Fix possible concurency issue in mesh normals computation.

Failure in own code from last December, thanks @sergey for finding it.

To be backported to 2.79a.

1 files changed, 5 insertions, 11 deletions

diff --git a/source/blender/blenkernel/intern/mesh_evaluate.c b/source/blender/blenkernel/intern/mesh_evaluate.c
index f95fad38c7c..425f28eb123 100644
--- a/source/blender/blenkernel/intern/mesh_evaluate.c
+++ b/source/blender/blenkernel/intern/mesh_evaluate.c
@@ -253,16 +253,6 @@ static void mesh_calc_normals_poly_prepare_cb(
 	}
 }
 
-static void mesh_calc_normals_poly_accum_cb(
-        void *__restrict userdata,
-        const int lidx,
-        const ParallelRangeTLS *__restrict UNUSED(tls))
-{
-	MeshCalcNormalsData *data = userdata;
-
-	add_v3_v3(data->vnors[data->mloop[lidx].v], data->lnors_weighted[lidx]);
-}
-
 static void mesh_calc_normals_poly_finalize_cb(
         void *__restrict userdata,
         const int vidx,
@@ -327,7 +317,11 @@ void BKE_mesh_calc_normals_poly(
 	BLI_task_parallel_range(0, numPolys, &data, mesh_calc_normals_poly_prepare_cb, &settings);
 
 	/* Actually accumulate weighted loop normals into vertex ones. */
-	BLI_task_parallel_range(0, numLoops, &data, mesh_calc_normals_poly_accum_cb, &settings);
+	/* Unfortunately, not possible to thread that (not in a reasonable, totally lock- and barrier-free fashion),
+	 * since several loops will point to the same vertex... */
+	for (int lidx = 0; lidx < numLoops; lidx++) {
+		add_v3_v3(vnors[mloop[lidx].v], data.lnors_weighted[lidx]);
+	}
 
 	/* Normalize and validate computed vertex normals. */
 	BLI_task_parallel_range(0, numVerts, &data, mesh_calc_normals_poly_finalize_cb, &settings);