diff --git a/src/Common/RWLock.cpp b/src/Common/RWLock.cpp
index 5a13bb83f29e..a3ba9523f6ce 100644
--- a/src/Common/RWLock.cpp
+++ b/src/Common/RWLock.cpp
@@ -177,6 +177,7 @@ RWLockImpl::getLock(RWLockImpl::Type type, const String & query_id, const std::c
     /// Lock is free to acquire
     if (rdlock_owner == readers_queue.end() && wrlock_owner == writers_queue.end())
     {
+        /// Set `rdlock_owner` or `wrlock_owner` and make it owner.
         (type == Read ? rdlock_owner : wrlock_owner) = it_group;  /// SM2: nothrow
         grantOwnership(it_group);
     }
@@ -341,13 +342,21 @@ void RWLockImpl::grantOwnershipToAllReaders() noexcept
 {
     if (rdlock_owner != readers_queue.end())
     {
+        size_t num_new_owners = 0;
+
         for (;;)
         {
+            if (!rdlock_owner->ownership)
+                ++num_new_owners;
             grantOwnership(rdlock_owner);
             if (std::next(rdlock_owner) == readers_queue.end())
                 break;
             ++rdlock_owner;
         }
+
+        /// There couldn't be more than one reader group which is not an owner.
+        /// (Because we add a new reader group only if the last reader group is already an owner - see the `can_use_last_group` variable.)
+        chassert(num_new_owners <= 1);
     }
 }
 
diff --git a/src/Common/tests/gtest_rw_lock.cpp b/src/Common/tests/gtest_rw_lock.cpp
index 7de3ced2d0df..16ba01d02c63 100644
--- a/src/Common/tests/gtest_rw_lock.cpp
+++ b/src/Common/tests/gtest_rw_lock.cpp
@@ -37,7 +37,7 @@ namespace
             if (timepoint.length() < 5)
                 timepoint.insert(0, 5 - timepoint.length(), ' ');
             std::lock_guard lock{mutex};
-            std::cout << timepoint << " : " << event << std::endl;
+            //std::cout << timepoint << " : " << event << std::endl;
             events.emplace_back(std::move(event));
         }
 
diff --git a/tests/integration/test_backup_restore_on_cluster/test_concurrency.py b/tests/integration/test_backup_restore_on_cluster/test_concurrency.py
index 65f05d186ceb..c08f3c9c2426 100644
--- a/tests/integration/test_backup_restore_on_cluster/test_concurrency.py
+++ b/tests/integration/test_backup_restore_on_cluster/test_concurrency.py
@@ -237,7 +237,8 @@ def truncate_tables():
         while time.time() < end_time:
             table_name = f"mydb.tbl{randint(1, num_nodes)}"
             node = nodes[randint(0, num_nodes - 1)]
-            # "TRUNCATE TABLE IF EXISTS" still can throw some errors (e.g. "WRITE locking attempt on node0 has timed out!")
+            # "TRUNCATE TABLE IF EXISTS" still can throw some errors
+            # (e.g. "WRITE locking attempt on node0 has timed out!" if the table engine is "Log").
             # So we use query_and_get_answer_with_error() to ignore any errors.
             # `lock_acquire_timeout` is reduced because we don't wait our test to wait too long.
             node.query_and_get_answer_with_error(