USB: Gadget: core: Help prevent panic during UVC unconfigure

Avichal Rakesh reported a kernel panic that occurred when the UVC
gadget driver was removed from a gadget's configuration.  The panic
involves a somewhat complicated interaction between the kernel driver
and a userspace component (as described in the Link tag below), but
the analysis did make one thing clear: The Gadget core should
accomodate gadget drivers calling usb_gadget_deactivate() as part of
their unbind procedure.

Currently this doesn't work.  gadget_unbind_driver() calls
driver->unbind() while holding the udc->connect_lock mutex, and
usb_gadget_deactivate() attempts to acquire that mutex, which will
result in a deadlock.

The simple fix is for gadget_unbind_driver() to release the mutex when
invoking the ->unbind() callback.  There is no particular reason for
it to be holding the mutex at that time, and the mutex isn't held
while the ->bind() callback is invoked.  So we'll drop the mutex
before performing the unbind callback and reacquire it afterward.

We'll also add a couple of comments to usb_gadget_activate() and
usb_gadget_deactivate().  Because they run in process context they
must not be called from a gadget driver's ->disconnect() callback,
which (according to the kerneldoc for struct usb_gadget_driver in
include/linux/usb/gadget.h) may run in interrupt context.  This may
help prevent similar bugs from arising in the future.

Reported-and-tested-by: Avichal Rakesh <arakesh@google.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 286d9975 ("usb: gadget: udc: core: Prevent soft_connect_store() race")
Link: https://lore.kernel.org/linux-usb/4d7aa3f4-22d9-9f5a-3d70-1bd7148ff4ba@google.com/
Cc: Badhri Jagan Sridharan <badhri@google.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/48b2f1f1-0639-46bf-bbfc-98cb05a24914@rowland.harvard.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>