Re: [PATCH] modules: add support for soft module dependencies

[Tom Gundersen]

On Tue, Sep 10, 2013 at 4:01 PM, Lucas De Marchi
<lucas.de.marchi@xxxxxxxxx> wrote:
> On Wed, Jul 24, 2013 at 11:03 PM, Herbert Xu
> <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
>> On Thu, Jul 25, 2013 at 09:32:02AM +0930, Rusty Russell wrote:
>>> Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> writes:
>>> > Hi Rusty:
>>> >
>>> > I don't know why this patch never went into the kernel, even
>>> > though the corresponding features have been added to modprobe
>>> > in most if not all distros.
>>>
>>> Because Andreas never sent me the patch?  This is the first I've *heard*
>>> of this feature.  Looks like it didn't hit lkml either.  And what was
>>> 2/2?
>>
>> 2/2 was the patch to actually use this in crc32c.
>>
>>> It's not how I would have done this: post-deps are more flexibly done at
>>> runtime, because the module may have to do work to figure out what to
>>> pull in.  But since it already exists, I'll apply this patch: it doesn't
>>> cost the kernel anything.
>
> But it did cause boot failures. The file modules.softdep file was
> supposed to be informational until now. That's why depmod put a
> comment saying to "copy on user's discretion to /etc/modules.d"
> instead of parsing it directly.
>
> If dependencies expressed with softdeps can be required dependencies
> now, depmod needs to be updated to work it out otherwise we will get
> missing dependencies as happened with the crypto stuff after this
> patch. I'm CC'ing Tom who had the boot failure; it looks like the same
> boot failure that caused this feature to get reverted the first time
> (the deps being required by sd_mod).

For the record: what happened was that I didn't get the "softdep'ed"
modules  in my initrd and when trying to modprobe sd_mod in the initrd
I got an error message about missing symbols (I forget the precise
wording). We cold certainly fix the initrd generation to include the
softdep modules, but it seems unexpected that missing softdeps should
cause failures like this.

> Even if it is an optional module, it would be great to have depmod
> updated so it parses the file directly now that we are exporting this
> info. I can do that once we agree what we want to do with softdeps
> coming directly from kernel itself.

I just submitted a patch to do this, I think it is much more useful
than expecting the packager/admin to copy modules.softdep to the
correct location.

> However looking at the only user right now, crct10dif: couldn't we
> detect at runtime if this module can be used instead of just trying to
> load it as a pre softdep and possibly failing?

Having softdeps statically specified is useful for inird generation.
Even if it turns out that the modules are truly optional, having the
possibility of automatically including all optional modules would
surely be useful.

Cheers,

Tom
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