Re: [PATCH v4 1/1] usb: gadget: add raw-gadget interface

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 13 Jan 2020, Andrey Konovalov wrote:

> I've also found an issue, but I'm not sure if that is the bug in Raw
> Gadget, or in the gadget layer (in the former case I'll add this fix
> to v5 as well). What I believe I'm seeing is
> __fput()->usb_gadget_unregister_driver()->usb_gadget_remove_driver()->gadget_unbind()
> racing with dummy_timer()->gadget_setup(). In my case it results in
> gadget_unbind() doing set_gadget_data(gadget, NULL), and then
> gadget_setup() dereferencing get_gadget_data(gadget).
> 
> Alan, does it look possible for those two functions to race? Should
> this be prevented by the gadget layer, or should I use some kind of
> locking in my gadget driver to prevent this?

In your situation this race shouldn't happen, because before
udc->driver->unbind() is invoked we call usb_gadget_disconnect().  If
that routine succeeds -- which it always does under dummy-hcd -- then
there can't be any more setup callbacks, because find_endpoint() will
always return NULL (the is_active() test fails; see the various
set_link_state* routines).  So I don't see how you could have ended up
with the race you describe.

However, a real UDC might not be able to perform a disconnect under
software control.  In that case usb_gadget_disconnect() would not
change the pullup state, and there would be a real possibility of a
setup callback racing with an unbind callback.  This seems like a 
genuine problem and I can't think of a solution offhand.

What we would need is a way to tell the UDC driver to stop invoking
gadget callbacks, _before_ the UDC driver's stop callback gets called.
Maybe this should be merged into the pullup callback somehow.

Alan Stern