Re: [PATCH 1/4] sync_file: add a mutex to protect fence and callback members.

[Chris Wilson <chris-Y6uKTt2uX1cEflXRtASbqLVCufUGDwFn@public.gmane.org>]

On Tue, Mar 14, 2017 at 10:13:37AM +0100, Christian König wrote:
> Am 14.03.2017 um 09:45 schrieb Daniel Vetter:
> >On Tue, Mar 14, 2017 at 10:50:51AM +1000, Dave Airlie wrote:
> >>From: Dave Airlie <airlied@redhat.com>
> >>
> >>This isn't needed currently, but to reuse sync file for Vulkan
> >>permanent shared semaphore semantics, we need to be able to swap
> >>the fence backing a sync file. This patch adds a mutex to the
> >>sync file and uses to protect accesses to the fence and cb members.
> >>
> >>Signed-off-by: Dave Airlie <airlied@redhat.com>
> >We've gone to pretty great lengths to rcu-protect all the fence stuff, so
> >that a peek-only is entirely lockless. Can we haz the same for this pls?
> 
> Yes, just wanted to suggest the same thing.
> 
> Basically you just need the following to retrieve the fence:
> 
> while (sync_file->fence && !(fence = fence_get_rcu(sync_file->fence));

We even have a helper for that:

	fence = dma_fence_get_rcu_safe(&sync_file->fence);

(Still going to suggest using a reservation_object rather than an
exclusive-only implementation.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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