How to prevent a module from unloading when in used

[Greg KH]

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