linux-kernel - Re: [PATCH] usb: dwc3: fix race of usb_gadget

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Date:   Mon, 28 Jun 2021 10:10:27 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     linyyuan@...eaurora.org
Cc:     Felipe Balbi <balbi@...nel.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jack Pham <jackp@...eaurora.org>
Subject: Re: [PATCH] usb: dwc3: fix race of usb_gadget_driver operation

On Mon, Jun 28, 2021 at 05:36:22PM +0800, linyyuan@...eaurora.org wrote:
> On 2021-06-27 22:09, Alan Stern wrote:
> > 
> > 	CPU0				CPU1
> > 	----				----
> > 					usb_gadget_remove_driver runs
> > 					  Calls synchronize_irq
> > 					    synchronize_irq returns
> > 					  Calls udc_driver_unbind
> > 	IRQ happens for disconnect
> > 	  Handler unlocks dwc->lock
> > 	  Calls dwc->gadget_driver->disconnect
> > 	    Gadget driver has already been unbound
> > 	      and is not prepared to handle a
> > 	      callback, so it crashes
> > 					  Calls usb_gadget_udc_stop
> > 					    dwc->gadget_driver is
> > 					      set to NULL
> > 
> > Without the async_callbacks mechanism, the gadget driver can get a
> > callback at the wrong time (after it has been unbound), which might
> > cause it to crash.
> 1. do you think we need to back to my original patch,
> https://lore.kernel.org/linux-usb/20210619154309.52127-1-linyyuan@codeaurora.org/T/#t

No, I think the async_callbacks approach is slightly better.

> i think we can add the spin lock or mutex lock to protect this kind of race
> from UDC layer, it will be easy understanding.

We don't actually need a lock or mutex to fix this problem.  We just 
need to make the remove_driver sequence issue two calls to the UDC 
driver rather than just one: async_callbacks and udc_stop.

> 2. if you insist this kind of change, how to change following code in dwc3 ?
> if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
> 
> 2.1 if (dwc->async_callbacks && dwc->gadget_driver->disconnect) {
> or
> 2.2 if (dwc->async_callbacks && vdwc->gadget_driver &&
> dwc->gadget_driver->disconnect) {

Either one would be okay.  If async_callbacks is enabled then 
dwc->gadget_driver should never be NULL, but it won't hurt to check.  
After all, disconnect does not occur often and it doesn't need to run 
extremely quickly.

Alan Stern