linux-kernel - Re: [RFC 1/8] dma-buf/fence: add fence

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Date:	Fri, 15 Apr 2016 21:25:50 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Daniel Stone <daniels@...labora.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Riley Andrews <riandrews@...roid.com>,
	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,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
Subject: Re: [RFC 1/8] dma-buf/fence: add fence_collection fences

On Fri, Apr 15, 2016 at 11:27:50AM -0700, Gustavo Padovan wrote:
> 2016-04-15 Christian König <christian.koenig@....com>:
> > Amdgpu also has an implementation for a fence collection which uses a a
> > hashtable to keep the fences grouped by context (e.g. only the latest fence
> > is keept for each context). See amdgpu_sync.c for reference.
> > 
> > We should either make the collection similar in a way that you can add as
> > many fences as you want (like the amdgpu implementation) or make it static
> > and only add a fixed number of fences right from the beginning.
> > 
> > I can certainly see use cases for both, but if you want to stick with a
> > static approach you should probably call the new object fence_array instead
> > of fence_collection and do as Daniel suggested.
> 
> Maybe we can go for something in between. Have fence_collection_init()
> need at least two fences to create the fence_collection. Then
> fence_collection_add() would add more dinamically.

The problem with adding fences later on is that it makes it trivial to add
deadlocks and loops. Just add the fence collection to itself, boom. From
that pov it's an unsafe api, and hence something to avoid.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch