Em Sat, 30 Apr 2022 14:04:59 +0200
Greg KH <[email protected]> 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
From: Mauro Carvalho Chehab
> Sent: 30 April 2022 14:38
>
> Em Sat, 30 Apr 2022 14:04:59 +0200
> Greg KH <[email protected]> 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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