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

[Mantas Mikulėnas]

If I remember correctly, udev (recent versions) takes a BSD lock using
flock(2) while processing the device, and tools are supposed to do the
same. The flock() call can be set to wait until the lock can be taken.

On Sun, Nov 26, 2023 at 1:40 AM 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.
>
> 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
>


-- 
Mantas Mikulėnas
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