On Mon, Apr 16, 2007 at 03:47:13PM -0400, Dmitry Torokhov wrote:
> On 4/16/07, Alan Stern <[email protected]> wrote:
> >On Mon, 16 Apr 2007, Dmitry Torokhov wrote:
> >
> >> On 4/16/07, Cornelia Huck <[email protected]> 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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