[systemd-devel] How to properly wait for udev?

[Lennart Poettering]

On So, 26.11.23 00:39, Richard Weinberger (richard.weinberger at gmail.com) wrote:

> 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.

Hmm, I have no experience with UBI, but are you sure we open that? why
would we? are such devices analyzed by blkid? We generally don't open
device nodes unless we have a reason to, such as doing blkid on it or
so.

What precisely fails for you? the open()? or some operation on the
opened fd?

>
> 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.

For block devices we implement this:

https://systemd.io/BLOCK_DEVICE_LOCKING

I understand UBI aren't block devices though?

If they conceptually should be considered block device equivalents, we
might want to extend the udev logic to such UBI devices too.  Patches
welcome.

We provide "udevadm lock" to lock a block device according to this
scheme from shell scripts.

> 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;
> }

This doesn't really work. udev might still process the device in the
background.

Lennart

--
Lennart Poettering, Berlin