diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
index 71548b59cc308..7a05a3d461370 100644
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -501,6 +501,8 @@ class PreambleThread {
       }
 
       {
+        // Add a lock guard to protect the critical section.
+        std::lock_guard<std::mutex> Lock(Mutex);
         WithContext Guard(std::move(CurrentReq->Ctx));
         // Note that we don't make use of the ContextProvider here.
         // Preamble tasks are always scheduled by ASTWorker tasks, and we
@@ -1329,6 +1331,8 @@ void ASTWorker::startTask(llvm::StringRef Name,
                           std::optional<UpdateType> Update,
                           TUScheduler::ASTActionInvalidation Invalidation) {
   if (RunSync) {
+    // Add a lock guard to protect the critical section.
+    std::lock_guard<std::mutex> Lock(Mutex);
     assert(!Done && "running a task after stop()");
     runTask(Name, Task);
     return;
diff --git a/clang-tools-extra/clangd/index/BackgroundRebuild.cpp b/clang-tools-extra/clangd/index/BackgroundRebuild.cpp
index 79383be012f83..b8677a32741b2 100644
--- a/clang-tools-extra/clangd/index/BackgroundRebuild.cpp
+++ b/clang-tools-extra/clangd/index/BackgroundRebuild.cpp
@@ -34,6 +34,8 @@ bool BackgroundIndexRebuilder::enoughTUsToRebuild() const {
 
 void BackgroundIndexRebuilder::indexedTU() {
   maybeRebuild("after indexing enough files", [this] {
+    // Add a lock guard to protect the critical section
+    std::lock_guard<std::mutex> Lock(Mu);
     ++IndexedTUs;
     if (Loading)
       return false;                      // rebuild once loading finishes
@@ -64,6 +66,8 @@ void BackgroundIndexRebuilder::loadedShard(size_t ShardCount) {
 }
 void BackgroundIndexRebuilder::doneLoading() {
   maybeRebuild("after loading index from disk", [this] {
+    // Add a lock guard to protect the critical section.
+    std::lock_guard<std::mutex> Lock(Mu);
     assert(Loading);
     --Loading;
     if (Loading)    // was loading multiple batches concurrently