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    SubjectRe: help needed with EXPORT_SYMBOL
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    On Mon, Aug 23, 2010 at 9:17 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > 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?
    >

    The assumption was that there was a single module using that hook.
    For multiple users you'd need to use a register function that adds the
    module to a linked list.

    --
    Brian Gerst
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