diff --git a/drama/src/rev-mc.c b/drama/src/rev-mc.c
index cdc1873..8ff89fb 100644
--- a/drama/src/rev-mc.c
+++ b/drama/src/rev-mc.c
@@ -372,7 +372,8 @@ find_row_mask(std::vector<set_t> &sets,
 
                 for (int i = 0; i < step && cur_mask < last_mask && !found.load(std::memory_order_relaxed); ++i) {
                     if (resolve(cur_mask)) {
-                        if (!found.exchange(true)) {
+                        bool expected = false;
+                        if (found.compare_exchange_strong(expected, true)) {
                             std::lock_guard<std::mutex> _(lock);
                             row_mask = cur_mask;
                             break;