On Mon, 16 Apr 2007 15:38:52 -0400 (EDT),
Alan Stern <[email protected]> wrote:
> On Mon, 16 Apr 2007, Dmitry Torokhov wrote:
> > 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.
My current preference is 2. (obviously :)). I don't like 3. too much
(too complicated code), but I think it would still be better than 1.
(And I agree, this will be harder to trigger with Tejun's patches.)
>
> (Also, doing this might be a good way to expose a lot of hidden
> refcounting bugs. They will become very obvious when rmmod hangs.)
Good point.
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