RE: [PATCH v2 1/2] module: update dependencies at try_module_get()

[David Laight <David.Laight@ACULAB.COM>]

From: Mauro Carvalho Chehab
> Sent: 30 April 2022 14:38
> 
> Em Sat, 30 Apr 2022 14:04:59 +0200
> Greg KH <gregkh@linuxfoundation.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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)