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

[Christian König]

Am 14.03.2017 um 10:29 schrieb Chris Wilson:
> 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 at 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 at 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.)

Yeah, thought about that as well. But the reservation object doesn't 
seem to match the required userspace semantic.

E.g. you actually don't want more than one fence it in as far as I 
understand it.

Christian.

> -Chris
>