Re: [RFD driver-core] Lifetime problems of the current driver model

Tejun Heo
2007-03-31T03:12:48Z

Cornelia Huck wrote:
> On Sat, 31 Mar 2007 00:08:19 +0900,
> Tejun Heo <htejun@gmail.com> wrote:
>
>> (3) make sure all existing kobjects are released by module exit function.
>>
>> For example, let's say there is a hypothetical disk device /dev/dk0
>> driven by a hypothetical driver mydrv. /dev/dk0 is represented like the
>> following in the sysfs tree.
>>
>> /sys/devices/pci0000:00/0000:00:1f.0/dk0/{myknob0,myknob1}
>>
>> Owner of both attrs myknob0 and myknob1 is mydrv and opening either
>> increases the reference counts of dk0 and mydrv and closing does the
>> opposite.
>>
>> * When there is no opener of either knob and the /dev/dk0 isn't used by
>> anyone. Reference count of dk0 is 1, mydrv 0.
>
> Hm, but as long as dk0 is registered, it can be looked up and someone
> could get a reference on it.

Yeah, exactly. That's why any getting any kobject reference backed by a
module must be accompanied by try_module_get().

int mydrv_get_dk(struct dk *dk)
{
rc = try_module_get(mydrv);
if (rc)
return rc;
kobject_get(&dk->kobj);
return 0;
}

>> * User issues rmmod mydrv. As mydrv's reference count is zero, unload
>> proceeds and mydrv's exit function is called.
>>
>> * mydrv's exit function looks like the following.
>>
>> mydrv_exit()
>> {
>> sysfs_remove_file(dk0, myknob0);
>> sysfs_remove_file(dk1, myknob1);
>> device_del(dk0);
>> deinit controller;
>> release all resources;
>> }
>>
>> The device_del(dk0) drops dk0's reference count to zero and its
>> ->release is invoked immediately.
>
> And here is the problem if someone else still has a reference. The
> module will be unloaded, but ->release will not be called until the
> "someone else" gives up the reference...

Exactly, in that case, module reference count must not be zero. You and
I are saying the same thing. Why are we running in circle?

--
tejun