[FFmpeg-devel] [PATCH v2] lavc/vvc: Fix race condition for MVs cropped to subpic
Nuo Mi
nuomi2021 at gmail.com
Sat Jan 4 15:51:48 EET 2025
On Fri, Jan 3, 2025 at 2:01 AM Frank Plowman <post at frankplowman.com> wrote:
> Thank you for your review.
>
> On 01/01/2025 04:30, Nuo Mi wrote:
> > 👍
> >
> > On Tue, Dec 31, 2024 at 2:02 AM Frank Plowman <post at frankplowman.com>
> wrote:
> >
> >> 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>
> >> ---
> >> Changes since v1:
> >> * Also clip max_y to the subpic_bottom, in order to prevent adding
> >> unecessary dependencies. max_y is also clipped to the picture bottom
> >> where it wasn't before. This doesn't make any difference to the
> >> function of the code as is, but this was only previously unnecessary
> >> due to a detail of how y-coordinates are mapped to CTU rows. To aid
> >> comprehensibility, the MV is now clipped to both the top and bottom
> edges
> >> of the picture.
> >> * Rename subpic_t to subpic_top to avoid clashing with conventions for
> >> type names.
> >> ---
> >> libavcodec/vvc/ctu.c | 34 +++++++++++++++++++++++-----------
> >> 1 file changed, 23 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/libavcodec/vvc/ctu.c b/libavcodec/vvc/ctu.c
> >> index f80bce637c..b82dece0dd 100644
> >> --- a/libavcodec/vvc/ctu.c
> >> +++ b/libavcodec/vvc/ctu.c
> >> @@ -2393,23 +2393,34 @@ 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 pic_height = pps->height;
> >> + const int subpic_idx = lc->sc->sh.r->curr_subpic_idx;
> >> + const int subpic_top = pps->subpic_y[subpic_idx];
> >> + const int subpic_bottom = subpic_top +
> pps->subpic_height[subpic_idx]
> >> - 1;
> >>
> > -1 is not needed.
>
> Are you sure? Would subpic_top + subpic_height not give the
>
Not entirely sure, but based on the previous code logic, -1 doesn’t seem
necessary.
We can submit another patch if we later decide that -1 isn’t harmful.
y-coordinate of the top edge of the subpic below the current one? And
> av_clip(a, amin, amax) returns aclip in the range amin <= aclip <= amax,
> so this would allow for y-coordinates in the lower subpic and so next
> CTU row.
>
>
> >
> >> + const int subpic_treated_as_pic =
> >> sps->r->sps_subpic_treated_as_pic_flag[subpic_idx];
> >> + const int lower_bound = subpic_treated_as_pic ? subpic_top : 0;
> >> + const int upper_bound = subpic_treated_as_pic ? subpic_bottom :
> >> pic_height;
> >> + return av_clip(y0 + (mv->y >> 4) + height, lower_bound,
> upper_bound);
> >> }
> >>
> > This can be further simplified as
> >
> > static int pred_get_y(const VVCLocalContext *lc, const int y0, const Mv
> *mv,
> > const int height, const VVCFrame *src_frame)
> > {
> > const VVCPPS *pps = src_frame->pps;
> > const int subpic_idx = lc->sc->sh.r->curr_subpic_idx;
> > const int top = pps->subpic_y[subpic_idx];
> > const int bottom = top + pps->subpic_height[subpic_idx];
> >
> > return av_clip(y0 + (mv->y >> 4) + height, top, bottom);
> > }
> >
> > since subpic_y and subpic_height have valid values even if we have no
> > subpic.
> > see static void pps_subpic(VVCPPS *pps, const VVCSPS *sps)
>
> Ah, I didn't realise this. Yes I agree this is cleaner. Would you like
> me to send a v3 or can you just use your local changes?
>
Sure. No need v3, I will make the change.
thank you
>
> >
> >>
> >> -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 +2435,9 @@ 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 +2464,7 @@ 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
> >>
> >>
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