Re: MOD_INC_USE_COUNT

["T.L.Madhu" <madhu.tarikere@wipro.com>]

On Mon, 9 Dec 2002, Edward Gerhold wrote:

> 
> 
> Jose Luis Alarcon wrote:
> 
> >   Hi, kernel developers.
> > 
> >   I has a big surprise when, adding at the beginning of init_module()
> > the macro MOD_INC_USE_COUNT, for have a module use counter. My program
> > get compile well and insmod work too.
> > 
> >   But the surprise was when i did rmmod, cos the output said:
> > 
> > FATAL: the module is in use
> > 
> >   I did lsmod, and i got that my module was used by [unsafe].
> > 
> >   Any kernel programmer with experience knows why happen this?.
> > 
> >   IMHO a use counter is a very few dangerous thing. So i was trying with.
> > 
> >   Thanks you, very much, in advance.
> > 
> >   Regards.
> > 
> >   Jose.
> > 
> 
> I don't know why this happens,
> but can it be, that you forgot MOD_DEC_USE_COUNT in your cleanup routine ?
> 
> bye
> Edward
> 

Basically, adding MOD_INC_USE_COUNT in init_module() is not a right
idea, usage counts are used to keep track of current users of the module &
the system doesn't allow removing the module if usage count is non-zero. 

In your case, 
- You have incremented usage count to 1 during init_module()
- When you run rmmod, it calls delete_module() system call which 
  checks for the usage count, if it is zero, then ONLY it calls the
  cleanup routine of corresponding module, if it is non-zero, system
  call returns EBUSY error thinking someone is using this module, so 
  even if you do add MOD_DEC_USE_COUNT in clenup routine, it's of no
  use. For more info. look for implementation of sys_delete_module().

bye,
Madhu

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