[FFmpeg-devel] [PATCH 4/6] avcodec/mpegpicture: Don't copy unnecessarily, fix race

[Michael Niedermayer]

On Sat, Aug 13, 2022 at 05:03:04PM +0200, Andreas Rheinhardt wrote:
> mpegvideo uses an array of Pictures and when it is done with using
> them, it only unreferences them incompletely: Some buffers are kept
> so that they can be reused lateron if the same slot in the Picture
> array is reused, making this a sort of a bufferpool.
> (Basically, a Picture is considered used if the AVFrame's buf is set.)
> Yet given that other pieces of the decoder may have a reference to
> these buffers, they need not be writable and are made writable using
> av_buffer_make_writable() when preparing a new Picture. This involves
> reading the buffer's data, although the old content of the buffer
> need not be retained.
> 
> Worse, this read can be racy, because the buffer can be used by another
> thread at the same time. This happens for Real Video 3 and 4.
> 
> This commit fixes this race by no longer copying the data;
> instead the old buffer is replaced by a new, zero-allocated buffer.
> 
> (Here are the details of what happens with three or more decoding threads
> when decoding rv30.rm from the FATE-suite as happens in the rv30 test:
> The first decoding thread uses the first slot of its picture array
> to store its current pic; update_thread_context copies this for the
> second thread that decodes a P-frame. It uses the second slot in its
> Picture array to store its P-frame. This arrangement is then copied
> to the third decode thread, which decodes a B-frame. It uses the third
> slot in its Picture array for its current frame.
> update_thread_context copies this to the next thread. It unreferences
> the third slot containing the other B-frame and then it reuses this
> slot for its current frame. Because the pic array slots are only
> incompletely unreferenced, the buffers of the previous B-frame are
> still in there and they are not writable; in fact the previous
> thread is concurrently writing to them, causing races when making
> the buffer writable.)
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
> ---
>  libavcodec/mpegpicture.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)

This causes a difference in some frames of: (i have not investigates why
just reporting as i noticed)
quite possibly thats just showing your bugfix in action

./ffmpeg -y -bitexact -i fate/svq3/Vertical400kbit.sorenson3.mov -ps 50 -bf 1 -bitexact -an -qscale 5  -ss 40 -error_rate 4 -threads 1 /tmp/out4.avi 
./ffmpeg -y -bitexact -v -1 -loglevel 0 -i /tmp/out4.avi -bitexact -vsync drop -f framecrc -

@@ -141,7 +141,7 @@
 0,         22,         22,        1,   115200, 0x4cc933e9
 0,         23,         23,        1,   115200, 0x693a40a9
 0,         24,         24,        1,   115200, 0x956e3b15
-0,         25,         25,        1,   115200, 0x61763ff4
+0,         25,         25,        1,   115200, 0xc9e53d1c
 0,         26,         26,        1,   115200, 0x5c5c3dfc
 0,         27,         27,        1,   115200, 0x7de641ea
 0,         28,         28,        1,   115200, 0xe6cc4136
@@ -187,7 +187,7 @@
 0,         68,         68,        1,   115200, 0x49dcbf4e
 0,         69,         69,        1,   115200, 0x1ea1c7d1
 0,         70,         70,        1,   115200, 0xdf77c67b
-0,         71,         71,        1,   115200, 0x7f6bd16d
+0,         71,         71,        1,   115200, 0x33d9d206
 0,         72,         72,        1,   115200, 0x5e37cb3a
 0,         73,         73,        1,   115200, 0x15abcda3
 0,         74,         74,        1,   115200, 0xbf4dcbd4
@@ -199,7 +199,7 @@
 0,         80,         80,        1,   115200, 0x17d1d667
 0,         81,         81,        1,   115200, 0x0c1fdf9c
 0,         82,         82,        1,   115200, 0x7eabde6b
-0,         83,         83,        1,   115200, 0xe623e7af
+0,         83,         83,        1,   115200, 0x3bf6e873
 0,         84,         84,        1,   115200, 0xf480dc82
 0,         85,         85,        1,   115200, 0x5fd6e098
 0,         86,         86,        1,   115200, 0xf520de95
@@ -211,7 +211,7 @@
 0,         92,         92,        1,   115200, 0x34cfe1c2
 0,         93,         93,        1,   115200, 0x1d94e1c3
 0,         94,         94,        1,   115200, 0x6d32e147
-0,         95,         95,        1,   115200, 0x7e40ee91
+0,         95,         95,        1,   115200, 0x09fbefd0
 0,         96,         96,        1,   115200, 0xa5f5eb43
 0,         97,         97,        1,   115200, 0x39b9ec3d
 0,         98,         98,        1,   115200, 0x3256ec18

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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