Re: kobject_put vs module unload

Mikulas Patocka
2011-01-08T13:19:30Z

On Fri, 7 Jan 2011, Greg KH wrote:

> On Fri, Jan 07, 2011 at 10:42:22PM -0500, Mikulas Patocka wrote:
> > Hi
> >
> > I'm looking at some kobject problem in device mapper and I came across
> > this problem:
> >
> > According to kobject interface specification, If we embed a kobject into
> > the device structure, we shouldn't free the device structure when the
> > device is unloaded, but we should register a "release" callback in
> > kobj_type that will actually free the device structure.
>
> That is correct.
>
> What does "device is unloaded" mean? That really doesn't mean anything
> in reference to the driver model/kobject code.
>
> > What happens in this scenario?:
> >
> > 1) someone references a device kobject
> > 2) the device is unloaded (but the device structure is still in memory
> > because of that reference)
>
> What do you mean here? How can a device be "unloaded"? Is it just
> unregistered?

Device mapper devices can be unloaded by the user with "dmsetup remove"
command. Other drivers (like md), offer other commands to unload devices,
but the principle is the same.

> > 3) the driver module is unloaded
>
> Oops, you just lost.

If you unload all device mappers's devices, the module reference count
drops to zero and you can unload the device mapper module.

> > 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.

> Hope this helps,
>
> greg k-h

Mikulas