diff --git a/src/clang_index.cc b/src/clang_index.cc
index 291b7cd1..41c7b65b 100644
--- a/src/clang_index.cc
+++ b/src/clang_index.cc
@@ -1,13 +1,22 @@
 #include "clang_index.h"
 
+#include <mutex>
+
 ClangIndex::ClangIndex() : ClangIndex(1, 0) {}
 
 ClangIndex::ClangIndex(int exclude_declarations_from_pch,
                        int display_diagnostics) {
+  // llvm::InitializeAllTargets (and possibly others) called by clang_createIndex
+  // transtively modifies/reads lib/Support/TargetRegistry.cpp FirstTarget.
+  // There will be a race condition if two threads call clang_createIndex
+  // concurrently.
+  static std::mutex mutex_;
+  std::lock_guard<std::mutex> lock(mutex_);
+
   cx_index =
       clang_createIndex(exclude_declarations_from_pch, display_diagnostics);
 }
 
 ClangIndex::~ClangIndex() {
   clang_disposeIndex(cx_index);
-}
\ No newline at end of file
+}
diff --git a/src/clang_translation_unit.cc b/src/clang_translation_unit.cc
index 4eb90582..ff49a8e4 100644
--- a/src/clang_translation_unit.cc
+++ b/src/clang_translation_unit.cc
@@ -14,16 +14,6 @@
 
 namespace {
 
-// We need to serialize requests to clang_parseTranslationUnit2FullArgv and
-// clang_reparseTranslationUnit. See
-// https://github.com/jacobdufault/cquery/issues/43#issuecomment-347614504.
-//
-// NOTE: This is disabled because it effectively serializes indexing, as a huge
-// chunk of indexing time is spent inside of these functions.
-//
-// std::mutex g_parse_translation_unit_mutex;
-// std::mutex g_reparse_translation_unit_mutex;
-
 void EmitDiagnostics(std::string path,
                      std::vector<const char*> args,
                      CXTranslationUnit tu) {
@@ -95,7 +85,6 @@ std::unique_ptr<ClangTranslationUnit> ClangTranslationUnit::Create(
   CXTranslationUnit cx_tu;
   CXErrorCode error_code;
   {
-    // std::lock_guard<std::mutex> lock(g_parse_translation_unit_mutex);
     error_code = clang_parseTranslationUnit2FullArgv(
         index->cx_index, filepath.c_str(), args.data(), (int)args.size(),
         unsaved_files.data(), (unsigned)unsaved_files.size(), flags, &cx_tu);
@@ -134,7 +123,6 @@ std::unique_ptr<ClangTranslationUnit> ClangTranslationUnit::Reparse(
     std::vector<CXUnsavedFile>& unsaved) {
   int error_code;
   {
-    // std::lock_guard<std::mutex> lock(g_reparse_translation_unit_mutex);
     error_code = clang_reparseTranslationUnit(
         tu->cx_tu, (unsigned)unsaved.size(), unsaved.data(),
         clang_defaultReparseOptions(tu->cx_tu));