linux-kernel - Re: [Patch -mm 0/3] RFC: module unloading vs. release function

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Message-ID: <d120d5000704161247v1ce98cc2v9ba53d91b166f935@mail.gmail.com>
Date:	Mon, 16 Apr 2007 15:47:13 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Alan Stern" <stern@...land.harvard.edu>
Cc:	"Cornelia Huck" <cornelia.huck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Greg K-H" <greg@...ah.com>, "Tejun Heo" <htejun@...il.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>
Subject: Re: [Patch -mm 0/3] RFC: module unloading vs. release function

On 4/16/07, Alan Stern <stern@...land.harvard.edu> wrote:
> On Mon, 16 Apr 2007, Dmitry Torokhov wrote:
>
> > On 4/16/07, Cornelia Huck <cornelia.huck@...ibm.com> wrote:
> > > Hi,
> > >
> > > based on the discussion in "How should an exit routine wait for
> > > release() callbacks?", I've cooked up some patches that make module
> > > unload wait until the last reference for a kobject has been dropped.
> > > This should plug the "release function in already deleted module" race;
> > > however, if the last kobject_put() from the module containing the
> > > release function is not in the module's exit function, there's still a
> > > small window (not sure if and how to plug this).
> >
> > Unfortunately all this "wait for refcount in module's exit" schemas
> > lead to the following deadlock:
> >
> >         rmmod my_module < /path/to/some/file/incrementing/my/refcount
>
> (Note that this problem will be a lot harder to provoke once Tejun's
> changes to sysfs are in place.  But it will still be possible, unless we
> make similar changes to all the other filesystems as well.)
>
> There are three possible approaches to this problem:
>
>     1. Ignore it, as we do now.  If someone actually tries running your
>        example above, an oops will result when the kobject's release
>        method is called after my_module has been unloaded from memory.
>
>     2. Do what Cornelia suggested, and allow the example to deadlock.
>
>     3. Change the module code so that rmmod can return _before_ the
>        module is actually unloaded from memory (but after the module's
>        exit routine has completed).  This will lead to more problems.
>        For example, what if someone tries to modprobe my_module back
>        again before it has finished unloading?
>
> My feeling is that either a deadlock or more complications with modprobe
> would be preferable to an oops.  Your opinion may differ.
>

What about 4:

     When registering an [k]object increment refcount of module that
provides ->release() function.

That would normally require ->release function to be placed on
subsystem level to allow unloading individual devices.

-- 
Dmitry
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