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• avcodec/mpeg4videodec : Move use_intra_dc_vlc to stack, fix data race

  26 janvier 2022, par Andreas Rheinhardt

  avcodec/mpeg4videodec : Move use_intra_dc_vlc to stack, fix data race
  use_intra_dc_vlc is currently kept in sync between frame threads
  
  in mpeg4_update_thread_context(), yet it is set when decoding
  
  blocks, i.e. after ff_thread_finish_setup(). This is a data race
  
  and therefore undefined behaviour.
  This race can be fixed easily by moving the variable from the context
  
  to the stack : use_intra_dc_vlc is only read in
  
  mpeg4_decode_block() and only if one is decoding an intra block.
  
  There are three callsites for this function : One in
  
  mpeg4_decode_partitioned_mb() which always sets use_intra_dc_vlc
  
  before the call and two in mpeg4_decode_mb(). One of these callsites
  
  is for intra blocks and use_intra_dc_vlc is set before it ;
  
  the last callsite is for non-intra blocks, where use_intra_dc_vlc
  
  is ignored. So if it is used, it always uses a new value and can
  
  therefore be moved to the stack.
  The above also explains why this data race did not lead to
  
  FATE-test failures.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/mpeg4video.h
  • [DH] libavcodec/mpeg4videodec.c

• avcodec/mpeg4videodec : Fix data race when initializing VLCs

  4 janvier 2022, par Andreas Rheinhardt

  avcodec/mpeg4videodec : Fix data race when initializing VLCs
  Both the MPEG-4 parser as well as the decoder initialized
  
  several VLCs. There is a "static int done = 0 ;" in order to
  
  guard against initializing these multiple times, but this does
  
  not work when several threads try to initialize these VLCs
  
  concurrently, which can happen when initializing several parsers
  
  at the same time (they don't use the global lock that is used
  
  for codecs without the FF_CODEC_CAP_INIT_THREADSAFE cap ; actually,
  
  they don't use any lock at all).
  Since ff_mpeg4_decode_picture_header() now aborts early when called
  
  from the parser, it no longer needs to have these VLCs initialized
  
  at all. This commit therefore does exactly this.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/mpeg4video.h
  • [DH] libavcodec/mpeg4video_parser.c
  • [DH] libavcodec/mpeg4videodec.c

• avformat/allformats : Fix data race when accessing devices lists

  1er octobre 2021, par Andreas Rheinhardt

  avformat/allformats : Fix data race when accessing devices lists
  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@outlook.com>
  

  • [DH] libavformat/allformats.c