From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: help needed with EXPORT_SYMBOL
Date: Mon, 23 Aug 2010 15:17:57 +0200
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To: Brian Gerst <brgerst@gmail.com>
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On Mon, 2010-08-23 at 07:48 -0400, Brian Gerst wrote:
> 
> Use an exported function pointer in the main kernel as a hook that the
> module sets when it is loaded.  Note, you must use module_get and
> module_put around the call to the module to prevent it from unloading
> while in use. 

Please don't do any such thing, its impossible to use correctly.

Suppose there are two modular users, A and B.

Both have something like:

extern void (*fptr)(void);

static void (*old_fptr)(void);

static void func(void)
{
	/* foo */
	if (old_fptr)
		old_fptr();
}

module_init()
{
	old_fptr = fptr;
	fptr = A_func;
}

Then you load A, load B and unload A, then guess what happens?