[FFmpeg-devel] [PATCH 13/20] avformat/allformats: Fix data race when accessing devices lists

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Up until now setting the input and output devices lists is guarded
> by a mutex. This prevents data races emanating from multiple concurrent
> calls to avpriv_register_devices() (triggered by multiple concurrent
> calls to avdevice_register_all()). Yet reading the lists pointers was
> done without any lock and with nonatomic variables. This means that
> there are data races in case of concurrent calls to
> av_(de)muxer_iterate() and avdevice_register_all() (but only if the
> iteration in av_(de)muxer_iterate exhausts the non-device (de)muxers).
> 
> This commit fixes this by putting said pointers into atomic objects.
> Due to the unavailability of _Atomic the object is an atomic_uintptr,
> leading to ugly casts. Switching to atomics also allowed to remove
> the mutex currently used in avpriv_register_devices().
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
> ---
>  libavformat/allformats.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/libavformat/allformats.c b/libavformat/allformats.c
> index 5471f7c16f..cadaf057fc 100644
> --- a/libavformat/allformats.c
> +++ b/libavformat/allformats.c
> @@ -19,7 +19,7 @@
>   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>   */
>  
> -#include "libavutil/thread.h"
> +#include <stdatomic.h>
>  #include "libavformat/internal.h"
>  #include "avformat.h"
>  
> @@ -534,18 +534,20 @@ extern const AVInputFormat  ff_vapoursynth_demuxer;
>  #include "libavformat/muxer_list.c"
>  #include "libavformat/demuxer_list.c"
>  
> -static const AVInputFormat * const *indev_list = NULL;
> -static const AVOutputFormat * const *outdev_list = NULL;
> +static atomic_uintptr_t indev_list_intptr  = ATOMIC_VAR_INIT(0);
> +static atomic_uintptr_t outdev_list_intptr = ATOMIC_VAR_INIT(0);
>  
>  const AVOutputFormat *av_muxer_iterate(void **opaque)
>  {
>      static const uintptr_t size = sizeof(muxer_list)/sizeof(muxer_list[0]) - 1;
>      uintptr_t i = (uintptr_t)*opaque;
>      const AVOutputFormat *f = NULL;
> +    uintptr_t tmp;
>  
>      if (i < size) {
>          f = muxer_list[i];
> -    } else if (outdev_list) {
> +    } else if (tmp = atomic_load_explicit(&outdev_list_intptr, memory_order_relaxed)) {
> +        const AVOutputFormat *const *outdev_list = (const AVOutputFormat *const *)tmp;
>          f = outdev_list[i - size];
>      }
>  
> @@ -559,10 +561,12 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
>      static const uintptr_t size = sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;
>      uintptr_t i = (uintptr_t)*opaque;
>      const AVInputFormat *f = NULL;
> +    uintptr_t tmp;
>  
>      if (i < size) {
>          f = demuxer_list[i];
> -    } else if (indev_list) {
> +    } else if (tmp = atomic_load_explicit(&indev_list_intptr, memory_order_relaxed)) {
> +        const AVInputFormat *const *indev_list = (const AVInputFormat *const *)tmp;
>          f = indev_list[i - size];
>      }
>  
> @@ -571,12 +575,8 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
>      return f;
>  }
>  
> -static AVMutex avpriv_register_devices_mutex = AV_MUTEX_INITIALIZER;
> -
>  void avpriv_register_devices(const AVOutputFormat * const o[], const AVInputFormat * const i[])
>  {
> -    ff_mutex_lock(&avpriv_register_devices_mutex);
> -    outdev_list = o;
> -    indev_list = i;
> -    ff_mutex_unlock(&avpriv_register_devices_mutex);
> +    atomic_store_explicit(&outdev_list_intptr, (uintptr_t)o, memory_order_relaxed);
> +    atomic_store_explicit(&indev_list_intptr,  (uintptr_t)i, memory_order_relaxed);
>  }
> 

Will apply this patch tomorrow unless there are objections.

- Andreas