Quoting Andreas Rheinhardt (2022-07-01 00:29:40) > The HEVC decoder has both HEVCContext and HEVCLocalContext > structures. The latter is supposed to be the structure > containing the per-slicethread state. > > Yet that is not how it is handled in practice: Each HEVCLocalContext > has a unique HEVCContext allocated for it and each of these > coincides with the main HEVCContext except in exactly one field: > The corresponding HEVCLocalContext. > This makes it possible to pass the HEVCContext everywhere where > logically a HEVCLocalContext should be used. > > This led to confusion in the first version of what eventually became > commit c8bc0f66a875bc3708d8dc11b757f2198606ffd7: > Before said commit, the initialization of the Rice parameter derivation > state was incorrect; the fix for single-threaded as well as > frame-threaded decoding was to add backup stats to HEVCContext > that are used when the cabac state is updated*, see > https://ffmpeg.org/pipermail/ffmpeg-devel/2020-August/268861.html > Yet due to what has been said above, this does not work for > slice-threading, because the each HEVCLocalContext has its own > HEVCContext, so the Rice parameter state would not be transferred > between threads. > > This is fixed in c8bc0f66a875bc3708d8dc11b757f2198606ffd7 > by a hack: It rederives what the previous thread was and accesses > the corresponding HEVCContext. > > Fix this by treating the Rice parameter state the same way > the ordinary CABAC parameters are shared between threads: > Make them part of the same struct that is shared between > slice threads. This does not cause races, because > the parts of the code that access these Rice parameters > are a subset of the parts of code that access the CABAC parameters. > > *: And if the persistent_rice_adaptation_enabled_flag is set. > > Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@outlook.com> > --- > libavcodec/hevc_cabac.c | 17 ++++++++--------- > libavcodec/hevcdec.c | 10 +++++----- > libavcodec/hevcdec.h | 10 +++++++--- > 3 files changed, 20 insertions(+), 17 deletions(-) > > diff --git a/libavcodec/hevc_cabac.c b/libavcodec/hevc_cabac.c > index a194f8a02a..985c97ef2a 100644 > --- a/libavcodec/hevc_cabac.c > +++ b/libavcodec/hevc_cabac.c > @@ -453,19 +453,18 @@ void ff_hevc_save_states(HEVCContext *s, int > ctb_addr_ts) > (ctb_addr_ts % s->ps.sps->ctb_width == 2 || > (s->ps.sps->ctb_width == 2 && > ctb_addr_ts % s->ps.sps->ctb_width == 0))) { > - memcpy(s->cabac_state, s->HEVClc->cabac_state, HEVC_CONTEXTS); > + memcpy(s->cabac->state, s->HEVClc->cabac_state, HEVC_CONTEXTS);
So if I'm reading this right, this copies the per-slice-context state into the decoder-global state. And it's done from slice threads with no locks. So how is this not racy? -- Anton Khirnov _______________________________________________ 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".
