linux-kernel - Re: [PATCH] media: usbtv: Remove useless locks in usbtv_video

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Date: Wed, 28 Feb 2024 12:33:16 +0100
From: Hans Verkuil <hverkuil-cisco@...all.nl>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>, mchehab@...nel.org,
 tfiga@...omium.org
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH] media: usbtv: Remove useless locks in usbtv_video_free()

On 28/02/2024 09:45, Hans Verkuil wrote:
> On 20/02/2024 09:06, Benjamin Gaignard wrote:
>> Remove locks calls in usbtv_video_free() because
>> are useless and may led to a deadlock as reported here:
>> https://syzkaller.appspot.com/x/bisect.txt?x=166dc872180000
>>
>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>> Fixes: c838530d230b ("media: media videobuf2: Be more flexible on the number of queue stored buffers")
> 
> Hmm, the code was really always wrong, it is just that before this commit
> it would only fail if you did a disconnect while streaming, and now it also
> fails if you disconnect when you are not streaming, so it is much more
> noticeable now.
> 
> That should probably be explained better in the commit log. You could add a
> second Fixes line:
> 
> Fixes: f3d27f34fdd7 ("[media] usbtv: Add driver for Fushicai USBTV007 video frame grabber")
> 
> but I am not sure how useful that is.
> 
>> ---
>>  drivers/media/usb/usbtv/usbtv-video.c | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> diff --git a/drivers/media/usb/usbtv/usbtv-video.c b/drivers/media/usb/usbtv/usbtv-video.c
>> index 62a583040cd4..96276358d116 100644
>> --- a/drivers/media/usb/usbtv/usbtv-video.c
>> +++ b/drivers/media/usb/usbtv/usbtv-video.c
>> @@ -963,15 +963,9 @@ int usbtv_video_init(struct usbtv *usbtv)
>>  
>>  void usbtv_video_free(struct usbtv *usbtv)
>>  {
>> -	mutex_lock(&usbtv->vb2q_lock);
>> -	mutex_lock(&usbtv->v4l2_lock);
>> -
>>  	usbtv_stop(usbtv);
> 
> This call should also be dropped...
> 
>>  	vb2_video_unregister_device(&usbtv->vdev);
> 
> ...since this function will call stop_streaming if streaming is in progress,
> which will call usbtv_stop with all the correct locks held.

With that change you can add my:

Tested-by: Hans Verkuil <hverkuil-cisco@...all.nl>

Regards,

	Hans

> 
> Regards,
> 
> 	Hans
> 
>>  	v4l2_device_disconnect(&usbtv->v4l2_dev);
>>  
>> -	mutex_unlock(&usbtv->v4l2_lock);
>> -	mutex_unlock(&usbtv->vb2q_lock);
>> -
>>  	v4l2_device_put(&usbtv->v4l2_dev);
>>  }
> 
>