[PATCH] drm/syncobj: Deal with signalled fences in transfer.

[Christian König]

Am 07.12.21 um 11:40 schrieb Bas Nieuwenhuizen:
> On Tue, Dec 7, 2021 at 8:21 AM Christian König <christian.koenig at amd.com> wrote:
>> Am 07.12.21 um 08:10 schrieb Lionel Landwerlin:
>>> On 07/12/2021 03:32, Bas Nieuwenhuizen wrote:
>>>> See the comments in the code. Basically if the seqno is already
>>>> signalled then we get a NULL fence. If we then put the NULL fence
>>>> in a binary syncobj it counts as unsignalled, making that syncobj
>>>> pretty much useless for all expected uses.
>>>>
>>>> Not 100% sure about the transfer to a timeline syncobj but I
>>>> believe it is needed there too, as AFAICT the add_point function
>>>> assumes the fence isn't NULL.
>>>>
>>>> Fixes: ea569910cbab ("drm/syncobj: add transition iotcls between
>>>> binary and timeline v2")
>>>> Cc: stable at vger.kernel.org
>>>> Signed-off-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
>>>> ---
>>>>    drivers/gpu/drm/drm_syncobj.c | 26 ++++++++++++++++++++++++++
>>>>    1 file changed, 26 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c
>>>> b/drivers/gpu/drm/drm_syncobj.c
>>>> index fdd2ec87cdd1..eb28a40400d2 100644
>>>> --- a/drivers/gpu/drm/drm_syncobj.c
>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
>>>> @@ -861,6 +861,19 @@ static int
>>>> drm_syncobj_transfer_to_timeline(struct drm_file *file_private,
>>>>                         &fence);
>>>>        if (ret)
>>>>            goto err;
>>>> +
>>>> +    /* If the requested seqno is already signaled
>>>> drm_syncobj_find_fence may
>>>> +     * return a NULL fence. To make sure the recipient gets
>>>> signalled, use
>>>> +     * a new fence instead.
>>>> +     */
>>>> +    if (!fence) {
>>>> +        fence = dma_fence_allocate_private_stub();
>>>> +        if (!fence) {
>>>> +            ret = -ENOMEM;
>>>> +            goto err;
>>>> +        }
>>>> +    }
>>>> +
>>>
>>> Shouldn't we fix drm_syncobj_find_fence() instead?
>> Mhm, now that you mention it. Bas, why do you think that
>> dma_fence_chain_find_seqno() may return NULL when the fence is already
>> signaled?
>>
>> Double checking the code that should never ever happen.
> Well, I tested the patch with
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fmesa%2Fmesa%2F-%2Fmerge_requests%2F14097%2Fdiffs%3Fcommit_id%3Dd4c5c840f4e3839f9f5c1747a9034eb2b565f5c0&data=04%7C01%7Cchristian.koenig%40amd.com%7Cc1ab29fc100842826f5d08d9b96e102a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637744705383763833%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=sXkTJWm%2FWm2xwgLGdepVWAOlqj%2FeArnvmMvnJpQ9YEs%3D&reserved=0
> so I'm pretty sure it happens, and this patch fixes  it, though I may
> have misidentified what the code should do.
>
> My reading is that the dma_fence_chain_for_each in
> dma_fence_chain_find_seqno will never visit a signalled fence (unless
> the top one is signalled), as dma_fence_chain_walk will never return a
> signalled fence (it only returns on NULL or !signalled).

Ah, yes that suddenly makes more sense.

> Happy to move this to drm_syncobj_find_fence.

No, I think that your current patch is fine.

That drm_syncobj_find_fence() only returns NULL when it can't find 
anything !signaled is correct behavior I think.

Going to push your original patch if nobody has any more objections.

But somebody might want to take care of the IGT as well.

Regards,
Christian.

>> Regards,
>> Christian.
>>
>>> By returning a stub fence for the timeline case if there isn't one.
>>>
>>>
>>> Because the same NULL fence check appears missing in amdgpu (and
>>> probably other drivers).
>>>
>>>
>>> Also we should have tests for this in IGT.
>>>
>>> AMD contributed some tests when this code was written but they never
>>> got reviewed :(
>>>
>>>
>>> -Lionel
>>>
>>>
>>>>        chain = kzalloc(sizeof(struct dma_fence_chain), GFP_KERNEL);
>>>>        if (!chain) {
>>>>            ret = -ENOMEM;
>>>> @@ -890,6 +903,19 @@ drm_syncobj_transfer_to_binary(struct drm_file
>>>> *file_private,
>>>>                         args->src_point, args->flags, &fence);
>>>>        if (ret)
>>>>            goto err;
>>>> +
>>>> +    /* If the requested seqno is already signaled
>>>> drm_syncobj_find_fence may
>>>> +     * return a NULL fence. To make sure the recipient gets
>>>> signalled, use
>>>> +     * a new fence instead.
>>>> +     */
>>>> +    if (!fence) {
>>>> +        fence = dma_fence_allocate_private_stub();
>>>> +        if (!fence) {
>>>> +            ret = -ENOMEM;
>>>> +            goto err;
>>>> +        }
>>>> +    }
>>>> +
>>>>        drm_syncobj_replace_fence(binary_syncobj, fence);
>>>>        dma_fence_put(fence);
>>>>    err:
>>>