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    SubjectRe: [PATCH 8/19]: SCST SYSFS interface implementation
    On Fri, Nov 12, 2010 at 2:23 AM, Dmitry Torokhov
    <> wrote:
    > On Thu, Nov 11, 2010 at 11:50:01PM +0300, Vladislav Bolkhovitin wrote:
    > > [ ... ]
    > >
    > > This is the last internal put. All other references are from outsiders.
    > > So, we are waiting for all them to put before we go on.
    > The question is why do you need to wait here? I presume it is module
    > unloading path, but then it is quite bad - you can easily wedge your
    > subsystem if you make something to take a reference to your kobject
    > while module is trying to be unloaded. Back when sysfs attributes tied
    > kobjects the easiest thing was to do:
    >        rmmod <module> < / sys/devices/..../attribute
    > If you are done with the kobject - just proceed with what you were doing
    > and let it die its own peaceful death some time later. You just need to
    > make sure release code sticks around to free it and your subsystem core
    > can be tasked with this. Use module counter to prevent unloading of the
    > subsystem core until all kobjects belonging to the subsystem are
    > destroyed.

    Do you mean keeping a kref object in the kernel module, invoking
    kref_get() every time a kobject has been created and invoking
    kref_put() from the kobject/ktype release method ? That would help to
    reduce the race window but would not eliminate all races: as soon as
    the last kref_put() has been invoked from the release method, the
    module can get unloaded. And module unloading involves freeing all
    module code sections, including the section that contains the
    implementation of the release method. Which is a race condition.

    I'm not sure that it is even possible with the current kobject
    implementation to solve this race. I haven't found any information
    about this race in Documentation/kobject.txt. And it seems to me that
    the code in samples/kobject/kobject-example.c is vulnerable to this
    race: methods like foo_show() and foo_store() can access statically
    allocated memory ("static int foo") after the module has been
    unloaded. Although the race window is small, this makes me wonder
    whether module unloading been overlooked at the time the kobject
    subsystem has been designed and implemented ?

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