[FFmpeg-devel] [PATCH 1/3 v2] avcodec/util: use a mutex instead of atomics in avcodec_register()

[James Almer]

On 1/5/2018 1:02 PM, wm4 wrote:
> On Thu,  4 Jan 2018 22:40:44 -0300
> James Almer <jamrial at gmail.com> wrote:
> 
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>>  libavcodec/utils.c | 12 +++++++++---
>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/libavcodec/utils.c b/libavcodec/utils.c
>> index dfbfe98d63..4d736d2e7d 100644
>> --- a/libavcodec/utils.c
>> +++ b/libavcodec/utils.c
>> @@ -26,7 +26,6 @@
>>   */
>>  
>>  #include "config.h"
>> -#include "libavutil/atomic.h"
>>  #include "libavutil/attributes.h"
>>  #include "libavutil/avassert.h"
>>  #include "libavutil/avstring.h"
>> @@ -127,17 +126,24 @@ int av_codec_is_decoder(const AVCodec *codec)
>>      return codec && (codec->decode || codec->receive_frame);
>>  }
>>  
>> +static AVMutex codec_register_mutex = AV_MUTEX_INITIALIZER;
>> +
>>  av_cold void avcodec_register(AVCodec *codec)
>>  {
>>      AVCodec **p;
>>      avcodec_init();
>> +
>> +    ff_mutex_lock(&codec_register_mutex);
>>      p = last_avcodec;
>> -    codec->next = NULL;
>>  
>> -    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, codec))
>> +    while (*p)
>>          p = &(*p)->next;
>> +    *p          = codec;
>> +    codec->next = NULL;
>>      last_avcodec = &codec->next;
>>  
>> +    ff_mutex_unlock(&codec_register_mutex);
>> +
>>      if (codec->init_static_data)
>>          codec->init_static_data(codec);
>>  }
> 
> Seems good, but should init_static_data really be outside of the lock?
> (Sure, it wasn't under a look before either, but still.)

I don't think calling avcodec_register() twice with the same AVCodec is
defined or supported at all, so outside of creating the linked list i
don't see any other potential race here. But i may be wrong.

In any case, that's something for a separate patch as it's a change in
behavior (and may make things slower), so pushed as is.