diff --git a/drama/src/rev-mc.c b/drama/src/rev-mc.c
index 0823a3f..ed00035 100644
--- a/drama/src/rev-mc.c
+++ b/drama/src/rev-mc.c
@@ -362,19 +362,24 @@ find_row_mask(std::vector<set_t> &sets,
                     }
                     diff -= std::min(inner_step, diff);
 
-                    std::lock_guard<std::mutex> _(lock);
                     if (end_pos - diff > g_base_pos) {
+                        std::lock_guard<std::mutex> _(lock);
                         if (found) { break; }
-                        g_base_pos = end_pos - diff;
-                        row_mask   = cur_mask;
+                        if (end_pos - diff > g_base_pos) {
+                            g_base_pos = end_pos - diff;
+                            row_mask   = cur_mask;
+                        }
                     } else if (end_pos >= g_base_pos) {
-                        diff     = end_pos - g_base_pos;
-                        cur_mask = row_mask;
+                        std::lock_guard<std::mutex> _(lock);
+                        if (end_pos >= g_base_pos) {
+                            diff     = end_pos - g_base_pos;
+                            cur_mask = row_mask;
+                        }
                     }
                 }
 
                 for (int i = outer_step; i > 0 && cur_mask < last_mask; --i) {
-                    if (resolve(cur_mask)) {
+                    if (resolve(cur_mask) && !found) {
                         bool expected = false;
                         if (found.compare_exchange_strong(expected, true)) {
                             std::lock_guard<std::mutex> _(lock);