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

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Message-ID: <460DD236.4030804@gmail.com>
Date:	Sat, 31 Mar 2007 12:15:02 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
CC:	gregkh@...e.de, hugh@...itas.com, dmitry.torokhov@...il.com,
	oneukum@...e.de, maneesh@...ibm.com, rpurdie@...ys.net,
	James.Bottomley@...elEye.com, Jeff Garzik <jgarzik@...ox.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	SCSI Mailing List <linux-scsi@...r.kernel.org>
Subject: Re: [RFD driver-core] Lifetime problems of the current driver model

Tejun Heo wrote:
> Cornelia Huck wrote:
>> On Sat, 31 Mar 2007 00:08:19 +0900,
>> Tejun Heo <htejun@...il.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;
> }

And one more thing just in case.  In the above code, try_module_get()
and kobject_get() must be and is atomic w.r.t. try_stop_module().
That's why we do the following.

  stop_machine_run(__try_stop_module, &sref, NR_CPUS);.

-- 
tejun
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