diff --git a/gnss/2.0/default/GnssMeasurement.cpp b/gnss/2.0/default/GnssMeasurement.cpp
index d778d50..a3ea807 100644
--- a/gnss/2.0/default/GnssMeasurement.cpp
+++ b/gnss/2.0/default/GnssMeasurement.cpp
@@ -49,8 +49,8 @@
 
 Return<void> GnssMeasurement::close() {
     ALOGD("close");
-    std::unique_lock<std::mutex> lock(mMutex);
     stop();
+    std::unique_lock<std::mutex> lock(mMutex);
     sCallback = nullptr;
     return Void();
 }
diff --git a/gnss/2.0/default/GnssMeasurement.h b/gnss/2.0/default/GnssMeasurement.h
index d8ffd59..151456c 100644
--- a/gnss/2.0/default/GnssMeasurement.h
+++ b/gnss/2.0/default/GnssMeasurement.h
@@ -61,10 +61,12 @@
     void stop();
     void reportMeasurement(const GnssData&);
 
+    // Guarded by mMutex
     static sp<IGnssMeasurementCallback> sCallback;
     std::atomic<long> mMinIntervalMillis;
     std::atomic<bool> mIsActive;
     std::thread mThread;
+    // Synchronization lock for sCallback
     mutable std::mutex mMutex;
 };
 
diff --git a/gnss/2.1/default/GnssAntennaInfo.cpp b/gnss/2.1/default/GnssAntennaInfo.cpp
index d32a0af..ed183a9 100644
--- a/gnss/2.1/default/GnssAntennaInfo.cpp
+++ b/gnss/2.1/default/GnssAntennaInfo.cpp
@@ -55,8 +55,8 @@
 
 Return<void> GnssAntennaInfo::close() {
     ALOGD("close");
-    std::unique_lock<std::mutex> lock(mMutex);
     stop();
+    std::unique_lock<std::mutex> lock(mMutex);
     sCallback = nullptr;
     return Void();
 }
diff --git a/gnss/2.1/default/GnssAntennaInfo.h b/gnss/2.1/default/GnssAntennaInfo.h
index f4bfd24..c512e4e 100644
--- a/gnss/2.1/default/GnssAntennaInfo.h
+++ b/gnss/2.1/default/GnssAntennaInfo.h
@@ -47,10 +47,12 @@
     void reportAntennaInfo(
             const hidl_vec<IGnssAntennaInfoCallback::GnssAntennaInfo>& antennaInfo) const;
 
+    // Guarded by mMutex
     static sp<IGnssAntennaInfoCallback> sCallback;
     std::atomic<long> mMinIntervalMillis;
     std::atomic<bool> mIsActive;
     std::thread mThread;
+    // Synchronization lock for sCallback
     mutable std::mutex mMutex;
 };
 
diff --git a/gnss/2.1/default/GnssMeasurement.cpp b/gnss/2.1/default/GnssMeasurement.cpp
index 34e20e5..63bbc0a 100644
--- a/gnss/2.1/default/GnssMeasurement.cpp
+++ b/gnss/2.1/default/GnssMeasurement.cpp
@@ -47,8 +47,8 @@
 
 Return<void> GnssMeasurement::close() {
     ALOGD("close");
-    std::unique_lock<std::mutex> lock(mMutex);
     stop();
+    std::unique_lock<std::mutex> lock(mMutex);
     sCallback_2_1 = nullptr;
     sCallback_2_0 = nullptr;
     return Void();
diff --git a/gnss/2.1/default/GnssMeasurement.h b/gnss/2.1/default/GnssMeasurement.h
index 3ed7bc5..410876d 100644
--- a/gnss/2.1/default/GnssMeasurement.h
+++ b/gnss/2.1/default/GnssMeasurement.h
@@ -66,11 +66,14 @@
     void reportMeasurement(const GnssDataV2_0&);
     void reportMeasurement(const GnssDataV2_1&);
 
+    // Guarded by mMutex
     static sp<V2_1::IGnssMeasurementCallback> sCallback_2_1;
+    // Guarded by mMutex
     static sp<V2_0::IGnssMeasurementCallback> sCallback_2_0;
     std::atomic<long> mMinIntervalMillis;
     std::atomic<bool> mIsActive;
     std::thread mThread;
+    // Synchronization lock for sCallback_2_1 and sCallback_2_0
     mutable std::mutex mMutex;
 };