linux-kernel - Re: [RFC v2 3/3] dma-buf/sync_file: rework fence storage in struct file

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Message-ID: <20160628150952.GE28577@nuc-i3427.alporthouse.com>
Date:	Tue, 28 Jun 2016 16:09:52 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Gustavo Padovan <gustavo.padovan@...labora.com>
Cc:	Gustavo Padovan <gustavo@...ovan.org>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Daniel Stone <daniels@...labora.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>,
	laurent.pinchart@...asonboard.com, seanpaul@...gle.com,
	marcheu@...gle.com, m.chehab@...sung.com,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Christian König <christian.koenig@....com>
Subject: Re: [RFC v2 3/3] dma-buf/sync_file: rework fence storage in struct
 file

On Tue, Jun 28, 2016 at 11:25:00AM -0300, Gustavo Padovan wrote:
> 2016-06-28 Chris Wilson <chris@...is-wilson.co.uk>:
> 
> > On Mon, Jun 27, 2016 at 04:29:22PM -0300, Gustavo Padovan wrote:
> > > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > > 
> > > Create sync_file->fence to abstract the type of fence we are using for
> > > each sync_file. If only one fence is present we use a normal struct fence
> > > but if there is more fences to be added to the sync_file a fence_array
> > > is created.
> > > 
> > > This change cleans up sync_file a bit. We don't need to have sync_file_cb
> > > array anymore. Instead, as we always have  one fence, only one fence
> > > callback is registered per sync_file.
> > > 
> > > Cc: Chris Wilson <chris@...is-wilson.co.uk>
> > > Cc: Christian König <christian.koenig@....com>
> > > Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > > ---
> > > @@ -76,21 +76,19 @@ struct sync_file *sync_file_create(struct fence *fence)
> > >  {
> > >  	struct sync_file *sync_file;
> > >  
> > > -	sync_file = sync_file_alloc(offsetof(struct sync_file, cbs[1]));
> > > +	sync_file = sync_file_alloc();
> > >  	if (!sync_file)
> > >  		return NULL;
> > >  
> > > -	sync_file->num_fences = 1;
> > > +	sync_file->fence = fence;
> > > +
> > >  	atomic_set(&sync_file->status, 1);
> > 
> > sync_file->status => fence_is_signaled(sync_file->fence);
> > 
> > Both should just be an atomic read, except fence_is_signaled() will then
> > do a secondary poll.
> 
> Not sure I follow. I set it to 1 here, but below when we call
> fence_add_callback() and the fence is already signalled atomic_dec sets
> sync_file->status to 0.

I'm just saying that usage sync_file->status is equivalent to
fence_is_signaled(), i.e. we reduce the amount of bookkeeping local to
sync_file.

> > >  	snprintf(sync_file->name, sizeof(sync_file->name), "%s-%s%llu-%d",
> > >  		 fence->ops->get_driver_name(fence),
> > >  		 fence->ops->get_timeline_name(fence), fence->context,
> > >  		 fence->seqno);
> > >  
> > > -	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))
> > > +	if (fence_add_callback(fence, &sync_file->cb, fence_check_cb_func))
> > >  		atomic_dec(&sync_file->status);
> > >  
> > >  	return sync_file;
> > > @@ -121,14 +119,42 @@ err:
> > >  	return NULL;
> > >  }
> > >  
> > > -static void sync_file_add_pt(struct sync_file *sync_file, int *i,
> > > -			     struct fence *fence)
> > > +static int sync_file_set_fence(struct sync_file *sync_file,
> > > +			       struct fence **fences, int num_fences)
> > >  {
> > > -	sync_file->cbs[*i].fence = fence;
> > > -	sync_file->cbs[*i].sync_file = sync_file;
> > > +	struct fence_array *array;
> > > +
> > > +	if (num_fences == 1) {
> > > +		sync_file->fence = fences[0];
> > 
> > This steals the references.
> > 
> > > +	} else {
> > > +		array = fence_array_create(num_fences, fences,
> > > +					   fence_context_alloc(1), 1, false);
> > 
> > This creates a reference.
> > 
> > When we call fence_put(sync_fence->fence) we release a reference we
> > never owned if num_fences == 1.
> 
> No, sync_file_merge() gets a new reference for each fence it is going to
> add to the new fence. So for num_fences == 1 when sync_file->fence is
> set we already hold a reference to it, so no matter if it is a fence or
> a array we own a reference.

Ugh. Root cause appears to be that fence_array_create() does not behave
how I would expect, in that it borrows references to the fences and
not own a reference to the fences in its array. I beg for a comment as
this function is very counter-intuitive for me.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre