[FFmpeg-devel] [PATCH] lavc/vvc: Fix race condition for MVs cropped to subpic

[Frank Plowman]

When the current subpicture has sps_subpic_treated_as_pic_flag equal to
1, motion vectors are cropped such that they cannot point to other
subpictures.  This was accounted for in the prediction logic, but not
in pred_get_y, which is used by the scheduling logic to determine which
parts of the reference pictures must have been reconstructed before
inter prediction of a subsequent frame may begin.  Consequently, where a
motion vector pointed to a location significantly above the current
subpicture, there was the possibility of a race condition.  Patch fixes
this by cropping the motion vector to the current subpicture in
pred_get_y.

Signed-off-by: Frank Plowman <post at frankplowman.com>
---
You can reproduce this issue with the bitstream available here:
https://chromium.googlesource.com/chromium/src/+/lkgr/media/test/data/vvc_frames_with_ltr.vvc?format=TEXT
Note this link downloads the file in base 64 format rather than binary.
You can use base64 -d <BASE 64 FILE> > <BINARY FILE> to convert it to
binary.  The issue does not reproduce very reliably with a normal number
of threads.  For better reproducibility, run ffmpeg with 100+ threads.
---
 libavcodec/vvc/ctu.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/libavcodec/vvc/ctu.c b/libavcodec/vvc/ctu.c
index f80bce637c..6857c79f2b 100644
--- a/libavcodec/vvc/ctu.c
+++ b/libavcodec/vvc/ctu.c
@@ -2393,23 +2393,30 @@ static int has_inter_luma(const CodingUnit *cu)
     return cu->pred_mode != MODE_INTRA && cu->pred_mode != MODE_PLT && cu->tree_type != DUAL_TREE_CHROMA;
 }
 
-static int pred_get_y(const int y0, const Mv *mv, const int height)
+static int pred_get_y(const VVCLocalContext *lc, const int y0, const Mv *mv, const int height, const VVCFrame *src_frame)
 {
-    return FFMAX(0, y0 + (mv->y >> 4) + height);
+    const VVCSPS *sps = src_frame->sps;
+    const VVCPPS *pps = src_frame->pps;
+    const int subpic_idx = lc->sc->sh.r->curr_subpic_idx;
+    const int subpic_t = pps->subpic_y[subpic_idx];
+    const int subpic_treated_as_pic = sps->r->sps_subpic_treated_as_pic_flag[subpic_idx];
+    return FFMAX(subpic_treated_as_pic ? subpic_t : 0, y0 + (mv->y >> 4) + height);
 }
 
-static void cu_get_max_y(const CodingUnit *cu, int max_y[2][VVC_MAX_REF_ENTRIES], const VVCFrameContext *fc)
+static void cu_get_max_y(const VVCLocalContext *lc, const CodingUnit *cu, int max_y[2][VVC_MAX_REF_ENTRIES])
 {
+    const VVCFrameContext *fc   = lc->fc;
     const PredictionUnit *pu    = &cu->pu;
 
     if (pu->merge_gpm_flag) {
         for (int i = 0; i < FF_ARRAY_ELEMS(pu->gpm_mv); i++) {
-            const MvField *mvf  = pu->gpm_mv + i;
-            const int lx        = mvf->pred_flag - PF_L0;
-            const int idx       = mvf->ref_idx[lx];
-            const int y         = pred_get_y(cu->y0, mvf->mv + lx, cu->cb_height);
+            const MvField *mvf    = pu->gpm_mv + i;
+            const int lx          = mvf->pred_flag - PF_L0;
+            const int idx         = mvf->ref_idx[lx];
+            const VVCRefPic *refp = lc->sc->rpl[lx].refs + idx;
+            const int y           = pred_get_y(lc, cu->y0, mvf->mv + lx, cu->cb_height, refp->ref);
 
-            max_y[lx][idx]      = FFMAX(max_y[lx][idx], y);
+            max_y[lx][idx]        = FFMAX(max_y[lx][idx], y);
         }
     } else {
         const MotionInfo *mi    = &pu->mi;
@@ -2424,8 +2431,9 @@ static void cu_get_max_y(const CodingUnit *cu, int max_y[2][VVC_MAX_REF_ENTRIES]
                 for (int lx = 0; lx < 2; lx++) {
                     const PredFlag mask = 1 << lx;
                     if (mvf->pred_flag & mask) {
-                        const int idx   = mvf->ref_idx[lx];
-                        const int y     = pred_get_y(y0, mvf->mv + lx, sbh);
+                        const int idx         = mvf->ref_idx[lx];
+                        const VVCRefPic *refp = lc->sc->rpl[lx].refs + idx;
+                        int y                 = pred_get_y(lc, y0, mvf->mv + lx, sbh, refp->ref);
 
                         max_y[lx][idx]  = FFMAX(max_y[lx][idx], y + max_dmvr_off);
                     }
@@ -2452,7 +2460,7 @@ static void ctu_get_pred(VVCLocalContext *lc, const int rs)
 
     while (cu) {
         if (has_inter_luma(cu)) {
-            cu_get_max_y(cu, ctu->max_y, fc);
+            cu_get_max_y(lc, cu, ctu->max_y);
             ctu->has_dmvr |= cu->pu.dmvr_flag;
         }
         cu = cu->next;
-- 
2.47.0