Re: [PATCH 1/1] usb: gadget: f_hid: Conduct proper refcounting on shared f_hidg pointer

From: Alan Stern
Date: Sun Nov 20 2022 - 15:46:33 EST


On Sun, Nov 20, 2022 at 05:22:19PM +0000, John Keeping wrote:
> On Fri, Nov 18, 2022 at 04:07:24PM -0500, Alan Stern wrote:
> > On Fri, Nov 18, 2022 at 04:37:32PM +0000, John Keeping wrote:
> > > I don't think it's at all simple to fix this - I posted a series
> > > addressing the lifetime issues here a few years ago but didn't chase it
> > > up and there was no feedback:
> > >
> > > https://lore.kernel.org/linux-usb/20191028114228.3679219-1-john@xxxxxxxxxxxx/
> > >
> > > That includes a patch to remove the embedded struct cdev and manage its
> > > lifetime separately, which I think is needed as there are two different
> > > struct device objects here and we cannot tie their lifetimes together.
> >
> > I still don't have a clear picture of what the real problem is. Lee's
> > original patch description just said "external references are presently
> > not tracked", with no details about what those external references are.
> > Why not add just proper cdev_get() and cdev_put() calls to whatever code
> > handles those external references, so that they _are_ tracked?
> >
> > What are the two different struct device objects? Why do their
> > lifetimes need to be tied together? If you do need to tie their
> > lifetimes somehow, why not simply make one of them (the one which is
> > logically allowed to be shorter-lived) hold a reference to the other?
>
> The problem is that we have a struct cdev embedded in f_hidg but the
> lifetime of f_hidg is not tied to any kobject so we can't solve this in
> the right way by setting the parent kobject of the cdev.
>
> While refcounting struct f_hidg is necessary, it's not sufficient
> because the only way to keep it alive long enough for the final
> kobject_put() on the embedded cdev is to tie the lifetime to a kobject
> of its own and there is no suitable object as this is not the model
> followed by gadget function instances.

I see. The solution is simple: Embed a struct device in struct f_hidg,
and call cdev_device_add() to add the device and the cdev. This will
automatically make the device the parent of the cdev, so the device's
refcount won't go to 0 until the cdev's refcount does. Then you can tie
the f_hidg's lifetime to the device's, so the device's release routine
can safely deallocate the entire f_hidg structure.

The parent of the new struct device should be set to &gadget->dev. If
you can't think of a better name for the device, you could simply append
":I" to the parent's name, where I is the interface number, or even
append ":C.I" where C is the config number (like we do on the host
side).

Alan Stern