[FFmpeg-devel] [PATCH 001/114] avcodec/bitstream: Add second function to create VLCs

[Anton Khirnov]

Quoting Andreas Rheinhardt (2020-11-12 21:51:28)
> Derek Buitenhuis:
> > On 10/11/2020 10:46, Andreas Rheinhardt wrote:
> >>  
> >> +#define INIT_VLC_STATIC_FROM_LENGTHS(vlc, bits, nb_codes, lens, len_wrap,  \
> >> +                                     symbols, symbols_wrap, symbols_size,  \
> >> +                                     offset, flags, static_size)           \
> >> +    do {                                                                   \
> >> +        static VLC_TYPE table[static_size][2];                             \
> >> +        (vlc)->table           = table;                                    \
> >> +        (vlc)->table_allocated = static_size;                              \
> >> +        ff_init_vlc_from_lengths(vlc, bits, nb_codes, lens, len_wrap,      \
> >> +                                 symbols, symbols_wrap, symbols_size,      \
> >> +                                 offset, flags | INIT_VLC_USE_NEW_STATIC); \
> >> +    } while (0)
> > 
> > If I am reading correctly, wherever you add/use this, you are adding non-thread
> > safe global init code to a decoder. This is a huge step back and not acceptable.
> > 
> > It should be made to properly use ff_thread_once if possible, or reworked.
> > 
> The ff_init_vlc_... functions are inherently thread-safe: Everything is
> modified only once and directly set to its final value; so it's no
> problem if two threads are initializing the same static VLC at the same
> time.

Strictly speaking it's still a race (and therefore UB), even if you
store the same values. I suspect tools like tsan will not like it
either. 

-- 
Anton Khirnov