[FFmpeg-devel] [PATCH] Don't needlessly reinitialize ff_cos_## tables.

[Ronald S. Bultje]

Hi,

On Fri, Oct 23, 2015 at 12:48 AM, Dale Curtis <dalecurtis at chromium.org>
wrote:

> On Wed, Oct 21, 2015 at 6:52 PM, Dale Curtis <dalecurtis at chromium.org>
> wrote:
>
> > On Tue, Oct 20, 2015 at 11:50 PM, Michael Niedermayer <
> > michael at niedermayer.cc> wrote:
> >>
> >> the last element to be written should be checked, so that if
> >> initialization is done by 2 threads at the same time, neither can
> >> return from this function without initialization having finished
> >>
> >> also the race detectors are broken if they complain about cases where
> >> a variable that has value a is set to value a, that cannot be part of
> >> a race, not even if a is written byte per byte instead of atomically
> >> unless theres something iam missing
> >> Is this something that can be fixed or disabled on the side of the
> >> race detectors?
> >> It might reduce false positives in FFmpeg and potentially other
> >> tools.
> >>
> >
> > We can suppress it, which I think is more reasonable then the overhead
> > it'd take to make this "race" go away. I notice the sin tables are
> > initialized within the fft context so there's no "race." Is there a
> reason
> > the cosine tables aren't done this way?
> >
>
> Actually it looks like sin may suffer from the same issues -- it doesn't
> make a copy like I was thinking it did. One of the TSAN folk detailed why
> suppression isn't a good idea here
> https://codereview.chromium.org/1412123007/#msg7 -- which roughly says we
> can't rely on the compiler to do the right thing here.
>
> What are your thoughts about using ff_thread_once() to build the full sin
> and cos tables all at once?


That sounds right. The reason that wasn't initially done is probably
because cos/sin tables existed before ff_thread_once did.

Ronald