[FFmpeg-devel] [PATCH v3] avcodec/ffv1dec: Don't set ThreadFrame properties, fix race
Andreas Rheinhardt
andreas.rheinhardt at outlook.com
Fri Mar 4 05:40:54 EET 2022
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 at outlook.com>
---
libavcodec/ffv1.h | 4 ++
libavcodec/ffv1dec.c | 130 ++++++++++++++++++++++++++++++++++++-------
2 files changed, 114 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..8a8ab90b2b 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 +826,28 @@ static av_cold int decode_init(AVCodecContext *avctx)
return 0;
}
+/* Macro to simplify comparisons of the rational values we deal with here.
+ * get_symbol() ensures that these fit into 32bits, so that one can just
+ * compare them in 64bits; they are also actually unsigned, so cast to that.
+ * Notice that av_image_check_sar() checks for these values to be
+ * positive integers. */
+#define CMP_Q(x, y) (FFDIFFSIGN((unsigned)(x).num * (uint64_t)(unsigned)(y).den,\
+ (unsigned)(y).num * (uint64_t)(unsigned)(x).den))
+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 fractions so that the former are at the end. */
+ if (x.den == 0 || y.den == 0)
+ return y.den - x.den;
+ cmp = 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 +986,82 @@ 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.den == 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.den == 0)
+ break;
+ if (CMP_Q(cur_sar, last_sar))
+ current_run = 1;
+ else
+ current_run++;
+ /* Using >= here has the effect of preferring
+ * bigger ratios if the runs are identical;
+ * this ensures that a known SAR is preferred over
+ * an unknown SAR in case there are equally many. */
+ 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
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