linux-kernel - Re: [RFC 0/5] rework fences on struct sync

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Message-ID: <20160624151932.GG2508@joana>
Date:	Fri, 24 Jun 2016 12:19:32 -0300
From:	Gustavo Padovan <gustavo.padovan@...labora.com>
To:	Christian König <christian.koenig@....com>
Cc:	Christian König <deathsimple@...afone.de>,
	Gustavo Padovan <gustavo@...ovan.org>,
	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,
	John Harrison <John.C.Harrison@...el.com>, m.chehab@...sung.com
Subject: Re: [RFC 0/5] rework fences on struct sync_file

2016-06-24 Christian König <christian.koenig@....com>:

> Am 24.06.2016 um 16:59 schrieb Gustavo Padovan:
> > 2016-06-24 Christian König <deathsimple@...afone.de>:
> > 
> > > Am 24.06.2016 um 15:17 schrieb Gustavo Padovan:
> > > > Hi Christian,
> > > > 
> > > > 2016-06-24 Christian König <christian.koenig@....com>:
> > > > 
> > > > > Am 23.06.2016 um 17:29 schrieb Gustavo Padovan:
> > > > > > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > > > > > 
> > > > > > Hi all,
> > > > > > 
> > > > > > This is an attempt to improve fence support on Sync File. The basic idea
> > > > > > is to have only sync_file->fence and store all fences there, either as
> > > > > > normal fences or fence_arrays. That way we can remove some potential
> > > > > > duplication when using fence_array with sync_file: the duplication of the array
> > > > > > of fences and the duplication of fence_add_callback() for all fences.
> > > > > > 
> > > > > > Now when creating a new sync_file during the merge process sync_file_set_fence()
> > > > > > will set sync_file->fence based on the number of fences for that sync_file. If
> > > > > > there is more than one fence a fence_array is created. One important advantage
> > > > > > approach is that we only add one fence callback now, no matter how many fences
> > > > > > there are in a sync_file - the individual callbacks are added by fence_array.
> > > > > > 
> > > > > > Two fence ops had to be created to help abstract the difference between handling
> > > > > > fences and fences_arrays: .teardown() and .get_fences(). The former run needed
> > > > > > on fence_array, and the latter just return a copy of all fences in the fence.
> > > > > > I'm not so sure about adding those two, speacially .get_fences(). What do you
> > > > > > think?
> > > > > Clearly not a good idea to add this a fence ops, cause those are specialized
> > > > > functions for only a certain fence implementation (the fence_array).
> > > > Are you refering only to .get_fences()?
> > > That comment was only for the get_fences() operation, but the teardown()
> > > callback looks very suspicious to me as well.
> > > 
> > > Can you explain once more why that should be necessary?
> > When the sync_file owner exits we need to clean up it and that means releasing
> > the fence too, however with fence_array we can't just call fence_put()
> > as a extra reference to array->base for each fence is held when enabling
> > signalling. Thus we need a prior step, that I called teardown(), to
> > remove the callback for not signaled fences and put the extra
> > references.
> > 
> > Another way to do this would be:
> > 
> > 	if (fence_is_array(sync_file->fence))
> > 		fence_array_destroy(to_fence_array(sync_file->fence));
> > 	else
> > 		fence_put(sync_file_fence);
> > 
> > This would avoid the extra ops, maybe we should go this way.
> 
> NAK on both approaches. The fence array grabs another reference on itself
> for each callback it registers, so this isn't necessary:
> 
> >         for (i = 0; i < array->num_fences; ++i) {
> >                 cb[i].array = array;
> >                 /*
> >                  * As we may report that the fence is signaled before all
> >                  * callbacks are complete, we need to take an additional
> >                  * reference count on the array so that we do not free
> > it too
> >                  * early. The core fence handling will only hold the
> > reference
> >                  * until we signal the array as complete (but that is now
> >                  * insufficient).
> >                  */
> >                 fence_get(&array->base);
> >                 if (fence_add_callback(array->fences[i], &cb[i].cb,
> >                                        fence_array_cb_func)) {
> >                         fence_put(&array->base);
> >                         if (atomic_dec_and_test(&array->num_pending))
> >                                 return false;
> >                 }
> >         }
> 
> So you can just use fence_remove_callback() and then fence_put() without
> worrying about the reference.

Yes. That is what I have in mind for fence_array_destroy() in the
snippet of code in the last e-mail. That plus the last fence_put() to
release the fence_array().

	Gustavo