How to prevent a module from unloading when in used

[greg@kroah.com (Greg KH)]

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@gmail.com> wrote:
> 
> 
> 
>     On Wed, Jul 16, 2014 at 6:39 PM, John de la Garza <john@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.

thanks,

greg k-h