diff --git a/spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java b/spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java
index 36d1c4e7c11..627b3276c6e 100644
--- a/spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java
+++ b/spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java
@@ -176,7 +176,9 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				requiredWriteSubscriber().onNext(item);
 				return;
 			}
-			// FIXME revisit in case of reentrant sync deadlock
+
+			boolean invokeWriteFunction = false;
+
 			synchronized (this) {
 				if (this.state == State.READY_TO_WRITE) {
 					requiredWriteSubscriber().onNext(item);
@@ -184,15 +186,7 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				else if (this.state == State.NEW) {
 					this.item = item;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					Publisher<Void> result;
-					try {
-						result = writeFunction.apply(this);
-					}
-					catch (Throwable ex) {
-						this.writeCompletionBarrier.onError(ex);
-						return;
-					}
-					result.subscribe(this.writeCompletionBarrier);
+					invokeWriteFunction = true;
 				}
 				else {
 					if (this.subscription != null) {
@@ -201,6 +195,20 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 					this.writeCompletionBarrier.onError(new IllegalStateException("Unexpected item."));
 				}
 			}
+
+			if (!invokeWriteFunction) {
+				return;
+			}
+
+			Publisher<Void> result;
+			try {
+				result = writeFunction.apply(this);
+			}
+			catch (Throwable ex) {
+				this.writeCompletionBarrier.onError(ex);
+				return;
+			}
+			result.subscribe(this.writeCompletionBarrier);
 		}
 
 		private Subscriber<? super T> requiredWriteSubscriber() {
@@ -234,6 +242,9 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				requiredWriteSubscriber().onComplete();
 				return;
 			}
+
+			boolean invokeWriteFunction = false;
+
 			synchronized (this) {
 				if (this.state == State.READY_TO_WRITE) {
 					requiredWriteSubscriber().onComplete();
@@ -241,20 +252,26 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				else if (this.state == State.NEW) {
 					this.completed = true;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					Publisher<Void> result;
-					try {
-						result = writeFunction.apply(this);
-					}
-					catch (Throwable ex) {
-						this.writeCompletionBarrier.onError(ex);
-						return;
-					}
-					result.subscribe(this.writeCompletionBarrier);
+					invokeWriteFunction = true;
 				}
 				else {
 					this.completed = true;
 				}
 			}
+
+			if (!invokeWriteFunction) {
+				return;
+			}
+
+			Publisher<Void> result;
+			try {
+				result = writeFunction.apply(this);
+			}
+			catch (Throwable ex) {
+				this.writeCompletionBarrier.onError(ex);
+				return;
+			}
+			result.subscribe(this.writeCompletionBarrier);
 		}
 
 		@Override
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java b/spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java
index 1baf031b20a..8c5f8893b64 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java
@@ -168,7 +168,9 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				requiredWriteSubscriber().onNext(item);
 				return;
 			}
-			// FIXME revisit in case of reentrant sync deadlock
+
+			boolean invokeWriteFunction = false;
+
 			synchronized (this) {
 				if (this.state == State.READY_TO_WRITE) {
 					requiredWriteSubscriber().onNext(item);
@@ -176,15 +178,7 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				else if (this.state == State.NEW) {
 					this.item = item;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					Publisher<Void> result;
-					try {
-						result = writeFunction.apply(this);
-					}
-					catch (Throwable ex) {
-						this.writeCompletionBarrier.onError(ex);
-						return;
-					}
-					result.subscribe(this.writeCompletionBarrier);
+					invokeWriteFunction = true;
 				}
 				else {
 					if (this.subscription != null) {
@@ -193,6 +187,20 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 					this.writeCompletionBarrier.onError(new IllegalStateException("Unexpected item."));
 				}
 			}
+
+			if (!invokeWriteFunction) {
+				return;
+			}
+
+			Publisher<Void> result;
+			try {
+				result = writeFunction.apply(this);
+			}
+			catch (Throwable ex) {
+				this.writeCompletionBarrier.onError(ex);
+				return;
+			}
+			result.subscribe(this.writeCompletionBarrier);
 		}
 
 		private Subscriber<? super T> requiredWriteSubscriber() {
@@ -226,6 +234,9 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				requiredWriteSubscriber().onComplete();
 				return;
 			}
+
+			boolean invokeWriteFunction = false;
+
 			synchronized (this) {
 				if (this.state == State.READY_TO_WRITE) {
 					requiredWriteSubscriber().onComplete();
@@ -233,20 +244,26 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				else if (this.state == State.NEW) {
 					this.completed = true;
 					this.state = State.FIRST_SIGNAL_RECEIVED;
-					Publisher<Void> result;
-					try {
-						result = writeFunction.apply(this);
-					}
-					catch (Throwable ex) {
-						this.writeCompletionBarrier.onError(ex);
-						return;
-					}
-					result.subscribe(this.writeCompletionBarrier);
+					invokeWriteFunction = true;
 				}
 				else {
 					this.completed = true;
 				}
 			}
+
+			if (!invokeWriteFunction) {
+				return;
+			}
+
+			Publisher<Void> result;
+			try {
+				result = writeFunction.apply(this);
+			}
+			catch (Throwable ex) {
+				this.writeCompletionBarrier.onError(ex);
+				return;
+			}
+			result.subscribe(this.writeCompletionBarrier);
 		}
 
 		@Override
diff --git a/spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java b/spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java
index ef730a3f6ed..f2d972ce9e1 100644
--- a/spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java
+++ b/spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java
@@ -21,6 +21,7 @@ import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
@@ -216,6 +217,44 @@ class ChannelSendOperatorTests {
 				.verify(Duration.ofMillis(5000));
 	}
 
+	@Test
+	void writeFunctionIsNotInvokedUnderMonitorLock() {
+		ChannelSendOperator<String> operator = new ChannelSendOperator<>(
+				Mono.just("one"),
+				publisher -> {
+					CountDownLatch acquired = new CountDownLatch(1);
+					Thread t = new Thread(() -> {
+						synchronized (publisher) {
+							acquired.countDown();
+						}
+					});
+					t.start();
+
+					try {
+						if (!acquired.await(1, TimeUnit.SECONDS)) {
+							throw new IllegalStateException("writeFunction appears to be invoked under monitor lock");
+						}
+					}
+					catch (InterruptedException ex) {
+						Thread.currentThread().interrupt();
+						throw new IllegalStateException(ex);
+					}
+					finally {
+						try {
+							t.join(1_000);
+						}
+						catch (InterruptedException ex) {
+							Thread.currentThread().interrupt();
+						}
+					}
+
+					return Mono.empty();
+				});
+
+		StepVerifier.create(operator)
+				.expectComplete()
+				.verify(Duration.ofSeconds(2));
+	}
 
 	private Mono<Void> sendOperator(Publisher<String> source){
 		return new ChannelSendOperator<>(source, writer::send);