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

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Message-ID: <878uyuzxp0.fsf@rustcorp.com.au>
Date:	Wed, 18 Sep 2013 11:40:51 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Lucas De Marchi <lucas.de.marchi@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Andreas Robinson <andr345@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zhao Hongjiang <zhaohongjiang@...wei.com>,
	David Miller <davem@...emloft.net>, tim.c.chen@...ux.intel.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	linux-crypto@...r.kernel.org,
	linux-modules <linux-modules@...r.kernel.org>,
	Tom Gundersen <teg@...m.no>
Subject: Re: [PATCH] modules: add support for soft module dependencies

Lucas De Marchi <lucas.de.marchi@...il.com> writes:
> On Thu, Sep 12, 2013 at 9:07 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>> Lucas De Marchi <lucas.de.marchi@...il.com> writes:
>>> On Wed, Jul 24, 2013 at 11:03 PM, Herbert Xu
>>> <herbert@...dor.apana.org.au> wrote:
>>>> On Thu, Jul 25, 2013 at 09:32:02AM +0930, Rusty Russell wrote:
>>>>> Herbert Xu <herbert@...dor.apana.org.au> 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.
>>
>> I'm happy to change this macro to create a modinfo line like
>> "softdep:<modname>"
>
> how is that solving the issue that this macro can be used to designate
> a mandatory or optional dependency
> (https://lkml.org/lkml/2013/9/10/371)? If we decide the dependency is
> mandatory we can very well let modprobe use that dependency during
> module load

I'm very close to sending Linus a revert commit at this point, since
there's no consensus on what it's for.

*Clearly* softdep shouldn't indicate a mandatory dependency.  We already
have a way (several) to make mandatory dependencies!

And the "pre:" vs "post:" thing is just weird.  If a module wants a post
dependency, you can request_module() it from a workqueue.

>> ie. tools like mkinitrd could pick it up and try to find a matching
>> module, but depmod would ignore it.
>
> Some mkinitrd-like use whatever depmod/modprobe tells them it's
> needed. So kmod still needs to know about it.

It sounds like we should create a separate tool, which takes a list of
modules and spits out the full pathname of all dependencies.  *That*
tool should include soft dependencies.

>> It's really up to Lucas, since this affects him.
>
> IMO  saying "this is an optional dependency and we can work without"
> doesn't buy us much. Distros will end up putting the soft dep in
> /etc/modules.d, kmod will always use them anyway and failing to load
> the soft dep will fail the module load. I'd like to have no distro
> files in /etc/modules.d in future.

I assumed modprobe would handle soft dependencies in modules and try to
pull them in, but *not* fail if they don't work.

The previous way of doing this was:
        install foo modprobe foo_softdep 2>/dev/null; modprobe --ignore-install foo $CMDLINE_OPTS

I agree this logic belongs in the kernel, we just have to figure out
exactly how.

Cheers,
Rusty.
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