diff --git a/src/runtime/hexagon/hexagon_thread_manager.cc b/src/runtime/hexagon/hexagon_thread_manager.cc
index 3658611cf00d..4f8ddd156b9f 100644
--- a/src/runtime/hexagon/hexagon_thread_manager.cc
+++ b/src/runtime/hexagon/hexagon_thread_manager.cc
@@ -265,9 +265,15 @@ void HexagonThreadManager::WaitOnThreads() {
 }
 
 void HexagonThreadManager::CheckSemaphore(unsigned syncID) {
+  // We want the success case to be fast, so do not lock the mutex
   if (semaphores_.find(syncID) == semaphores_.end()) {
-    semaphores_[syncID] = reinterpret_cast<qurt_sem_t*>(malloc(sizeof(qurt_sem_t)));
-    qurt_sem_init_val(semaphores_[syncID], 0);
+    // If we don't find it, lock the mutex, make sure it hasn't
+    // been added by another thread before creating it.
+    std::lock_guard<std::mutex> lock(semaphores_mutex_);
+    if (semaphores_.find(syncID) == semaphores_.end()) {
+      semaphores_[syncID] = reinterpret_cast<qurt_sem_t*>(malloc(sizeof(qurt_sem_t)));
+      qurt_sem_init_val(semaphores_[syncID], 0);
+    }
   }
 }
 
diff --git a/src/runtime/hexagon/hexagon_thread_manager.h b/src/runtime/hexagon/hexagon_thread_manager.h
index c911d1326a39..9bf6bb6efe64 100644
--- a/src/runtime/hexagon/hexagon_thread_manager.h
+++ b/src/runtime/hexagon/hexagon_thread_manager.h
@@ -213,6 +213,9 @@ class HexagonThreadManager {
   //! \brief Semaphores used by `Signal` and `Wait` mapped by ID.
   std::unordered_map<unsigned, qurt_sem_t*> semaphores_;
 
+  //! \brief Protects updates to semaphores_
+  std::mutex semaphores_mutex_;
+
   //! \brief Start semaphore created at time of construction; signled by `Start`.
   qurt_sem_t start_semaphore_;