[PR] [release/5.1] avcodec/h264_direct: do not read the other field's rows of colocated mb_type (PR #24274)

PR #24274 opened by ffmpeg-devel
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24274
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24274.patch

**Backport:** https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24155




From 9748b8900b3f0b5825368cd3e2d38ae59e17e828 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kacper=20Michaj=C5=82ow?= <[email protected]>
Date: Thu, 30 Jul 2026 17:29:04 +0200
Subject: [PATCH 1/2] avcodec/h264_slice: clear the ER picture when starting a
 second field
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

h264_frame_start() clears h->er.cur_pic, and decode_nal_units() only sets
it again for pictures that are not field coded, right before running error
concealment. A second field goes through neither, so h->er.cur_pic keeps
pointing at whatever frame coded picture the context decoded earlier.

With frame threading the two fields of a pair are decoded by different
contexts, so whether this triggers depends on what the context that picks
up the second field decoded before, which is why the number of bogus
warnings varies between runs.

Signed-off-by: Kacper Michajłow <[email protected]>
(cherry picked from commit f1d318e5eb2eb487d2be4b4b721b93ae76fd5ba7)
---
 libavcodec/h264_slice.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
index 117a780235..af69064e9b 100644
--- a/libavcodec/h264_slice.c
+++ b/libavcodec/h264_slice.c
@@ -1759,6 +1759,10 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
         int field = h->picture_structure == PICT_BOTTOM_FIELD;
         release_unused_pictures(h, 0);
         h->cur_pic_ptr->tf.owner[field] = h->avctx;
+        /* h264_frame_start(), which clears this for every other picture, is
+         * not called for a second field. */
+        if (CONFIG_ERROR_RESILIENCE)
+            ff_h264_set_erpic(&h->er.cur_pic, NULL);
     }
     /* Some macroblocks can be accessed before they're available in case
     * of lost slices, MBAFF or threading. */
-- 
2.52.0


From 7499ccc61aaf933956b5979477b58d28a1354eb3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kacper=20Michaj=C5=82ow?= <[email protected]>
Date: Sat, 15 Aug 2026 00:36:22 +0200
Subject: [PATCH 2/2] avcodec/h264_direct: do not read the other field's rows
 of colocated mb_type
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

To determine whether the colocated picture is field coded, the temporal
and spatial direct setup peek at
sl->ref_list[1][0].parent->mb_type[mb_xy] before mb_xy has been remapped
to the colocated field's parity. That row belongs to the current field's
parity, while await_reference_mb_row() above waited on the parity of the
referenced field, so whenever the two differ the load is unsynchronised
and with frame threading can observe macroblocks of a field that is
still being decoded.

Every macroblock of a field picture has MB_TYPE_INTERLACED set, so
checking field_picture first gives the same result in all correctly
synchronized cases without touching the mb_type array.

This also fixes few TSAN reported warnings.

Fixes: https://trac.ffmpeg.org/ticket/10891
Signed-off-by: Kacper Michajłow <[email protected]>
(cherry picked from commit c3a8a223b29260dc170d41c62f9b3b33f6b97b2d)
---
 libavcodec/h264_direct.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libavcodec/h264_direct.c b/libavcodec/h264_direct.c
index 1bc28333ce..20bac73961 100644
--- a/libavcodec/h264_direct.c
+++ b/libavcodec/h264_direct.c
@@ -292,7 +292,8 @@ static void pred_spatial_direct_motion(const H264Context *const h, H264SliceCont
         return;
     }
 
-    if (IS_INTERLACED(sl->ref_list[1][0].parent->mb_type[mb_xy])) { // AFL/AFR/FR/FL -> AFL/FL
+    if (sl->ref_list[1][0].parent->field_picture ||
+        IS_INTERLACED(sl->ref_list[1][0].parent->mb_type[mb_xy])) { // AFL/AFR/FR/FL -> AFL/FL
         if (!IS_INTERLACED(*mb_type)) {                    //     AFR/FR    -> AFL/FL
             mb_y  = (sl->mb_y & ~1) + sl->col_parity;
             mb_xy = sl->mb_x +
@@ -510,7 +511,8 @@ static void pred_temp_direct_motion(const H264Context *const h, H264SliceContext
     await_reference_mb_row(h, &sl->ref_list[1][0],
                            sl->mb_y + !!IS_INTERLACED(*mb_type));
 
-    if (IS_INTERLACED(sl->ref_list[1][0].parent->mb_type[mb_xy])) { // AFL/AFR/FR/FL -> AFL/FL
+    if (sl->ref_list[1][0].parent->field_picture ||
+        IS_INTERLACED(sl->ref_list[1][0].parent->mb_type[mb_xy])) { // AFL/AFR/FR/FL -> AFL/FL
         if (!IS_INTERLACED(*mb_type)) {                    //     AFR/FR    -> AFL/FL
             mb_y  = (sl->mb_y & ~1) + sl->col_parity;
             mb_xy = sl->mb_x +
-- 
2.52.0

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