Lock and unlock both cells when trying to run the gravity M-M task.

src/task.c

@@ -317,11 +317,15 @@ void task_unlock(struct task *t) {
       cell_munlocktree(ci);
       break;
 
-    case task_type_grav_mm:
     case task_type_grav_long_range:
       cell_munlocktree(ci);
       break;
 
+    case task_type_grav_mm:
+      cell_munlocktree(ci);
+      cell_munlocktree(cj);
+      break;
+
     default:
       break;
   }
@@ -444,12 +448,20 @@ int task_lock(struct task *t) {
       break;
 
     case task_type_grav_long_range:
-    case task_type_grav_mm:
       /* Lock the m-poles */
       if (ci->mhold) return 0;
       if (cell_mlocktree(ci) != 0) return 0;
       break;
 
+    case task_type_grav_mm:
+      /* Lock both m-poles */
+      if (ci->mhold || cj->mhold) return 0;
+      if (cell_mlocktree(ci) != 0) return 0;
+      if (cell_mlocktree(cj) != 0) {
+        cell_munlocktree(ci);
+        return 0;
+      }
+
     default:
       break;
   }