diff --git a/lldb/include/lldb/Target/ThreadCollection.h b/lldb/include/lldb/Target/ThreadCollection.h
index 5b0267d76d42..e3965b57e7a6 100644
--- a/lldb/include/lldb/Target/ThreadCollection.h
+++ b/lldb/include/lldb/Target/ThreadCollection.h
@@ -48,11 +48,11 @@ public:
     return ThreadIterable(m_threads, GetMutex());
   }
 
-  virtual std::recursive_mutex &GetMutex() { return m_mutex; }
+  virtual std::recursive_mutex &GetMutex() const { return m_mutex; }
 
 protected:
   collection m_threads;
-  std::recursive_mutex m_mutex;
+  mutable std::recursive_mutex m_mutex;
 };
 
 } // namespace lldb_private
diff --git a/lldb/include/lldb/Target/ThreadList.h b/lldb/include/lldb/Target/ThreadList.h
index d0779d810163..9e3c940c3c2d 100644
--- a/lldb/include/lldb/Target/ThreadList.h
+++ b/lldb/include/lldb/Target/ThreadList.h
@@ -135,7 +135,7 @@ public:
 
   void SetStopID(uint32_t stop_id);
 
-  std::recursive_mutex &GetMutex() override;
+  std::recursive_mutex &GetMutex() const override;
 
   void Update(ThreadList &rhs);
 
diff --git a/lldb/source/Target/ThreadList.cpp b/lldb/source/Target/ThreadList.cpp
index 15388a5afa1e..1e474518aace 100644
--- a/lldb/source/Target/ThreadList.cpp
+++ b/lldb/source/Target/ThreadList.cpp
@@ -44,6 +44,7 @@ const ThreadList &ThreadList::operator=(const ThreadList &rhs) {
     // Lock both mutexes to make sure neither side changes anyone on us
     // while the assignment occurs
     std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    std::lock_guard<std::recursive_mutex> rhs_guard(rhs.GetMutex());
 
     m_process = rhs.m_process;
     m_stop_id = rhs.m_stop_id;
@@ -749,7 +750,7 @@ void ThreadList::Flush() {
     (*pos)->Flush();
 }
 
-std::recursive_mutex &ThreadList::GetMutex() {
+std::recursive_mutex &ThreadList::GetMutex() const {
   return m_process->m_thread_mutex;
 }