEAD:/lib/libubi.c#l994

-- 
Thanks,
//richard