[Intel-gfx] [PATCH 8/8] dma-buf: nuke reservation_object seq number

[Koenig, Christian]

Am 07.08.19 um 14:19 schrieb Chris Wilson:
> Quoting Christian König (2019-08-07 13:08:38)
>> Am 06.08.19 um 21:57 schrieb Chris Wilson:
>>> If we add to shared-list during the read, ... Hmm, actually we should
>>> return num_list, i.e.
>>>
>>> do {
>>>        *list = rcu_dereference(obj->fence);
>>>        num_list = *list ? (*list)->count : 0;
>>>        smp_rmb();
>>> } while (...)
>>>
>>> return num_list.
>>>
>>> as the relationship between the count and the fence entries is also
>>> determined by the mb in add_shared_fence.
>> I've read that multiple times now, but can't follow. Why should we do this?
>>
>> The only important thing is that the readers see the new fence before
>> the increment of the number of fences.
> Exactly. We order the store so that the fence is in the list before we
> update the count (so that we don't read garbage because the fence isn't
> there yet).
>
> But we don't have the equivalent here for the read once the rmb is
> removed from the seqcount_read_begin/end looping. We need to see the
> update in the same order as was stored, and only use the coherent
> portion of the list.

Ok that makes sense. Going to fix up the code regarding to that.

Christian.

> -Chris