How to prevent a module from unloading when in used

[Chetan Nanda]

On Fri, Jul 18, 2014 at 9:47 AM, Greg KH <greg at kroah.com> wrote:

> On Fri, Jul 18, 2014 at 09:43:48AM +0530, Chetan Nanda wrote:
> >
> >
> >
> > On Wed, Jul 16, 2014 at 9:51 PM, Greg KH <greg at kroah.com> wrote:
> >
> >     On Wed, Jul 16, 2014 at 08:57:38PM +0530, Chetan Nanda wrote:
> >     >
> >     >
> >     >
> >     > On Wed, Jul 16, 2014 at 8:49 PM, Chetan Nanda <
> chetannanda at gmail.com>
> >     wrote:
> >     >
> >     >
> >     >
> >     >     On Wed, Jul 16, 2014 at 6:39 PM, John de la Garza <
> john at jjdev.com>
> >     wrote:
> >     >
> >     >         On Wed, Jul 16, 2014 at 04:00:18PM +0530, Chetan Nanda
> wrote:
> >     >         > A depends on B, so B is automatically loaded when A is
> loaded.
> >     >         > B module is also directly being used by the user side
> code via
> >     misc
> >     >         > interface.
> >     >         >.
> >     >         > Now when I am unloading module A, via "modprobe -r A" it
> is
> >     also
> >     >         unloading
> >     >         > the module B which is being used by the application and
> >     resulting in
> >     >         the
> >     >         > kernel crash.
> >     >
> >     >         You said that A depends on B, right?  Why do you have A
> dependng
> >     on B?
> >     >         If it A needs to have B then it makes sense that you can
> not
> >     remove A
> >     >         while
> >     >         B is in use.  If A doesn't need B, why not remove the
> dependency.
> >     >
> >     >
> >     > A is calling few APIs defined by B.
> >     >
> >     > But why when user space application is already using  module B.
> (it has
> >     already
> >     > open its device fd) kernel allows to remove it.
> >     >
> >     > I tried with doing try_module_get() in the module's open function,
> it
> >     prevent
> >     > module B unloading but cause thread doing modprobe -r to hang
> >     > Is there any other way to mark module as busy when being used by
> user
> >     > application?
> >
> >     Never use try_module_get(), that is racy.
> >
> >     What is the user/kernel interface you are using, and why doesn't it
> >     automatically increase the module count when userspace opens the
> >     interface?  It should all be done in a way that your module doesn't
> need
> >     to do anything special.
> >
> >
> > Hi Greg,
> >
> > Thanks for your mail.
> >
> > Module is using misc driver interface to export its functionality to
> > userspace,
> >
> > Need to debug further why module count is not getting incremented
> automatically
> > when module is open by userspace application via open system call.
>
> Are you properly setting the .owner field of your file operations
> structure to be THIS_MODULE?  If not, try fixing that up.  If you are,
> try posting your code for review.
>
> greg k-h
>

Hi Greg,

Thanks for the hint, indeed that was the issue, .owner field was not set in
file operation structure.
After setting that, 'modprobe -r A' is hanging. As the module B in use and
can't be removed.

I am using busybox on embedded Linux, I think this could be a modprobe
utility issue.
Ideally modprobe should not try to remove the module in used.

Thanks,
Chetan Nanda
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