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

[Andreas Rheinhardt]

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.

- Andreas