Hi Hillf

Thanks for the heads up

On Tue, 3 Jan 2023 at 03:25, Hillf Danton hdanton@sina.com wrote:

On 02 Jan 2023 15:48:01 +0100 Ricardo Ribalda ribalda@chromium.org

...

--- a/drivers/media/usb/uvc/uvc_ctrl.c +++ b/drivers/media/usb/uvc/uvc_ctrl.c @@ -1442,6 +1442,9 @@ static void uvc_ctrl_status_event_work(struct work_struct *work)

  uvc_ctrl_status_event(w->chain, w->ctrl, w->data);

• if (dev->flush_status)
  

•         return;
  

• /* Resubmit the URB. */
  w->urb->interval = dev->int_ep->desc.bInterval;
  ret = usb_submit_urb(w->urb, GFP_KERNEL);
  

diff --git a/drivers/media/usb/uvc/uvc_status.c b/drivers/media/usb/uvc/uvc_status.c index 7518ffce22ed..e457889345a3 100644 --- a/drivers/media/usb/uvc/uvc_status.c +++ b/drivers/media/usb/uvc/uvc_status.c @@ -6,6 +6,7 @@

•      Laurent Pinchart (laurent.pinchart@ideasonboard.com)
  

*/

+#include <asm/barrier.h> #include <linux/kernel.h> #include <linux/input.h> #include <linux/slab.h> @@ -309,5 +310,44 @@ int uvc_status_start(struct uvc_device *dev, gfp_t flags)

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 */
  

• dev->flush_status = true;
  

• /*
  

•  * 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_mb();
  

Given unpaired mb, take a look at the release/acquire memory barrier pairing in c5b2cbdbdac5 ("ipc/mqueue.c: update/document memory barriers")

Would it work? to replace:

dev->flush_status = true; smp_mb();

with: smp_store_release(&dev->flush_status, 1);

and then read it always with:

smp_load_acquire(&dev->flush_status);

Thanks!

...

• /* If there is any status event on the queue, process it. */
  

• 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, this is, no events will be queued until uvc_status_start()
  

•  * is called.
  

•  */
  

• dev->flush_status = false;
  

• /*
  

•  * Write to memory the value of flush_status before uvc_status_start()
  

•  * is called again.
  

•  */
  

• smp_mb();
  

}