Re: [PATCH] dma-buf: fix reservation_object_wait_timeout_rcu to wait correctly

[zhoucm1 <david1.zhou@amd.com>]

On 2017年07月24日 19:57, Daniel Vetter wrote:
> On Mon, Jul 24, 2017 at 11:51 AM, Christian König
> <deathsimple@vodafone.de> wrote:
>> Am 24.07.2017 um 10:33 schrieb Daniel Vetter:
>>> On Fri, Jul 21, 2017 at 06:20:01PM +0200, Christian König wrote:
>>>> From: Christian König <christian.koenig@amd.com>
>>>>
>>>> With hardware resets in mind it is possible that all shared fences are
>>>> signaled, but the exlusive isn't. Fix waiting for everything in this
>>>> situation.
>>> How did you end up with both shared and exclusive fences on the same
>>> reservation object? At least I thought the point of exclusive was that
>>> it's exclusive (and has an implicit barrier on all previous shared
>>> fences). Same for shared fences, they need to wait for the exclusive one
>>> (and replace it).
>>>
>>> Is this fallout from the amdgpu trickery where by default you do all
>>> shared fences? I thought we've aligned semantics a while back ...
>>
>> No, that is perfectly normal even for other drivers. Take a look at the
>> reservation code.
>>
>> The exclusive fence replaces all shared fences, but adding a shared fence
>> doesn't replace the exclusive fence. That actually makes sense, cause when
>> you want to add move shared fences those need to wait for the last exclusive
>> fence as well.
> Hm right.
>
>> Now normally I would agree that when you have shared fences it is sufficient
>> to wait for all of them cause those operations can't start before the
>> exclusive one finishes. But with GPU reset and/or the ability to abort
>> already submitted operations it is perfectly possible that you end up with
>> an exclusive fence which isn't signaled and a shared fence which is signaled
>> in the same reservation object.
> How does that work? The batch(es) with the shared fence are all
> supposed to wait for the exclusive fence before they start, which
> means even if you gpu reset and restart/cancel certain things, they
> shouldn't be able to complete out of order.
Hi Daniel,

Do you mean exclusive fence must be signalled before any shared fence? 
Where could I find this restriction?

Thanks,
David Zhou
>
> If you outright cancel a fence then you're supposed to first call
> dma_fence_set_error(-EIO) and then complete it. Note that atm that
> part might be slightly overengineered and I'm not sure about how we
> expose stuff to userspace, e.g. dma_fence_set_error(-EAGAIN) is (or
> soon, has been) used by i915 for it's internal book-keeping, which
> might not be the best to leak to other consumers. But completing
> fences (at least exported ones, where userspace or other drivers can
> get at them) shouldn't be possible.
> -Daniel