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    Subject[PATCH] Module : call synchronize_sched() between module exit() and free.
    > Hi Mathieu,
    > Yes: stop_machine is merely used to atomically check the module refcount
    > for zero and set the state so it can't be incremented again (ie.
    > try_module_get will fail).
    > So placing a tracepoint or marker in a module does not bump the module
    > refcount? If that's true, then there needs to be some kind of
    > remove_markers_from_module() call after module->exit(), which should do the
    > synchronize_sched() or whatever, right?
    > Rusty.

    Actually, it's not placing a marker/tracepoint in a module which causes
    a problem, this is a simple function call after all, and correctly dealt
    with by current module.c code.

    The problem comes from a probe function (the callback) that would be
    registered to be called from a marker and would sit in an unloadable
    kernel module. I would not want to tie the refcount of the probe modules
    to the fact that they are connected to a marker because it would then
    become impossible to unload them due to the fact that unregistration is
    done in module exit().

    This is one of the reasons why I disable preemption around the marker
    site (the function call) : to make sure I can can unregister the
    callback, wait for a quiescent state (with synchronize_sched()) and then
    free the module memory.

    This would give the following supplementary guarantee about module
    teardown : every function called with preemption off and unregistered in
    the module exit() would reach a quiescent state before the module is
    freed. Given this does apply to rarely used code (module unload), I
    think it might be ok to simply add a call to synchronize_sched() before
    the module memory is freed. Not tying this to markers/tracepoints would
    keep the behavior consistant across various build options, which is IMHO
    a good thing.

    I could also just document that a mandatory "synchronize_sched()" should
    be called at the end of the probe module exit() function which makes
    sure the probes has reached a quiescent state.

    I don't want to add a synchronize_sched() into the marker/tracepoint
    probe unregistration code because I want to keep batch probe
    unregistration fast enough so it does no take ~5 seconds to unload ~100
    probes. (may take longer on a loaded SMP system)


    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

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