Re: [PATCH v2] media: uvcvideo: Fix deadlock if uvc_status_stop is called from async_ctrl.work

[Sean Anderson <sean.anderson@linux.dev>]

On 3/23/26 19:52, Laurent Pinchart wrote:
> Hi Sean,
> 
> Thank you for the patch.
> 
> On Mon, Mar 16, 2026 at 11:58:22AM -0400, Sean Anderson wrote:
>> If a UVC camera has an asynchronous control, uvc_status_stop may be
>> called from async_ctrl.work:
>> 
>> uvc_ctrl_status_event_work()
>>     uvc_ctrl_status_event()
>>         uvc_ctrl_clear_handle()
>> 	    uvc_pm_put()
>> 	        uvc_status_put()
>> 		    uvc_status_stop()
>> 		        cancel_work_sync()
>> 
>> This will cause a deadlock, since cancel_work_sync will wait for
>> uvc_ctrl_status_event_work to complete before returning.
>> 
>> Fix this by returning early from uvc_status_stop if we are currently in
>> the work function. flush_status now remains false until uvc_status_start
>> is called again, ensuring that uvc_ctrl_status_event_work won't resubmit
>> the URB.
>> 
>> Fixes: a32d9c41bdb8 ("media: uvcvideo: Make power management granular")
>> Closes: https://lore.kernel.org/all/6733bdfb-3e88-479f-8956-ab09c04c433e@linux.dev/
>> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
>> ---
>> 
>> Changes in v2:
>> - Update comments with review feedback
>> - Use flush_work instead of cancel_work_sync since the work should never
>>   be rescheduled.
>> 
>>  drivers/media/usb/uvc/uvc_status.c | 27 ++++++++++++++++++---------
>>  1 file changed, 18 insertions(+), 9 deletions(-)
>> 
>> diff --git a/drivers/media/usb/uvc/uvc_status.c b/drivers/media/usb/uvc/uvc_status.c
>> index 231cfee8e7c2c..ea45b11642e59 100644
>> --- a/drivers/media/usb/uvc/uvc_status.c
>> +++ b/drivers/media/usb/uvc/uvc_status.c
>> @@ -316,6 +316,15 @@ static int uvc_status_start(struct uvc_device *dev, gfp_t flags)
>>  	if (!dev->int_urb)
>>  		return 0;
>>  
>> +	/*
>> +	 * If the work called uvc_status_stop it may still be running. Wait for
>> +	 * it to finish before we submit the urb.
>> +	 */
>> +	flush_work(&dev->async_ctrl.work);
> 
> I assume the only way this can happen is for uvc_status_start() to run
> after the uvc_status_put() call in
> 
> uvc_ctrl_status_event_work()
>     uvc_ctrl_status_event()
>         uvc_ctrl_clear_handle()
> 	    uvc_pm_put()
> 	        uvc_status_put()
> 
> returns. Is that right ? If so I'd like to capture that better in the
> comment, as I have a feeling we'll need to revisit this code at some
> point.
> 
> 	/*
> 	 * If the previous uvc_status_stop() call was from the async work, the
> 	 * work may still be running. Wait for it to finish before we submit the
> 	 * urb.
> 	 */
> 
> And if that's not correct, there's something I don't get and a better
> comment is even more important :-)
> 
> If the above change is fine, I can update the comment when applying, and
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Fine by me

> This being said, I wonder if we could have a simpler implementation if
> we always stopped the work and URB when the last userspace user goes
> away, instead of keeping the device active until we get the asynchronous
> control notification.
> 
>> +
>> +	/* Clear the flush status if we were previously stopped. */
>> +	smp_store_release(&dev->flush_status, false);
>> +
>>  	return usb_submit_urb(dev->int_urb, flags);
>>  }
>>  
>> @@ -336,6 +345,15 @@ static void uvc_status_stop(struct uvc_device *dev)
>>  	 */
>>  	smp_store_release(&dev->flush_status, true);
>>  
>> +	/*
>> +	 * If we are called from the event work function, the URB is guaranteed
>> +	 * to not be in flight as it has completed and has not been resubmitted.
>> +	 * There's no need to cancel the work (which would deadlock), or to kill
>> +	 * the URB.
>> +	 */
>> +	if (current_work() == &w->work)
>> +		return;
>> +
>>  	/*
>>  	 * Cancel any pending asynchronous work. If any status event was queued,
>>  	 * process it synchronously.
>> @@ -354,15 +372,6 @@ static void uvc_status_stop(struct uvc_device *dev)
>>  	 */
>>  	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);
>>  }
>>  
>>  int uvc_status_resume(struct uvc_device *dev)
>