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.rheinha...@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 > > [...] >
Decoding this sample uses earlier values of mbskip_table. If I zero mbskip_table_buf in every ff_alloc_picture(), nothing changes due to this patch and (most importantly) the output of decoding does not depend upon the number of threads used (which it currently does -- with and without this patch). I don't know exactly where the bug is or whether there is a cheaper way to mitigate it than just to zero the buffer every time. I guess there are more such bugs affecting damaged samples than we are currently aware of. - Andreas _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
