[PATCH] drm/syncobj: remove boring message

[Koenig, Christian]

Am 02.08.2019 18:28 schrieb Jason Ekstrand <jason at jlekstrand.net>:
On Thu, Aug 1, 2019 at 9:05 AM Koenig, Christian <Christian.Koenig at amd.com<mailto:Christian.Koenig at amd.com>> wrote:
Am 01.08.19 um 15:45 schrieb Lionel Landwerlin:
> Just had a few exchanges with Chris about this.
> Chris suggests that if we're about to add a point to the timeline in
> an unordered fashion, actually better not add it at all.
>
> What's your take on this?

That is a clear NAK. See not adding a point at all means we lose some
synchronization and that is not something we can do here.

In other words syncing to much if userspace does something nasty is ok
and defensive programmed, but not syncing at all could have unforeseen
consequences.

So if process A signals 7, process B detects that and signals 3 and then process A tries to insert something which waits on 7 and signals 8, what happens?  My understanding is that it "breaks" the timeline and so, from the perspective of process A, its signal operation on 7 is gone and it's attempt to wait on 7 will either -EINVAL because the kernel can't find the time point or else just sit there.  Am I understanding this correctly?

Nope, what happens is that we note the largest signaled seqno and wait for everything without returning an error.

For example if we have signaled 7 and then 3 then any waiting for 7 we would wait for both 3 and 7.

  If so, it sounds more like an attack vector than defensive programming to me.

Yeah, completely agree. That's why I also rejected the idea to return an error on wait.


Yes, more syncornization is generally better than less.  However, if you're screwing up your syncronization from userspace and getting wrong rendering results, that's your fault.  If you're causing your compositor to suddenly start seeing -EINVAL which gets turned into VK_ERROR_DEVICE_LOST, that's a lot worse IMO.  I'm not saying that we shouldn't try to be robust in this case; I'm just concerned that the suggest solution isn't.

Completely agree as well.

The key point is we need to find a balance between keeping things working and signaling that something is wrong.

I mean the two options we have is to either ignore such errors and do the most defensive thing we can. And the current solution is already pretty good at that.

Or we can signal those errors but risk that it can be used for a deny of service.

Regards,
Christian.


--Jason


Another idea would be to add the fence, but also set an error flag and
deny any further signaling on that syncobj.

Regards,
Christian.

> I'm fine with this, but rather than add another variant of add_point()
> maybe we change change the existing.
>
> Thanks,
>
> -Lionel
>
> On 29/07/2019 11:20, Chunming Zhou wrote:
>> It is normal that binary syncobj replaces the underlying fence.
>>
>> Signed-off-by: Chunming Zhou <david1.zhou at amd.com<mailto:david1.zhou at amd.com>>
>> ---
>>   drivers/gpu/drm/drm_syncobj.c | 3 ---
>>   1 file changed, 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_syncobj.c
>> b/drivers/gpu/drm/drm_syncobj.c
>> index 929f7c64f9a2..bc7ec1679e4d 100644
>> --- a/drivers/gpu/drm/drm_syncobj.c
>> +++ b/drivers/gpu/drm/drm_syncobj.c
>> @@ -151,9 +151,6 @@ void drm_syncobj_add_point(struct drm_syncobj
>> *syncobj,
>>       spin_lock(&syncobj->lock);
>>         prev = drm_syncobj_fence_get(syncobj);
>> -    /* You are adding an unorder point to timeline, which could
>> cause payload returned from query_ioctl is 0! */
>> -    if (prev && prev->seqno >= point)
>> -        DRM_ERROR("You are adding an unorder point to timeline!\n");
>>       dma_fence_chain_init(chain, prev, fence, point);
>>       rcu_assign_pointer(syncobj->fence, &chain->base);
>
>

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