media: uvcvideo: Fix race condition with usb_kill_urb

usb_kill_urb warranties that all the handlers are finished when it
returns, but does not protect against threads that might be handling
asynchronously the urb.

For UVC, the function uvc_ctrl_status_event_async() takes care of
control changes asynchronously.

If the code is executed in the following order:

CPU 0 CPU 1
===== =====
uvc_status_complete()
uvc_status_stop()
uvc_ctrl_status_event_work()
uvc_status_start() -> FAIL

Then uvc_status_start will keep failing and this error will be shown:

<4>[ 5.540139] URB 0000000000000000 submitted while active
drivers/usb/core/urb.c:378 usb_submit_urb+0x4c3/0x528

Let's improve the current situation, by not re-submiting the urb if
we are stopping the status event. Also process the queued work
(if any) during stop.

CPU 0 CPU 1
===== =====
uvc_status_complete()
uvc_status_stop()
uvc_status_start()
uvc_ctrl_status_event_work() -> FAIL

Hopefully, with the usb layer protection this should be enough to cover
all the cases.

Cc: [e-mail]
Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
Reviewed-by: Yunke Cao <[e-mail]>
Signed-off-by: Ricardo Ribalda <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>

drivers/media/usb/uvc/uvc_ctrl.c

···
  *          Laurent Pinchart ([e-mail])
  */
 
+#include <asm/barrier.h>
 #include <linux/bitops.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
···
 	int ret;
 
 	uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
+
+	/* The barrier is needed to synchronize with uvc_status_stop(). */
+	if (smp_load_acquire(&dev->flush_status))
+		return;
 
 	/* Resubmit the URB. */
 	w->urb->interval = dev->int_ep->desc.bInterval;

drivers/media/usb/uvc/uvc_status.c

···
  *          Laurent Pinchart ([e-mail])
  */
 
+#include <asm/barrier.h>
 #include <linux/kernel.h>
 #include <linux/input.h>
 #include <linux/slab.h>
···
 
 void uvc_status_stop(struct uvc_device *dev)
 {
+	struct uvc_ctrl_work *w = &dev->async_ctrl;
+
+	/*
+	 * Prevent the asynchronous control handler from requeing the URB. The
+	 * barrier is needed so the flush_status change is visible to other
+	 * CPUs running the asynchronous handler before usb_kill_urb() is
+	 * called below.
+	 */
+	smp_store_release(&dev->flush_status, true);
+
+	/*
+	 * Cancel any pending asynchronous work. If any status event was queued,
+	 * process it synchronously.
+	 */
+	if (cancel_work_sync(&w->work))
+		uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
+
+	/* Kill the urb. */
 	usb_kill_urb(dev->int_urb);
+
+	/*
+	 * The URB completion handler may have queued asynchronous work. This
+	 * won't resubmit the URB as flush_status is set, but it needs to be
+	 * cancelled before returning or it could then race with a future
+	 * uvc_status_start() call.
+	 */
+	if (cancel_work_sync(&w->work))
+		uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
+
+	/*
+	 * From this point, there are no events on the queue and the status URB
+	 * is dead. No events will be queued until uvc_status_start() is called.
+	 * The barrier is needed to make sure that flush_status is visible to
+	 * uvc_ctrl_status_event_work() when uvc_status_start() will be called
+	 * again.
+	 */
+	smp_store_release(&dev->flush_status, false);
 }

drivers/media/usb/uvc/uvcvideo.h

···
 	struct usb_host_endpoint *int_ep;
 	struct urb *int_urb;
 	struct uvc_status *status;
+	bool flush_status;
 
 	struct input_dev *input;
 	char input_phys[64];