Re: Kernel modules - documentation?

[Paul Goyette <paul%whooppee.com@localhost>]

On Sat, 22 May 2010, Adam Hamsik wrote:

> > My specific questions:
> >
> > What actually triggers an autoload of a module?  (There seem to be 
> > very few places where module_autoload() is called.)
>
> Device module_autoloading is done in specfs_open.

OK.  So I guess there's no generic "Gee, the kernel just tried to 
reference something that's not here, so maybe we can find a module to 
resolve that reference."

> > What is the semantic difference between module_autoload() and a 
> > "normal" module_load()?
>
> module_autoload happens automatically when you try to open device node 
> which doesn't have device driver in kernel.

My question was more along the lines of "what is the _difference_ 
between module_load() vs module_autoload()?"  Or even simpler, "Why do 
both routines exist?"  :)

> > Does the code which calls either of these routines need to be 
> > concerned with whether the module has been previously loaded?  Is it 
> > OK to load a module that has already been loaded?
>
> I think that this is not possible and these routines return an error 
> when you try to do that.

That's OK, I'd expect the error.  I can ignore that.  I just need to be 
sure that the previously-loaded module doesn't get "screwed up" from the 
attempt to load it the second time.

> > Given that there is a kernel thread that runs around and attempts to 
> > unload any unreferenced modules that have been loaded "for a while", 
> is it ever necessary or desirable to explicitly unload a module?
>
> It doesn't work this way there is a thread(workqueue) which try to 
> unload module in first 300 seconds or mili seconds I can't remember 
> now. But this thread or whatever is it doesn't unload modules older 
> that set limit AFAIK.

Hmmm, I must have misunderstood this code - time to go look again.

> > What happens if a global symbol referenced by a module doesn't exist? 
> Does the module get loaded anyway, leaving the reference unresolved?
>
> If you have a module which uses foo routine and it is not defined in 
> kernel or loaded modules your module will simply not load.

This is actually what I was hoping would happen.


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