diff --git a/web/src/main/java/org/springframework/security/web/server/ObservationWebFilterChainDecorator.java b/web/src/main/java/org/springframework/security/web/server/ObservationWebFilterChainDecorator.java
index b414758e53..cb5c8fe179 100644
--- a/web/src/main/java/org/springframework/security/web/server/ObservationWebFilterChainDecorator.java
+++ b/web/src/main/java/org/springframework/security/web/server/ObservationWebFilterChainDecorator.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,10 +19,6 @@ package org.springframework.security.web.server;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.ListIterator;
-import java.util.concurrent.atomic.AtomicInteger;
-import java.util.concurrent.atomic.AtomicReference;
-import java.util.concurrent.locks.Lock;
-import java.util.concurrent.locks.ReentrantLock;
 
 import io.micrometer.common.KeyValue;
 import io.micrometer.common.KeyValues;
@@ -258,12 +254,13 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 		class SimpleAroundWebFilterObservation implements AroundWebFilterObservation {
 
+			private final Object lock = new Object();
+
 			private final PhasedObservation before;
 
 			private final PhasedObservation after;
 
-			private final AtomicReference<PhasedObservation> currentObservation = new AtomicReference<>(
-					PhasedObservation.NOOP);
+			private volatile PhasedObservation currentObservation = PhasedObservation.NOOP;
 
 			SimpleAroundWebFilterObservation(Observation before, Observation after) {
 				this.before = new PhasedObservation(before);
@@ -272,22 +269,26 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 			@Override
 			public Observation start() {
-				if (this.currentObservation.compareAndSet(PhasedObservation.NOOP, this.before)) {
-					this.before.start();
-					return this.before.observation;
-				}
-				if (this.currentObservation.compareAndSet(this.before, this.after)) {
-					this.before.stop();
-					this.after.start();
-					return this.after.observation;
+				synchronized (this.lock) {
+					if (this.currentObservation == PhasedObservation.NOOP) {
+						this.before.start();
+						this.currentObservation = this.before;
+						return this.currentObservation;
+					}
+					if (this.currentObservation == this.before) {
+						this.before.stop();
+						this.after.start();
+						this.currentObservation = this.after;
+						return this.currentObservation;
+					}
 				}
 				return Observation.NOOP;
 			}
 
 			@Override
 			public Observation error(Throwable ex) {
-				this.currentObservation.get().error(ex);
-				return this.currentObservation.get().observation;
+				this.currentObservation.error(ex);
+				return this.currentObservation.observation;
 			}
 
 			@Override
@@ -303,42 +304,42 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 			@Override
 			public Observation contextualName(String contextualName) {
-				return this.currentObservation.get().observation.contextualName(contextualName);
+				return this.currentObservation.observation.contextualName(contextualName);
 			}
 
 			@Override
 			public Observation parentObservation(Observation parentObservation) {
-				return this.currentObservation.get().observation.parentObservation(parentObservation);
+				return this.currentObservation.observation.parentObservation(parentObservation);
 			}
 
 			@Override
 			public Observation lowCardinalityKeyValue(KeyValue keyValue) {
-				return this.currentObservation.get().observation.lowCardinalityKeyValue(keyValue);
+				return this.currentObservation.observation.lowCardinalityKeyValue(keyValue);
 			}
 
 			@Override
 			public Observation highCardinalityKeyValue(KeyValue keyValue) {
-				return this.currentObservation.get().observation.highCardinalityKeyValue(keyValue);
+				return this.currentObservation.observation.highCardinalityKeyValue(keyValue);
 			}
 
 			@Override
 			public Observation observationConvention(ObservationConvention<?> observationConvention) {
-				return this.currentObservation.get().observation.observationConvention(observationConvention);
+				return this.currentObservation.observation.observationConvention(observationConvention);
 			}
 
 			@Override
 			public Observation event(Event event) {
-				return this.currentObservation.get().observation.event(event);
+				return this.currentObservation.observation.event(event);
 			}
 
 			@Override
 			public Context getContext() {
-				return this.currentObservation.get().observation.getContext();
+				return this.currentObservation.observation.getContext();
 			}
 
 			@Override
 			public Scope openScope() {
-				return this.currentObservation.get().observation.openScope();
+				return this.currentObservation.observation.openScope();
 			}
 
 			@Override
@@ -386,7 +387,7 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 			@Override
 			public String toString() {
-				return this.currentObservation.get().observation.toString();
+				return this.currentObservation.observation.toString();
 			}
 
 		}
@@ -665,9 +666,9 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 		private static final PhasedObservation NOOP = new PhasedObservation(Observation.NOOP);
 
-		private final Lock lock = new ReentrantLock();
+		private final Object lock = new Object();
 
-		private final AtomicInteger phase = new AtomicInteger(0);
+		private volatile int phase = 0;
 
 		private final Observation observation;
 
@@ -717,57 +718,41 @@ public final class ObservationWebFilterChainDecorator implements WebFilterChainP
 
 		@Override
 		public PhasedObservation start() {
-			try {
-				this.lock.lock();
-				if (this.phase.compareAndSet(0, 1)) {
+			synchronized (this.lock) {
+				if (this.phase == 0) {
 					this.observation.start();
+					this.phase = 1;
 				}
 			}
-			finally {
-				this.lock.unlock();
-			}
 			return this;
 		}
 
 		@Override
 		public PhasedObservation error(Throwable ex) {
-			try {
-				this.lock.lock();
-				if (this.phase.get() == 1) {
+			synchronized (this.lock) {
+				if (this.phase == 1) {
 					this.observation.error(ex);
 				}
 			}
-			finally {
-				this.lock.unlock();
-			}
 			return this;
 		}
 
 		@Override
 		public void stop() {
-			try {
-				this.lock.lock();
-				if (this.phase.compareAndSet(1, 2)) {
+			synchronized (this.lock) {
+				if (this.phase == 1) {
 					this.observation.stop();
+					this.phase = 2;
 				}
 			}
-			finally {
-				this.lock.unlock();
-			}
 		}
 
 		void close() {
-			try {
-				this.lock.lock();
-				if (this.phase.compareAndSet(1, 3)) {
+			synchronized (this.lock) {
+				if (this.phase == 1) {
 					this.observation.stop();
 				}
-				else {
-					this.phase.set(3);
-				}
-			}
-			finally {
-				this.lock.unlock();
+				this.phase = 3;
 			}
 		}