Re: [PATCH v4 1/4] [media] davinci: vpif_capture: don't lock over s_stream

[Kevin Hilman <khilman@baylibre.com>]

Laurent Pinchart <laurent.pinchart@ideasonboard.com> writes:

> Hi Kevin,
>
> Thank you for the patch.
>
> On Tuesday 29 Nov 2016 15:57:09 Kevin Hilman wrote:
>> Video capture subdevs may be over I2C and may sleep during xfer, so we
>> cannot do IRQ-disabled locking when calling the subdev.
>> 
>> Signed-off-by: Kevin Hilman <khilman@baylibre.com>
>> ---
>>  drivers/media/platform/davinci/vpif_capture.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/drivers/media/platform/davinci/vpif_capture.c
>> b/drivers/media/platform/davinci/vpif_capture.c index
>> 5104cc0ee40e..9f8f41c0f251 100644
>> --- a/drivers/media/platform/davinci/vpif_capture.c
>> +++ b/drivers/media/platform/davinci/vpif_capture.c
>> @@ -193,7 +193,10 @@ static int vpif_start_streaming(struct vb2_queue *vq,
>> unsigned int count) }
>>  	}
>> 
>> +	spin_unlock_irqrestore(&common->irqlock, flags);
>>  	ret = v4l2_subdev_call(ch->sd, video, s_stream, 1);
>> +	spin_lock_irqsave(&common->irqlock, flags);
>
> I always get anxious when I see a spinlock being released randomly with an 
> operation in the middle of a protected section. Looking at the code it looks 
> like the spinlock is abused here. irqlock should only protect the dma_queue 
> and should thus only be taken around the following code:
>
> spin_lock_irqsave(&common->irqlock, flags);
> /* Get the next frame from the buffer queue */
> common->cur_frm = common->next_frm = list_entry(common->dma_queue.next,
>                             struct vpif_cap_buffer, list);
> /* Remove buffer from the buffer queue */
> list_del(&common->cur_frm->list);
> spin_unlock_irqrestore(&common->irqlock, flags);

Yes, that looks correct.  Will update.

> The code that is currently protected by the lock in the start and stop 
> streaming functions should be protected by a mutex instead.

I tried taking the mutex here, but lockdep pointed out a deadlock.  I
may not be fully understanding the V4L2 internals here, but it seems
that the ioctl is already taking a mutex, so taking it again in
start/stop streaming is a deadlock.  Unless you think the locking should
be nested here, it seems to me that the mutex isn't needed.

Kevin