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 > --- > 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.