linux-kernel - Re: kobject_put vs module unload

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Date:	Sat, 8 Jan 2011 19:09:03 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com
Subject: Re: kobject_put vs module unload

On Sat, Jan 8, 2011 at 19:01, Mikulas Patocka <mpatocka@...hat.com> wrote:
> On Sat, 8 Jan 2011, Kay Sievers wrote:
>> On Sat, Jan 8, 2011 at 14:19, Mikulas Patocka <mpatocka@...hat.com> wrote:
>> > On Fri, 7 Jan 2011, Greg KH wrote:
>>
>> >> > 4) the reference obtained at point 1) is dropped, kobject reference count
>> >> > reaches zero and the release callback is called. But that callback points
>> >> > to an unloaded module and causes a crash.
>> >> >
>> >> > How is it solved? Am I missing something?
>> >>
>> >> You have the code that creates and frees the object, to not be in the
>> >> module that could have been unloaded.  It's really just that simple.
>> >
>> > If the whole device mapper subsystem can be a module, where can I put the
>> > code?
>> >
>> > I think a similar bug exists in md, it can also be unloaded as a module
>> > and it has "release" method in its module code.
>>
>> If you still have data structures hanging around, these structures are
>> supposed to take a reference on the module -- and you can not unload
>> the module as long as this is the case.
>>
>> Kay
>
> kobject references don't increase module reference count. If kobject held
> module references, it would be a fix for this bug, but could cause other
> bugs (some modules could be unloadable due to self-references to its own
> kobjects).

Sure, they don't automatically take references. It's the job of the
driver/module, to make sure to pin the module for any data of it,
which might be still in use.

Kay
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