Re: udevadm settle persistently failing

[Nix <nix@esperi.org.uk>]

On 16 May 2011, Kay Sievers outgrape:
> With devtmpfs you get the "early" /dev for free, and you preserve the
> "early" /dev in the real root. Instead of mounting a 'tmpfs' just
> mount a 'devtmpfs' at /dev, and it will be magically filled already,
> that's the only difference.
> 
> When mounting the real root, just mount --move it over to the real
> root before switching to the real root.

I'll have to see what busybox's switch_root does. I know it deletes
the entire contents of the root directory it switches away from: I'll
have to make sure that doesn't somehow include moved-away mountpoints.
(I doubt it does.)

> Maybe you can run:
>   udevadm monitor
> in the background writing to a file, and get timestamps of your commands too.

They're fine.

After a bunch of straces, I think the problem is different. As I
suspected from its intermittent nature, it's a race.

When udevd starts up, it looks for an old database and converts it into
a new-style /run database. This takes nonzero time, and is done *after*
daemonization. Unfortunately, in the same time window it is quite
possible to fit a pair of 'udevadm trigger's and a 'udevadm settle': and
when udevadm settle kicks up and finds that udev hasn't even initialized
yet, what does it do? Well, unless I misread the code
udev_queue_get_queue_is_empty() returns 1 in this situation, so 'udevadm
settle' returns EXIT_SUCCESS, and boom.

My evidence for this is very thin: it's no more than a hypothesis
really. My only evidence for this so far is that I haven't managed to
get the failure to happen unless I mkdir the old udev directories first
to force udev to do an old->new db conversion. It would be nice to
gather more data, but it's hard to do that without perturbing the race
:/ even an ls of /run slows it down enough that it doesn't happen
anymore. Hm, perhaps an echo would be fast enough, it's a shell
builtin after all...

If I'm right (and I'm not sure I am, but it feels plausible), then I
suspect the only fix is to move the db conversion so that it happens
before daemonization.

-- 
NULL && (void)