diff --git a/lldb/include/lldb/Target/ThreadPlanStack.h b/lldb/include/lldb/Target/ThreadPlanStack.h
--- a/lldb/include/lldb/Target/ThreadPlanStack.h
+++ b/lldb/include/lldb/Target/ThreadPlanStack.h
@@ -123,11 +123,13 @@
               bool check_for_new = true);
 
   void AddThread(Thread &thread) {
+    std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
     lldb::tid_t tid = thread.GetID();
     m_plans_list.emplace(tid, thread);
   }
 
   bool RemoveTID(lldb::tid_t tid) {
+    std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
     auto result = m_plans_list.find(tid);
     if (result == m_plans_list.end())
       return false;
@@ -137,6 +139,7 @@
   }
 
   ThreadPlanStack *Find(lldb::tid_t tid) {
+    std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
     auto result = m_plans_list.find(tid);
     if (result == m_plans_list.end())
       return nullptr;
@@ -154,6 +157,7 @@
   }
 
   void Clear() {
+    std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
     for (auto &plan : m_plans_list)
       plan.second.ThreadDestroyed(nullptr);
     m_plans_list.clear();
@@ -172,8 +176,10 @@
 
 private:
   Process &m_process;
+  mutable std::recursive_mutex m_stack_map_mutex;
   using PlansList = std::unordered_map<lldb::tid_t, ThreadPlanStack>;
   PlansList m_plans_list;
+  
 };
 
 } // namespace lldb_private
diff --git a/lldb/source/Target/ThreadPlanStack.cpp b/lldb/source/Target/ThreadPlanStack.cpp
--- a/lldb/source/Target/ThreadPlanStack.cpp
+++ b/lldb/source/Target/ThreadPlanStack.cpp
@@ -400,6 +400,7 @@
                                 bool delete_missing,
                                 bool check_for_new) {
 
+  std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
   // Now find all the new threads and add them to the map:
   if (check_for_new) {
     for (auto thread : current_threads.Threads()) {
@@ -434,6 +435,7 @@
                                    lldb::DescriptionLevel desc_level,
                                    bool internal, bool condense_if_trivial,
                                    bool skip_unreported) {
+  std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
   for (auto &elem : m_plans_list) {
     lldb::tid_t tid = elem.first;
     uint32_t index_id = 0;
@@ -470,6 +472,7 @@
                                          bool internal,
                                          bool condense_if_trivial,
                                          bool skip_unreported) {
+  std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
   uint32_t index_id = 0;
   ThreadSP thread_sp = m_process.GetThreadList().FindThreadByID(tid);
 
@@ -509,6 +512,7 @@
 
 bool ThreadPlanStackMap::PrunePlansForTID(lldb::tid_t tid) {
   // We only remove the plans for unreported TID's.
+  std::lock_guard<std::recursive_mutex> guard(m_stack_map_mutex);
   ThreadSP thread_sp = m_process.GetThreadList().FindThreadByID(tid);
   if (thread_sp)
     return false;