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

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Date:	Mon, 16 Apr 2007 12:52:29 -0700
From:	Greg KH <greg@...ah.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tejun Heo <htejun@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [Patch -mm 0/3] RFC: module unloading vs. release function

On Mon, Apr 16, 2007 at 03:47:13PM -0400, Dmitry Torokhov wrote:
> 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.

But that would also mean that a lot of modules that want to be able to
be released whenever they want to today, not be allowed to (network
drivers, etc.)

thanks,

greg k-h
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