Re: [PATCH] usb: dwc3: fix race of usb_gadget_driver operation

[linyyuan]

On 2021-06-26 23:03, Alan Stern wrote:
> On Sat, Jun 26, 2021 at 09:16:25AM +0800, linyyuan@xxxxxxxxxxxxxx 
> wrote:
> > On 2021-06-26 00:37, Alan Stern wrote:
> > > On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:
>
> > > > --- a/drivers/usb/dwc3/ep0.c
> > > > +++ b/drivers/usb/dwc3/ep0.c
> > > > @@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3
> > > > *dwc, struct usb_ctrlrequest *ctrl)
> > > >
> > > >  static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct
> > > > usb_ctrlrequest *ctrl)
> > > >  {
> > > > -	int ret;
> > > > +	int ret = 0;
> > > >
> > > >  	spin_unlock(&dwc->lock);
> > > > -	ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> > > > +	if (dwc->async_callbacks)
> > > > +		ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> > > >  	spin_lock(&dwc->lock);
> > >
> > > Here and in the other places, you should test dwc->async_callbacks
> > > _before_ dropping the spinlock.  Otherwise there is a race (the flag
> > > could be written at about the same time it is checked).
> > thanks for your comments,
> >
> > if you think there is race here, how to make sure gadget_driver 
> > pointer is
> > safe,
> > this is closest place where we can confirm it is non-NULL by checking
> > async_callbacks ?
>
> I explained this twice already: We know that gadget_driver is not
> NULL because usb_gadget_remove_driver calls synchronize_irq before
> doing usb_gadget_udc_stop.
>
> Look at this timing diagram:
>
> 	CPU0				CPU1
> 	----				----
> 	IRQ happens for setup packet
> 	  Handler sees async_callbacks
> 	    is enabled
> 	  Handler unlocks dwc->lock
> 					usb_gadget_remove_driver runs
> 					  Disables async callbacks
> 					  Calls synchronize_irq
> 	  Handler calls dwc->		  . waits for IRQ handler to
> 	    gadget_driver->setup	  .   return
> 	  Handler locks dwc-lock	  .
> 	  ...				  .
> 	  Handler returns		  .
> 					  . synchronize_irq returns
> 					  Calls usb_gadget_udc_stop
> 					    dwc->gadget_driver is
> 					      set to NULL
>
> As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
> even though async_callbacks gets cleared during the time when the
> lock is released.
thanks for your patient explanation,
but from this part, seem it is synchronize_irq() help to avoid NULL 
pointer crash.

can you also explain how async_callbacks flag help here  ?
> Alan Stern