linux-kernel - Re: kobject_put vs module unload

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Date:	Sat, 8 Jan 2011 13:01:21 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Kay Sievers <kay.sievers@...y.org>
cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com
Subject: Re: kobject_put vs module unload



On Sat, 8 Jan 2011, Kay Sievers wrote:

> On Sat, Jan 8, 2011 at 14:19, Mikulas Patocka <mpatocka@...hat.com> wrote:
> > On Fri, 7 Jan 2011, Greg KH wrote:
> 
> >> > 4) the reference obtained at point 1) is dropped, kobject reference count
> >> > reaches zero and the release callback is called. But that callback points
> >> > to an unloaded module and causes a crash.
> >> >
> >> > How is it solved? Am I missing something?
> >>
> >> You have the code that creates and frees the object, to not be in the
> >> module that could have been unloaded.  It's really just that simple.
> >
> > If the whole device mapper subsystem can be a module, where can I put the
> > code?
> >
> > I think a similar bug exists in md, it can also be unloaded as a module
> > and it has "release" method in its module code.
> 
> If you still have data structures hanging around, these structures are
> supposed to take a reference on the module -- and you can not unload
> the module as long as this is the case.
> 
> Kay

kobject references don't increase module reference count. If kobject held 
module references, it would be a fix for this bug, but could cause other 
bugs (some modules could be unloadable due to self-references to its own 
kobjects).

Mikulas