[FFmpeg-devel] [RFC] d3dva security hw+threads

[Lukas Fellechner]

> Gesendet: Freitag, 02. September 2022 um 01:46 Uhr
> Von: "Timo Rothenpieler" <timo at rothenpieler.org>
> An: ffmpeg-devel at ffmpeg.org
> Betreff: Re: [FFmpeg-devel] [RFC] d3dva security hw+threads
> On 02.09.2022 01:32, Michael Niedermayer wrote:
>> Hi all
>>
>> Theres a use after free issue in H.264 Decoding on d3d11va with multiple threads
>> I dont have the hardware/platform nor do i know the hw decoding code so i made
>> no attempt to fix this beyond asking others to ...
>
> hwaccel with multiple threads being broken is not exactly a surprise.
> So we could just disable that, and always have it be one single thread?

I am using FFmpeg as decoder library in a video player, either with sw decoding
or d3d11va. Originally, I had threads set to auto in all cases. While it worked
for some codecs such as H.264, it produced garbage output for other codecs.
I think VP8/VP9 are severly broken with threading+d3d11va. So I had to manually
set threads to 1, if using hwaccel. Only then I had stable output for all codecs.

Using multithreading together with hwaccel really does not make any sense.
All the work is done by the GPU, not by the CPU. And the GPU will parallelize
internally where possible. Adding CPU multithreading will not help at all.

IMHO the best would be to completely disable threading when hwaccel is in use.