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

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Date:	Mon, 16 Apr 2007 12:11:20 -0700
From:	Greg KH <greg@...ah.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	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 03:03:16PM -0400, Dmitry Torokhov wrote:
> On 4/16/07, Greg KH <greg@...ah.com> wrote:
> >On Mon, Apr 16, 2007 at 02:30:17PM -0400, 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
> >
> >No, it should just return "module in use" as the reference count it
> >grabbed before rmmod is called.
> >
> 
> No, because it it were module's refcount we woudl not have problem
> with ->release() to begin with. It is object's refcount.

Yes, but with these patches, we are incrementing that reference count
when the kobject is created, which will cause this to fail.

> >But either way, that's just foolish to try to prevent that from failing
> >:)
> 
> Why? It works now for most of teh subsystems.

That's because it is buggy :)

thanks,

greg k-h
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