Re: [PATCH 6/6] v4l: disable lockdep on vb2_fop_mmap()

[Hans Verkuil <hverkuil@xs4all.nl>]

Hi Antti,

Is this still needed after this commit was merged?

http://git.linuxtv.org/media_tree.git/commit/b18a8ff29d80b132018d33479e86ab8ecaee6b46

Regards,

	Hans

On 12/29/2013 05:51 AM, Antti Palosaari wrote:
> Avoid that lockdep warning:
> 
> [ INFO: possible circular locking dependency detected ]
> 3.13.0-rc1+ #77 Tainted: G         C O
> -------------------------------------------------------
> video_source:sr/32072 is trying to acquire lock:
>  (&dev->mutex#2){+.+.+.}, at: [<ffffffffa073fde3>] vb2_fop_mmap+0x33/0x90 [videobuf2_core]
> 
>                                                 but task is already holding lock:
>  (&mm->mmap_sem){++++++}, at: [<ffffffff8117825f>] vm_mmap_pgoff+0x6f/0xc0
> 
>  Possible unsafe locking scenario:
>        CPU0                    CPU1
>        ----                    ----
>   lock(&mm->mmap_sem);
>                                lock(&dev->mutex#2);
>                                lock(&mm->mmap_sem);
>   lock(&dev->mutex#2);
>                                                  *** DEADLOCK ***
> 
> Signed-off-by: Antti Palosaari <crope@iki.fi>
> ---
>  drivers/media/v4l2-core/videobuf2-core.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> index 12df9fd..2a74295 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -2641,12 +2641,24 @@ int vb2_fop_mmap(struct file *file, struct vm_area_struct *vma)
>  	struct video_device *vdev = video_devdata(file);
>  	struct mutex *lock = vdev->queue->lock ? vdev->queue->lock : vdev->lock;
>  	int err;
> +	/*
> +	 * FIXME: Ugly hack. Disable possible lockdep as it detects possible
> +	 * deadlock. "INFO: possible circular locking dependency detected"
> +	 */
> +	lockdep_off();
>  
> -	if (lock && mutex_lock_interruptible(lock))
> +	if (lock && mutex_lock_interruptible(lock)) {
> +		lockdep_on();
>  		return -ERESTARTSYS;
> +	}
> +
>  	err = vb2_mmap(vdev->queue, vma);
> +
>  	if (lock)
>  		mutex_unlock(lock);
> +
> +	lockdep_on();
> +
>  	return err;
>  }
>  EXPORT_SYMBOL_GPL(vb2_fop_mmap);
>