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

From: Gustavo Padovan <gustavo@padovan.org>
To: Chris Wilson <chris@chris-wilson.co.uk>,
	dri-devel@lists.freedesktop.org, marcheu@google.com,
	Daniel Stone <daniels@collabora.com>,
	seanpaul@google.com, Daniel Vetter <daniel.vetter@ffwll.ch>,
	linux-kernel@vger.kernel.org, laurent.pinchart@ideasonboard.com,
	Gustavo Padovan <gustavo.padovan@collabora.co.uk>,
	John Harrison <John.C.Harrison@Intel.com>,
	m.chehab@samsung.com
Subject: Re: [RFC 5/5] dma-buf/sync_file: rework fence storage in struct file
Date: Fri, 24 Jun 2016 10:23:46 -0300	[thread overview]
Message-ID: <20160624132346.GC2503@joana> (raw)
In-Reply-To: <20160623212724.GD1086@nuc-i3427.alporthouse.com>

2016-06-23 Chris Wilson <chris@chris-wilson.co.uk>:

> 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;

Yes. That is much cleaner solution. I did this initially but then tried
to come up with .get_fences(), but that was the wrong road.

> 
> >  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.)

Maybe. I'll look into ways to identify that. What I did during the
development of this patch was to have a fence_array_destroy(), but then
I moved to .teardown() in the hope to abstract the diff between fences
and fence_arrays.

	Gustavo
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