linux-kernel - Re: [RFC v2 2/3] dma-buf/fence-array: add fence_array

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Message-ID: <20160628141703.GJ2508@joana>
Date:	Tue, 28 Jun 2016 11:17:03 -0300
From:	Gustavo Padovan <gustavo.padovan@...labora.com>
To:	Christian König <christian.koenig@....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>,
	Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [RFC v2 2/3] dma-buf/fence-array: add fence_array_teardown()

2016-06-28 Christian König <christian.koenig@....com>:

> Am 27.06.2016 um 21:29 schrieb Gustavo Padovan:
> > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > 
> > As the array of fence callbacks held by an active struct fence_array
> > each has a reference to the struct fence_array, when the owner of the
> > fence_array is freed it must dispose of the callback references before
> > it can free the fence_array. This can not happen simply during
> > fence_release() because of the extra references and so we need a new
> > function to run before the final fence_put().
> 
> As I said previously as well, this is completely superfluous.
> 
> The fence array keeps a reference to itself as long as not all callbacks are
> signaled.
> 
> So you only need to unregister your callback from the array itself and drop
> your reference when you don't need it any more in the sync file.

Exactly, this should be called from sync_file_free() because of the
following use case:

	1. You create 2 sync_file with 1 fence each
	2. Merge both fences, which creates a fence array
	3. Close the sync_file fd without waiting for the fences to
	signal

At this point you leak the fence-array because the final fence_put() 
does not release it because of the extra references from the non
signalled fences so we need to clean up this somehow.

	Gustavo