[FFmpeg-devel] [PATCH] avutil/eval: make av_expr_eval more thread safe

[Marton Balint]

On Sun, 15 Dec 2019, Michael Niedermayer wrote:

> On Sun, Dec 15, 2019 at 01:52:52PM +0100, Marton Balint wrote:
>>
>>
>> On Sun, 15 Dec 2019, Michael Niedermayer wrote:
>>
>>> On Sun, Dec 15, 2019 at 02:16:49AM +0100, Marton Balint wrote:
>>>> Unfortunately the ld() and st() operations store the variables in the AVExpr
>>>> context and not in the parser in order to keep the stored values between
>>>> evaluations (which is a rarely(?) used undocumented(?) feature of AVExpr).
>>>>
>>>> This causes data race issues when the same expression is evaluated from
>>>> multiple threads. Let's use the Parser as variable store during evaluation and
>>>> only sync the AVExpr context variables with the Parser before and after
>>>> evaluation. This keeps the feature working for single threaded cases and fixes
>>>> using ld() and st() usage for the "normal" use case when the ld()-s only
>>>> reference st()-d values from the same evaluation.
>>>>
>>>

[...]

>>
>> The confusion is caused by mixing the state with the parsed expression.
>> Maybe we should decouple them, or at least add the possibility to the user
>> to use a custom state.
>
> yes decoupling them / giving the user more control over the state makes sense
>
>
>>
>> Anyway, I am inclined to accept what Gyan proposed, to determine if the
>> expression uses lt() st() or rand() and if it does fall back to 1 thread. Or
>> implement something like av_expr_is_stateless() to do this in a helper
>> function.
>
> This solution kind of sucks because it forces a heap of cases to run
> single threaded which would work find with multiple threads
> OTOH with one AVExpr or AVExprState per thread it would work multi threaded

That would resolve the data race issue in the ticket. However, if the 
expression depends on previous state then the result will be 
undeterministic because it is random which slices get assigned to which 
threads, so the initial AVExprState they see will be random as well.

If you force single thread, the result will be deterministic.

>
> The one case the state per thread will need to take a bit of care of is the
> random seed, which we would want to be different per thread

We can just say that it is up to the user to initialize the random seed if 
needed, that is the case for single thread as well.

Regards,
Marton