diff --git a/src/main/java/org/jboss/netty/channel/socket/nio/NioDatagramWorker.java b/src/main/java/org/jboss/netty/channel/socket/nio/NioDatagramWorker.java
index 6bfa5a57d7..5c1d3e2234 100644
--- a/src/main/java/org/jboss/netty/channel/socket/nio/NioDatagramWorker.java
+++ b/src/main/java/org/jboss/netty/channel/socket/nio/NioDatagramWorker.java
@@ -645,6 +645,13 @@ class NioDatagramWorker implements Runnable {
         boolean connected = channel.isConnected();
         boolean bound = channel.isBound();
         try {
+            // It is necessary to cancel all keys before closing a socket
+            // because the shutdown flag in the Selector loop is set only when
+            // all keys are cancelled.  Thus, SocketChannel.close() and
+            // SelectionKey.cancel() must be placed in a synchronized block.
+            // Otherwise SocketChannel.register() in RegisterTask can be called
+            // after cancel(), but before close(), resulting in the infinite
+            // Selector loop that refuses to shut down due to the dangling keys.
             synchronized (channel.interestOpsLock) {
                 SelectionKey key = channel.getDatagramChannel().keyFor(selector);
                 if (key != null) {
diff --git a/src/main/java/org/jboss/netty/channel/socket/nio/NioWorker.java b/src/main/java/org/jboss/netty/channel/socket/nio/NioWorker.java
index ca947eb193..94466628ea 100644
--- a/src/main/java/org/jboss/netty/channel/socket/nio/NioWorker.java
+++ b/src/main/java/org/jboss/netty/channel/socket/nio/NioWorker.java
@@ -559,6 +559,13 @@ class NioWorker implements Runnable {
         boolean connected = channel.isConnected();
         boolean bound = channel.isBound();
         try {
+            // It is necessary to cancel all keys before closing a socket
+            // because the shutdown flag in the Selector loop is set only when
+            // all keys are cancelled.  Thus, SocketChannel.close() and
+            // SelectionKey.cancel() must be placed in a synchronized block.
+            // Otherwise SocketChannel.register() in RegisterTask can be called
+            // after cancel(), but before close(), resulting in the infinite
+            // Selector loop that refuses to shut down due to the dangling keys.
             synchronized (channel.interestOpsLock) {
                 SelectionKey key = channel.socket.keyFor(selector);
                 if (key != null) {