Re: udev breakages - was: Re: Need of an ".async_probe()" type of callback at driver's core - Was: Re: [PATCH] [media] drxk: change it to use request_firmware_nowait()

[Kay Sievers <kay@vrfy.org>]

On Wed, Oct 3, 2012 at 6:57 PM, Greg KH <gregkh@linuxfoundation.org> wrote:

>> It's the same in the current release, we still haven't wrapped our
>> head around how to fix it/work around it.
>
> Ick, as this is breaking people's previously-working machines, shouldn't
> this be resolved quickly?

Nothing really "breaks", It's "slow" and it will surely be fixed when
we know what's the right fix, which we haven't sorted out at this
moment.

> module_init() can do lots of "bad" things, sleeping, asking for
> firmware, and lots of other things.  To have userspace block because of
> this doesn't seem very wise.

Not saying that it is right or nice, but it's the kernel itself that
blocks. Run init=/bin/sh and do a modprobe of one of these drivers and
it hangs un-interruptible until the kernel's internal firmware loading
request times out, just because userspace is not there.

> But previously this all "just worked" as we ran 'modprobe' in a new
> thread/process right?

No, we used to un-queue events which had a timeout specified in the
environment, that code caused other issues and was removed.

> it can do without worrying about stopping anything else in the system that might
> want to happen at the same time (like load multiple modules in a row).

It should not be an issue, the serialization is strictly parent <->
child, everything else runs in parallel.

>> If that unfortunate module_init() lockup can't be solved properly in
>> the kernel, we need to find out if we need to make the modprobe
>> handling in udev async, or let firmware events bypass dependency
>> resolving. As mentioned, we haven't decided as of now which road to
>> take here.
>
> It's not a lockup, there have never been rules about what a driver could
> and could not do in its module_init() function.  Sure, there are some
> not-nice drivers out there, but don't halt the whole system just because
> of them.

It is a kind of lock up, just try modprobe with the init=/bin/sh boot.

> I recommend making module loading async, like it used to be, and then
> all should be fine, right?

That's the current idea, and Tom is looking into it how it could look like.

I also have no issues at all if the kernel does load the firmware from
the filesystem on its own; it sounds like the simplest and most robust
solution from a general look at the problem. It would also make the
difference between in-kernel firmware and out-of-kernel firmware less
visible, which sounds good.
Honestly, requiring firmware-loading userspace-transactions to
successfully link a module into the kernel sounds like a pretty bad
idea to start with. Unlike module loading which needs the depmod alias
database and userspace configuration; with firmware loading, there is
no policy involved where userspace would add any single additional
value to that step.

Thanks,
Kay