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

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

On 2017年07月25日 14:50, Daniel Vetter wrote:
> On Tue, Jul 25, 2017 at 02:16:55PM +0800, zhoucm1 wrote:
>>
>> 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?
> Yes, Christian also described it above. Could be that we should have
> better kerneldoc to document this ...
Is that a known assumption? if that way, it doesn't matter even that we 
always wait exclusive fence, right? Just one more line checking.

Thanks,
David Zhou
> -Daniel
>
>> 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