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

[Richard Weinberger]

On Wed, Nov 29, 2023 at 9:48 AM Lennart Poettering
<lennart at poettering.net> wrote:
> > Why doesn't udev flock() every device it is probing?
> > Or asked differently, why is this feature opt-in instead of opt-out?
>
> Some software really doesn't like it if we take BSD locks on their
> devices, hence we don't take it blanket everywhere. And what's more
> important even: for various devices it simply isn't safe to just
> willy-nilly even open them (tape drivers and things, which might start
> to pull in a tape if we do). For others we might not be able to even
> open thing at all with the wrong flags (for example, because they are
> output only).
>
> Bock devices have relatively well defined semantics, there it's
> generally safe to do this, hence we do.

I see.

> Hence, it might be safe for UBI, but for the general case it might
> not be.
>
> That said, would BSD locking even address your issue? If you devices
> are exclusive access things and we first open() them and then flock()
> them, then that's not atomic. So if your test cases open the devices,
> then flock() them you might still get into conflict udev because it
> just open()ed the device, but didn#t get to call flock() yet.
>
> Doesn't UBI have something like O_EXCL-behaviour that grants true
> exclusive access?

It has in-kernel support for that but not from userspace, I'm
currently evaluating
whether it's worth exposing exclusive access to userspace.

Stay tuned. :-)

-- 
Thanks,
//richard