diff --git a/clang/tools/clang-scan-deps/ClangScanDeps.cpp b/clang/tools/clang-scan-deps/ClangScanDeps.cpp
--- a/clang/tools/clang-scan-deps/ClangScanDeps.cpp
+++ b/clang/tools/clang-scan-deps/ClangScanDeps.cpp
@@ -260,6 +260,8 @@
     mergeDeps(std::move(TUDeps.ModuleGraph), InputIndex);
     ID.DriverCommandLine = std::move(TUDeps.DriverCommandLine);
     ID.Commands = std::move(TUDeps.Commands);
+
+    std::unique_lock<std::mutex> ul(Lock);
     Inputs.push_back(std::move(ID));
   }