diff --git a/lldb/source/Symbol/TypeSystem.cpp b/lldb/source/Symbol/TypeSystem.cpp
--- a/lldb/source/Symbol/TypeSystem.cpp
+++ b/lldb/source/Symbol/TypeSystem.cpp
@@ -217,11 +217,21 @@
 
 void TypeSystemMap::ForEach(
     std::function<bool(lldb::TypeSystemSP)> const &callback) {
-  std::lock_guard<std::mutex> guard(m_mutex);
+
+  // The callback may call into this function again causing
+  // us to lock m_mutex twice if we held it across the callback.
+  // Since we just care about guarding access to 'm_map', make
+  // a local copy and iterate over that instead.
+  collection map_snapshot;
+  {
+      std::lock_guard<std::mutex> guard(m_mutex);
+      map_snapshot = m_map;
+  }
+
   // Use a std::set so we only call the callback once for each unique
   // TypeSystem instance.
   llvm::DenseSet<TypeSystem *> visited;
-  for (auto &pair : m_map) {
+  for (auto &pair : map_snapshot) {
     TypeSystem *type_system = pair.second.get();
     if (!type_system || visited.count(type_system))
       continue;