[FFmpeg-devel] [PATCH v2] avcodec/ffv1dec: Don't set ThreadFrame properties, fix race

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Each FFV1 slice has its own SAR and picture structure encoded;
when a slice header was parsed, the relevant fields of a ThreadFrame's
AVFrame were directly set based upon the parsed values. This is
a data race in case slice threading is in use because of the concurrent
writes. In case of frame threading, it is also a data race because
the writes happen after ff_thread_finish_setup(), so that the reads
performed by ff_thread_ref_frame() are unsynchronized with the writes
performed when parsing the header.

This commit fixes these issues by not writing to the ThreadFrame at all;
instead the raw data is read into the each SliceContext first; after
decoding the current frame and creating the actual output frame these
values are compared to each other. If they are valid and coincide, the
derived value is written directly to the output frame, not to the
ThreadFrame, thereby avoiding data races; in case they are not valid
or inconsistent the most commonly used valid value is used instead.

This fixes most FFV1 FATE-tests completely when using slice threading;
the exceptions are fate-vsynth3-ffv1, vsynth3-ffv1-v3-yuv420p and
vsynth3-ffv1-v3-yuv422p10. (In these tests the partitioning into slices
does not honour chroma subsampling; as a result, chroma pixels at slice
borders get set by more than one thread without any synchronization.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavcodec/ffv1.h    |   4 ++
 libavcodec/ffv1dec.c | 119 +++++++++++++++++++++++++++++++++++--------
 2 files changed, 103 insertions(+), 20 deletions(-)

diff --git a/libavcodec/ffv1.h b/libavcodec/ffv1.h
index ac80fa85ce..f640d5a710 100644
--- a/libavcodec/ffv1.h
+++ b/libavcodec/ffv1.h
@@ -91,6 +91,8 @@ typedef struct FFV1Context {
     struct FFV1Context *fsrc;
 
     AVFrame *cur;
+    int picture_structure;
+    AVRational sample_aspect_ratio;
     int plane_count;
     int ac;                              ///< 1=range coder <-> 0=golomb rice
     int ac_byte_count;                   ///< number of bytes used for AC coding
@@ -132,6 +134,8 @@ typedef struct FFV1Context {
     int slice_coding_mode;
     int slice_rct_by_coef;
     int slice_rct_ry_coef;
+
+    AVRational slice_sample_aspect_ratios[MAX_SLICES];
 } FFV1Context;
 
 int ff_ffv1_common_init(AVCodecContext *avctx);
diff --git a/libavcodec/ffv1dec.c b/libavcodec/ffv1dec.c
index 201630167d..bda8a5a2f1 100644
--- a/libavcodec/ffv1dec.c
+++ b/libavcodec/ffv1dec.c
@@ -167,7 +167,7 @@ static int decode_slice_header(const FFV1Context *f, FFV1Context *fs)
 {
     RangeCoder *c = &fs->c;
     uint8_t state[CONTEXT_SIZE];
-    unsigned ps, i, context_count;
+    unsigned i, context_count;
     memset(state, 128, sizeof(state));
 
     av_assert0(f->version > 2);
@@ -205,25 +205,17 @@ static int decode_slice_header(const FFV1Context *f, FFV1Context *fs)
         p->context_count = context_count;
     }
 
-    ps = get_symbol(c, state, 0);
-    if (ps == 1) {
-        f->cur->interlaced_frame = 1;
-        f->cur->top_field_first  = 1;
-    } else if (ps == 2) {
-        f->cur->interlaced_frame = 1;
-        f->cur->top_field_first  = 0;
-    } else if (ps == 3) {
-        f->cur->interlaced_frame = 0;
-    }
-    f->cur->sample_aspect_ratio.num = get_symbol(c, state, 0);
-    f->cur->sample_aspect_ratio.den = get_symbol(c, state, 0);
-
-    if (av_image_check_sar(f->width, f->height,
-                           f->cur->sample_aspect_ratio) < 0) {
+    fs->picture_structure       = get_symbol(c, state, 0);
+    fs->sample_aspect_ratio.num = get_symbol(c, state, 0);
+    fs->sample_aspect_ratio.den = get_symbol(c, state, 0);
+    /* Num or den being zero means unknown for FFV1; our unknown is 0 / 1. */
+    if (fs->sample_aspect_ratio.num == 0 || fs->sample_aspect_ratio.den == 0) {
+        fs->sample_aspect_ratio = (AVRational) { 0, 1 };
+    } else if (av_image_check_sar(f->width, f->height,
+                                  fs->sample_aspect_ratio) < 0) {
         av_log(f->avctx, AV_LOG_WARNING, "ignoring invalid SAR: %u/%u\n",
-               f->cur->sample_aspect_ratio.num,
-               f->cur->sample_aspect_ratio.den);
-        f->cur->sample_aspect_ratio = (AVRational){ 0, 1 };
+               fs->sample_aspect_ratio.num, fs->sample_aspect_ratio.den);
+        fs->sample_aspect_ratio = (AVRational) { 0, 0 };
     }
 
     if (fs->version > 3) {
@@ -251,6 +243,9 @@ static int decode_slice(AVCodecContext *c, void *arg)
     AVFrame * const p = f->cur;
     int i, si;
 
+    fs->picture_structure   = 0;
+    fs->sample_aspect_ratio = (AVRational){ 0, 0 };
+
     for( si=0; fs != f->slice_context[si]; si ++)
         ;
 
@@ -831,6 +826,21 @@ static av_cold int decode_init(AVCodecContext *avctx)
     return 0;
 }
 
+static int compare_sar(const void *a, const void *b)
+{
+    const AVRational x = *(const AVRational*)a;
+    const AVRational y = *(const AVRational*)b;
+    int cmp;
+
+    /* 0/0 means invalid and 0/1 means unknown.
+     * Order the ratios so that these values are at the end. */
+    if (x.num == 0 || y.num == 0)
+        return y.num - x.num;
+    cmp = av_cmp_q(x, y);
+    /* For definiteness, order equivalent fractions by "lower terms last". */
+    return cmp ? cmp : y.num - x.num;
+}
+
 static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPacket *avpkt)
 {
     uint8_t *buf        = avpkt->data;
@@ -969,9 +979,78 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
 
     if (f->last_picture.f)
         ff_thread_release_ext_buffer(avctx, &f->last_picture);
-    if ((ret = av_frame_ref(data, f->picture.f)) < 0)
+    p = data;
+    if ((ret = av_frame_ref(p, f->picture.f)) < 0)
         return ret;
 
+    if (f->version > 2) {
+        AVRational sar = f->slice_context[0]->sample_aspect_ratio;
+        int pic_structures[4] = { 0 }, picture_structure;
+        int inconsistent_sar = 0, has_sar = 0;
+
+        for (int i = 0; i < f->slice_count; i++) {
+            const FFV1Context *const fs = f->slice_context[i];
+            int slice_picture_structure = (unsigned)fs->picture_structure > 3 ?
+                                                    0 : fs->picture_structure;
+
+            pic_structures[slice_picture_structure]++;
+
+            if (fs->sample_aspect_ratio.num != sar.num ||
+                fs->sample_aspect_ratio.den != sar.den)
+                inconsistent_sar = 1;
+            sar = fs->sample_aspect_ratio;
+            if (sar.num == 0)
+                inconsistent_sar = 1;
+            else
+                has_sar = 1;
+            f->slice_sample_aspect_ratios[i] = sar;
+        }
+        if (has_sar) {
+            if (inconsistent_sar) {
+                AVRational last_sar = (AVRational){ 0, 0 };
+                qsort(f->slice_sample_aspect_ratios, f->slice_count,
+                      sizeof(f->slice_sample_aspect_ratios[0]), compare_sar);
+
+                for (int i = 0, current_run = 0, longest_run = 0;
+                     i < f->slice_count; i++) {
+                    AVRational cur_sar = f->slice_sample_aspect_ratios[i];
+
+                    if (cur_sar.num == 0)
+                        break;
+                    if (av_cmp_q(cur_sar, last_sar))
+                        current_run = 1;
+                    else
+                        current_run++;
+                    if (current_run > longest_run) {
+                        longest_run = current_run;
+                        sar         = cur_sar;
+                    }
+                    last_sar = cur_sar;
+                }
+                av_log(avctx, AV_LOG_WARNING, "SAR inconsistent, using %u/%u\n",
+                       sar.num, sar.den);
+            }
+            p->sample_aspect_ratio = sar;
+        }
+
+        for (int i = 0, count = 0; i < FF_ARRAY_ELEMS(pic_structures); i++) {
+            if (pic_structures[i] > count) {
+                picture_structure = i;
+                count = pic_structures[i];
+            }
+        }
+        /* Set picture structure based upon the most commonly encountered
+         * picture structure. */
+        if (picture_structure == 1) {
+            p->interlaced_frame = 1;
+            p->top_field_first  = 1;
+        } else if (picture_structure == 2) {
+            p->interlaced_frame = 1;
+            p->top_field_first  = 0;
+        } else if (picture_structure == 3)
+            p->interlaced_frame = 0;
+    }
+
     *got_frame = 1;
 
     return buf_size;
-- 
2.32.0

_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel

To unsubscribe, visit link above, or email
ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".