linux-kernel - Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same gadget device

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Date:   Wed, 5 Apr 2017 10:12:28 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Felipe Balbi <balbi@...nel.org>
cc:     Roger Quadros <rogerq@...com>, <vivek.gautam@...eaurora.org>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same
 gadget device

On Wed, 5 Apr 2017, Felipe Balbi wrote:

> >> >> --- a/drivers/usb/gadget/udc/core.c
> >> >> +++ b/drivers/usb/gadget/udc/core.c
> >> >> @@ -1273,6 +1273,7 @@ void usb_del_gadget_udc(struct usb_gadget *gadget)
> >> >>  	flush_work(&gadget->work);
> >> >>  	device_unregister(&udc->dev);
> >> >>  	device_unregister(&gadget->dev);
> >> >> +	memset(&gadget->dev, 0x00, sizeof(gadget->dev));
> >> >>  }
> >> >>  EXPORT_SYMBOL_GPL(usb_del_gadget_udc);
> >> >
> >> > Isn't this dangerous?  It's quite possible that the device_unregister() 
> >> 
> >> not on the gadget API, no.
> >> 
> >> > call on the previous line invokes the gadget->dev.release callback, 
> >> > which might deallocate gadget.  If that happens, your new memset will 
> >> > oops.
> >> 
> >> that won't happen. struct usb_gadget is a member of the UDC's private
> >> structure, like this:
> >> 
> >> struct dwc3 {
> >> 	[...]
> >> 	struct usb_gadget	gadget;
> >> 	struct usb_gadget_driver *gadget_driver;
> >> 	[...]
> >> };
> >
> > Yes.  So what?  Can't the UDC driver use the refcount inside struct 
> > usb_gadget to control the lifetime of its private structure?
> 
> nope, not being used. At least not yet.

I'm not convinced (yet)...

> > (By the way, can you tell what's going on in net2280.c?  I must be
> > missing something; it looks like gadget_release() would quickly run
> > into problems because it calls dev_get_drvdata() for &gadget->dev, but
> > net2280_probe() never calls dev_set_drvdata() for that device.  
> > Furthermore, net2280_remove() continues to reference the net2280 struct
> > after calling usb_del_gadget_udc(), and it never does seem to do a
> > final put.)
> 
> static int net2280_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> {
> 	struct net2280		*dev;
> 	unsigned long		resource, len;
> 	void			__iomem *base = NULL;
> 	int			retval, i;
> 
> 	/* alloc, and start init */
> 	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
> 	if (dev == NULL) {
> 		retval = -ENOMEM;
> 		goto done;
> 	}
> 
> 	pci_set_drvdata(pdev, dev);
> 	^^^^^^^^^^^^^^^^^^^^^^^^^^^

That sets the driver data in the struct pci_dev, not in
dev->gadget.dev.  As far as I can see, _nothing_ in the driver sets the 
driver data in dev->gadget.dev.

(Even after all these years, I still get bothered by the way Dave 
Brownell used to call everything "dev"...  IIRC, at one time he had a 
line of code that went something like:  dev->dev.dev = &pdev->dev !)

> >> I'm actually thinking that struct usb_gadget shouldn't have a struct
> >> device at all. Just a pointer to a device, that would solve all these
> >> issues.
> >
> > A pointer to which device?  The UDC?  That would change the directory 
> > layout in sysfs.
> 
> indeed. Would that be a problem?

Possibly for some userspace tool.

> > Or a pointer to a separate dynamically allocated device (the way struct 
> > usb_hcd contains a pointer to the root hub device)?  That would work.  
> > If the UDC driver wanted to re-register the gadget, it would have to 
> > allocate a new device.
> 
> That could be done as well, if maintaining the directory structure is a
> must.

Alan Stern