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

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Date:   Sun, 1 May 2022 13:23:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mauro Carvalho Chehab' <mchehab@...nel.org>,
        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" <alsa-devel@...a-project.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        "mauro.chehab@...ux.intel.com" <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()

From: Mauro Carvalho Chehab
> Sent: 30 April 2022 14:38
> 
> 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.

You could add a compile-time check.
But a run-time one seems unnecessary.
Clearly try_module_get(THIS_MODULE) usually succeeds.

I think I can invent a case where it can fail:
The module count must be zero, and a module unload in progress.
The thread doing the unload is blocked somewhere.
Another thread makes a callback into the module for some request
that (for instance) would need to create a kernel thread.
It tries to get a reference for the thread.
So try_module_get(THIS_MODULE) is the right call - and will fail here.

	David

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