linux-kernel - Re: [RFC 5/5] dma-buf/sync_file: rework fence storage in struct file

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Date:	Thu, 23 Jun 2016 22:27:24 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Gustavo Padovan <gustavo@...ovan.org>
Cc:	dri-devel@...ts.freedesktop.org, marcheu@...gle.com,
	Daniel Stone <daniels@...labora.com>, seanpaul@...gle.com,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org, laurent.pinchart@...asonboard.com,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	John Harrison <John.C.Harrison@...el.com>, m.chehab@...sung.com
Subject: Re: [RFC 5/5] dma-buf/sync_file: rework fence storage in struct file

On Thu, Jun 23, 2016 at 12:29:50PM -0300, Gustavo Padovan wrote:
> -static void sync_file_add_pt(struct sync_file *sync_file, int *i,
> +static int sync_file_set_fence(struct sync_file *sync_file,
> +			       struct fence **fences)
> +{
> +	struct fence_array *array;
> +
> +	if (sync_file->num_fences == 1) {
> +		sync_file->fence = fences[0];

Straightforward pointer assignment.

> +	} else {
> +		array = fence_array_create(sync_file->num_fences, fences,
> +					   fence_context_alloc(1), 1, false);
> +		if (!array)
> +			return -ENOMEM;
> +
> +		sync_file->fence = &array->base;

New reference.

Imbalance will promptly go bang after we release the single fence[0].

Would fence_array_create(1, fence) returning fence_get(fence) be too
much of a hack?

I would suggest dropping the exported fence_get_fences() and use a local
instead that could avoid the copy, e.g.

static struct fence *get_fences(struct fence **fence,
				unsigned int *num_fences)
{
	if (fence_is_array(*fence)) {
		struct fence_array *array = to_fence_array(*fence);
		*num_fences = array->num_fences;
		return array->fences;
	} else {
		*num_fences = 1;
		return fence;
	}
}

sync_file_merge() {
	int num_fences, num_a_fences, num_b_fences;
	struct fence **fences, **a_fences, **b_fences;

	a_fences = get_fences(&a, &num_a_fences);
	b_fences = get_fences(&b, &num_b_fences);

	num_fences = num_a_fences + num_b_fences;

>  static void sync_file_free(struct kref *kref)
>  {
>  	struct sync_file *sync_file = container_of(kref, struct sync_file,
>  						     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);
> -		fence_put(sync_file->cbs[i].fence);
> -	}
>  
> +	fence_remove_callback(sync_file->fence, &sync_file->cb);
> +	fence_teardown(sync_file->fence);

Hmm. Could we detect the removal of the last callback and propagate that
to the fence_array? (Rather then introduce a manual call to
fence_teardown.)

-- 
Chris Wilson, Intel Open Source Technology Centre