Re: usb/gadget: stalls in dummy_timer

[Alan Stern]

On Tue, 19 Sep 2017, Andrey Konovalov wrote:

> On Fri, Sep 15, 2017 at 8:57 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, 14 Sep 2017, Andrey Konovalov wrote:
> >
> >> On Thu, Sep 14, 2017 at 7:49 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> >> > On Thu, 14 Sep 2017, Andrey Konovalov wrote:
> >> >
> >> >> Looked at this a little more.
> >> >>
> >> >> dummy_timer() stucks in an infinite loop. It calls
> >> >> usb_hcd_giveback_urb(), which in turn calls usbtouch_irq(), which
> >> >> calls usb_submit_urb(), which calls dummy_urb_enqueue() and puts urb
> >> >> back into dummy urb queue. dummy_timer() then does goto restart, finds
> >> >> the urb and calls usb_hcd_giveback_urb() again. And this process goes
> >> >> on again and again. It seems that something should either process the
> >> >> urb and set urb->status or it should just expire.
> >> >
> >> > There is some throttling code, but it applies only to bulk transfers.
> >> > Probably because the bandwidth limits for other types are slightly
> >> > different.  However, I don't think we need to worry about this level of
> >> > detail, since the driver makes a number of other approximations anyway.
> >> >
> >> > Try the patch below; it should fix the problem.
> >>
> >> Hi Alan,
> >>
> >> Just tried your patch, my reproducer still hangs the kernel until all
> >> memory is exhausted.
> >>
> >> Thanks!
> >
> > Hmmm.  Your reproducer doesn't run on my system.  The mmap system call
> > fails, perhaps because this laptop has a 32-bit kernel.  So I can't
> > tell what's going on.
> >
> > Can you collect a usbmon trace that shows what happens while the
> > reproducer runs?  If it turns out to be extremely large, just post an
> > initial portion of it.
> 
> I've attached the usbmon trace. It's actually quite short, probably
> due to the fact that the kernel enters infinite loop.
> 
> I've also attached a reproducer that should compile on a 32 bit
> system, however I haven't tested whether it reproduces the issue.

Got it, thanks.  Can you test the patch below in place of (or in 
addition to) the earlier patch?

Alan Stern



Index: usb-4.x/drivers/usb/gadget/udc/dummy_hcd.c
===================================================================
--- usb-4.x.orig/drivers/usb/gadget/udc/dummy_hcd.c
+++ usb-4.x/drivers/usb/gadget/udc/dummy_hcd.c
@@ -237,6 +237,8 @@ struct dummy_hcd {
 
 	struct usb_device		*udev;
 	struct list_head		urbp_list;
+	struct urbp			*next_frame_urbp;
+
 	u32				stream_en_ep;
 	u8				num_stream[30 / 2];
 
@@ -1252,6 +1254,8 @@ static int dummy_urb_enqueue(
 
 	list_add_tail(&urbp->urbp_list, &dum_hcd->urbp_list);
 	urb->hcpriv = urbp;
+	if (!dum_hcd->next_frame_urbp)
+		dum_hcd->next_frame_urbp = urbp;
 	if (usb_pipetype(urb->pipe) == PIPE_CONTROL)
 		urb->error_count = 1;		/* mark as a new urb */
 
@@ -1768,6 +1772,7 @@ static void dummy_timer(unsigned long _d
 		spin_unlock_irqrestore(&dum->lock, flags);
 		return;
 	}
+	dum_hcd->next_frame_urbp = NULL;
 
 	for (i = 0; i < DUMMY_ENDPOINTS; i++) {
 		if (!ep_info[i].name)
@@ -1784,6 +1789,10 @@ restart:
 		int			type;
 		int			status = -EINPROGRESS;
 
+		/* stop when we reach URBs queued after the timer interrupt */
+		if (urbp == dum_hcd->next_frame_urbp)
+			break;
+
 		urb = urbp->urb;
 		if (urb->unlinked)
 			goto return_urb;