Re: [PATCH 1/3] dma-buf/sync_file: only enable fence signalling during wait

[Daniel Vetter]

On Thu, Jun 09, 2016 at 12:05:28PM -0300, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> 
> Signalling doesn't need to be enabled at sync_file creation, it is only
> required if userspace waiting the fence to signal through poll().
> 
> Thus we delay fence_add_callback() until poll is called. It only adds the
> callback the first time poll() is called. This avoid re-adding the same
> callback multiple times.
> 
> Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> ---
>  drivers/dma-buf/sync_file.c | 36 +++++++++++++++++++++++-------------
>  include/linux/fence.h       | 12 ++++++++++++
>  2 files changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> index 00e9186..59f3445 100644
> --- a/drivers/dma-buf/sync_file.c
> +++ b/drivers/dma-buf/sync_file.c
> @@ -91,9 +91,7 @@ struct sync_file *sync_file_create(struct fence *fence)
>  
>  	sync_file->cbs[0].fence = fence;
>  	sync_file->cbs[0].sync_file = sync_file;
> -	if (fence_add_callback(fence, &sync_file->cbs[0].cb,
> -			       fence_check_cb_func))
> -		atomic_dec(&sync_file->status);
> +	INIT_LIST_HEAD(&sync_file->cbs[0].cb.node);
>  
>  	sync_file_debug_add(sync_file);
>  	return sync_file;
> @@ -130,8 +128,7 @@ static void sync_file_add_pt(struct sync_file *sync_file, int *i,
>  	sync_file->cbs[*i].fence = fence;
>  	sync_file->cbs[*i].sync_file = sync_file;
>  
> -	if (!fence_add_callback(fence, &sync_file->cbs[*i].cb,
> -				fence_check_cb_func)) {
> +	if (!fence_is_signaled(fence)) {
>  		fence_get(fence);
>  		(*i)++;
>  	}
> @@ -212,11 +209,8 @@ static void sync_file_free(struct kref *kref)
>  						     kref);
>  	int i;
>  
> -	for (i = 0; i < sync_file->num_fences; ++i) {
> -		fence_remove_callback(sync_file->cbs[i].fence,
> -				      &sync_file->cbs[i].cb);
> +	for (i = 0; i < sync_file->num_fences; ++i)
>  		fence_put(sync_file->cbs[i].fence);
> -	}
>  
>  	sync_file_debug_remove(sync_file);
>  	kfree(sync_file);
> @@ -233,17 +227,33 @@ static int sync_file_release(struct inode *inode, struct file *file)
>  static unsigned int sync_file_poll(struct file *file, poll_table *wait)
>  {
>  	struct sync_file *sync_file = file->private_data;
> -	int status;
> +	int status, i;
>  
>  	poll_wait(file, &sync_file->wq, wait);
>  
> +	for (i = 0 ; i < sync_file->num_fences ; i++) {
> +		if (fence_is_enabled(sync_file->cbs[i].fence))

If some other sync_file (or anyone else really) added enabling, then
you'll miss adding a callback. I don't think this check here works. You
probably also need more testcases in the debugfs-based tests if this falls
through the cracks ;-)
-Daniel

> +			continue;
> +
> +		if (fence_add_callback(sync_file->cbs[i].fence,
> +				       &sync_file->cbs[i].cb,
> +				       fence_check_cb_func))
> +			atomic_dec(&sync_file->status);
> +	}
> +
>  	status = atomic_read(&sync_file->status);
>  
> -	if (!status)
> -		return POLLIN;
> +	if (status > 0)
> +		return 0;
> +
> +	for (i = 0 ; i < sync_file->num_fences ; i++)
> +		fence_remove_callback(sync_file->cbs[i].fence,
> +				      &sync_file->cbs[i].cb);
> +
>  	if (status < 0)
>  		return POLLERR;
> -	return 0;
> +
> +	return POLLIN;
>  }
>  
>  static long sync_file_ioctl_merge(struct sync_file *sync_file,
> diff --git a/include/linux/fence.h b/include/linux/fence.h
> index 523ea3f..95a70b2 100644
> --- a/include/linux/fence.h
> +++ b/include/linux/fence.h
> @@ -283,6 +283,18 @@ fence_is_signaled(struct fence *fence)
>  }
>  
>  /**
> + * fence_is_enabled - return an indication if signalling is enabled
> + * @fence:	[in]	the fence to check
> + *
> + * Returns true if signalling was already enabled, false if not.
> + */
> +static inline bool
> +fence_is_enabled(struct fence *fence)
> +{
> +	return test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags);
> +}
> +
> +/**
>   * fence_is_later - return if f1 is chronologically later than f2
>   * @f1:	[in]	the first fence from the same context
>   * @f2:	[in]	the second fence from the same context
> -- 
> 2.5.5
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch