From 41af27c582ec21e65ff3f835754c7b0bcf6d3be7 Mon Sep 17 00:00:00 2001
From: Brecht Van Lommel <brecht@blender.org>
Date: Mon, 21 Jun 2021 18:44:40 +0200
Subject: [PATCH] Fix deadlocks in mesh modifier evaluation and particles

The recent task isolation changes missed two mutex locks that also need task
isolation.

Ref D11603, T89194
---
 source/blender/blenkernel/intern/DerivedMesh.cc    | 13 +++++++++----
 source/blender/blenkernel/intern/particle_system.c | 11 ++++++++++-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/source/blender/blenkernel/intern/DerivedMesh.cc b/source/blender/blenkernel/intern/DerivedMesh.cc
index 4cd611d2fa8..3150e096a73 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.cc
+++ b/source/blender/blenkernel/intern/DerivedMesh.cc
@@ -42,6 +42,7 @@
 #include "BLI_linklist.h"
 #include "BLI_math.h"
 #include "BLI_task.h"
+#include "BLI_task.hh"
 #include "BLI_utildefines.h"
 #include "BLI_vector.hh"
 
@@ -1463,10 +1464,14 @@ static void mesh_calc_modifiers(struct Depsgraph *depsgraph,
       BLI_assert(runtime->eval_mutex != nullptr);
       BLI_mutex_lock((ThreadMutex *)runtime->eval_mutex);
       if (runtime->mesh_eval == nullptr) {
-        mesh_final = BKE_mesh_copy_for_eval(mesh_input, true);
-        mesh_calc_modifier_final_normals(mesh_input, &final_datamask, sculpt_dyntopo, mesh_final);
-        mesh_calc_finalize(mesh_input, mesh_final);
-        runtime->mesh_eval = mesh_final;
+        /* Isolate since computing normals is multithreaded and we are holding a lock. */
+        blender::threading::isolate_task([&] {
+          mesh_final = BKE_mesh_copy_for_eval(mesh_input, true);
+          mesh_calc_modifier_final_normals(
+              mesh_input, &final_datamask, sculpt_dyntopo, mesh_final);
+          mesh_calc_finalize(mesh_input, mesh_final);
+          runtime->mesh_eval = mesh_final;
+        });
       }
       BLI_mutex_unlock((ThreadMutex *)runtime->eval_mutex);
     }
diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index 149e345e501..2dc752d57e1 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -1320,6 +1320,14 @@ void psys_get_pointcache_start_end(Scene *scene, ParticleSystem *psys, int *sfra
   *efra = min_ii((int)(part->end + part->lifetime + 1.0f), max_ii(scene->r.pefra, scene->r.efra));
 }
 
+/* BVH tree balancing inside a mutex lock must be run in isolation. Balancing
+ * is multithreaded, and we do not want the current thread to start another task
+ * that may involve acquiring the same mutex lock that it is waiting for. */
+static void bvhtree_balance_isolated(void *userdata)
+{
+  BLI_bvhtree_balance((BVHTree *)userdata);
+}
+
 /************************************************/
 /*          Effectors                           */
 /************************************************/
@@ -1356,7 +1364,8 @@ static void psys_update_particle_bvhtree(ParticleSystem *psys, float cfra)
           }
         }
       }
-      BLI_bvhtree_balance(psys->bvhtree);
+
+      BLI_task_isolate(bvhtree_balance_isolated, psys->bvhtree);
 
       psys->bvhtree_frame = cfra;