linux-kernel - Re: [PATCH v2 1/2] module: update dependencies at try_module

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Message-ID: <20220430143814.7184bd93@sal.lan>
Date:   Sat, 30 Apr 2022 14:38:14 +0100
From:   Mauro Carvalho Chehab <mchehab@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jaroslav Kysela <perex@...ex.cz>,
        Kai Vehmanen <kai.vehmanen@...el.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...el.com>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org,
        mauro.chehab@...ux.intel.com,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 1/2] module: update dependencies at try_module_get()

Em Sat, 30 Apr 2022 14:04:59 +0200
Greg KH <gregkh@...uxfoundation.org> escreveu:

> On Sat, Apr 30, 2022 at 11:30:58AM +0100, Mauro Carvalho Chehab wrote:

> Did you run checkpatch on this?  Please do :)
> 
> > +
> > +	if (mod == this)
> > +		return 0;  
> 
> How can this happen?
> When people mistakenly call try_module_get(THIS_MODULE)?  

Yes. There are lots of place where this is happening:

	$ git grep try_module_get\(THIS_MODULE|wc -l
	82

> We should
> throw up a big warning when that happens anyway as that's always wrong.
> 
> But that's a different issue from this change, sorry for the noise.

It sounds very weird to use try_module_get(THIS_MODULE).

We could add a WARN_ON() there - or something similar - but I would do it 
on a separate patch.

> 
> > +
> > +	mutex_lock(&module_mutex);
> > +
> > +	ret = ref_module(this, mod);
> > +
> > +#ifdef CONFIG_MODULE_UNLOAD
> > +	if (ret)
> > +		goto ret;
> > +
> > +	ret = sysfs_create_link(mod->holders_dir,
> > +				&this->mkobj.kobj, this->name);  
> 
> Meta comment, why do we only create links if we can unload things?

Good question. I don't know for certain. This is the already existing 
pattern at add_usage_links() - see kernel/module/sysfs.c.

Also, lsmod uses sysfs links when showing dependencies.

Regards,
Mauro