[dm-devel] kobject_put vs module unload

[Greg KH]

On Sat, Jan 08, 2011 at 01:50:50PM -0500, Mikulas Patocka wrote:
> 
> 
> On Sat, 8 Jan 2011, Kay Sievers wrote:
> 
> > On Sat, Jan 8, 2011 at 19:01, Mikulas Patocka <mpatocka at redhat.com> wrote:
> > > On Sat, 8 Jan 2011, Kay Sievers wrote:
> > >> On Sat, Jan 8, 2011 at 14:19, Mikulas Patocka <mpatocka at redhat.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).
> > 
> > Sure, they don't automatically take references. It's the job of the
> > driver/module, to make sure to pin the module for any data of it,
> > which might be still in use.
> > 
> > Kay
> 
> A module can't unpin itself. If it does, the module can be unloaded 
> immediatelly after module_put(THIS_MODULE) call and the function that 
> called module_put crashes.
> 
> A module must be unpinned by someone else. The question is: who unpins the 
> module when kobject references are gone?

Who ever you decide to have unpin it.

Seriously, it's up to you to handle this properly, it's not something
built into the kobject/driver model at all due to it being something
that can be done in a number of different ways.

thanks,

greg k-h