diff --git a/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java b/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
index 151f4d92685..4fd2d221127 100644
--- a/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
@@ -19,6 +19,8 @@
 
 package org.elasticsearch.cluster;
 
+import org.apache.logging.log4j.message.ParameterizedMessage;
+import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ElasticsearchTimeoutException;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
@@ -36,6 +38,7 @@ import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.transport.BoundTransportAddress;
 import org.elasticsearch.common.transport.TransportAddress;
 import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.junit.annotations.TestLogging;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.ConnectTransportException;
@@ -57,8 +60,10 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.BrokenBarrierException;
 import java.util.concurrent.CyclicBarrier;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Predicate;
 
@@ -215,6 +220,7 @@ public class NodeConnectionsServiceTests extends ESTestCase {
         assertConnectedExactlyToNodes(targetNodes);
     }
 
+    @TestLogging("org.elasticsearch.cluster.NodeConnectionsService:TRACE") // for https://github.com/elastic/elasticsearch/issues/40170
     public void testOnlyBlocksOnConnectionsToNewNodes() throws Exception {
         final NodeConnectionsService service = new NodeConnectionsService(Settings.EMPTY, threadPool, transportService);
 
@@ -227,7 +233,7 @@ public class NodeConnectionsServiceTests extends ESTestCase {
         assertConnectedExactlyToNodes(nodes0);
 
         // connection attempts to node0 block indefinitely
-        final CyclicBarrier connectionBarrier = new CyclicBarrier(2);
+        final CyclicBarrier connectionBarrier = new VerboseCyclicBarrier(2);
         try {
             nodeConnectionBlocks.put(node0, connectionBarrier::await);
             transportService.disconnectFromNode(node0);
@@ -297,6 +303,33 @@ public class NodeConnectionsServiceTests extends ESTestCase {
         }
     }
 
+    // tracing barrier usage for https://github.com/elastic/elasticsearch/issues/40170
+    private class VerboseCyclicBarrier extends CyclicBarrier {
+        VerboseCyclicBarrier(int parties) {
+            super(parties);
+        }
+
+        @Override
+        public int await() throws InterruptedException, BrokenBarrierException {
+            final String waitUUID = UUIDs.randomBase64UUID(random());
+            logger.info(new ParameterizedMessage("--> wait[{}] starting", waitUUID),
+                new ElasticsearchException("stack trace for CyclicBarrier#await()"));
+            final int result = super.await();
+            logger.info("--> wait[{}] returning [{}]", waitUUID, result);
+            return result;
+        }
+
+        @Override
+        public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException {
+            final String waitUUID = UUIDs.randomBase64UUID(random());
+            logger.info(new ParameterizedMessage("--> wait[{}] starting", waitUUID),
+                new ElasticsearchException("stack trace for CyclicBarrier#await(" + timeout + ", " + unit + ')'));
+            final int result = super.await(timeout, unit);
+            logger.info("--> wait[{}] returning [{}]", waitUUID, result);
+            return result;
+        }
+    }
+
     private void runTasksUntil(DeterministicTaskQueue deterministicTaskQueue, long endTimeMillis) {
         while (deterministicTaskQueue.getCurrentTimeMillis() < endTimeMillis) {
             if (deterministicTaskQueue.hasRunnableTasks() && randomBoolean()) {