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>

1 files changed, 0 insertions, 1 deletions

diff --git a/libavcodec/mpeg4video.h b/libavcodec/mpeg4video.h
index 9fc79b1a22..14fc5e1396 100644
--- a/libavcodec/mpeg4video.h
+++ b/libavcodec/mpeg4video.h
@@ -94,7 +94,6 @@ typedef struct Mpeg4DecContext {
     int new_pred;
     int enhancement_type;
     int scalability;
-    int use_intra_dc_vlc;
 
     /// QP above which the ac VLC should be used for intra dc
     int intra_dc_threshold;