diff --git a/drama/src/rev-mc.c b/drama/src/rev-mc.c
index 400c10a..08e3177 100644
--- a/drama/src/rev-mc.c
+++ b/drama/src/rev-mc.c
@@ -360,13 +360,18 @@ find_row_mask(std::vector<set_t> &sets,
                     }
                     step_count -= std::min(step_count, step);
 
-                    std::lock_guard<std::mutex> _(lock);
                     if (my_pos > g_base_pos) {
-                        g_base_pos = my_pos;
-                        row_mask   = cur_mask;
+                        std::lock_guard<std::mutex> _(lock);
+                        if (my_pos > g_base_pos) {
+                            g_base_pos = my_pos;
+                            row_mask   = cur_mask;
+                        }
                     } else if (g_base_pos <= g_cur_pos) {
-                        cur_mask = row_mask;
-                        my_pos   = g_base_pos;
+                        std::lock_guard<std::mutex> _(lock);
+                        if (g_base_pos < g_cur_pos) {
+                            cur_mask = row_mask;
+                            my_pos   = g_base_pos;
+                        }
                     }
                 }