diff --git a/compiler-rt/lib/hwasan/hwasan.cpp b/compiler-rt/lib/hwasan/hwasan.cpp
--- a/compiler-rt/lib/hwasan/hwasan.cpp
+++ b/compiler-rt/lib/hwasan/hwasan.cpp
@@ -345,7 +345,7 @@
 
   // Needs to be called here because flags()->random_tags might not have been
   // initialized when InitInstrumentation() was called.
-  GetCurrentThread()->InitRandomState();
+  GetCurrentThread()->EnsureRandomStateInited();
 
   SetPrintfAndReportCallback(AppendToErrorMessageBuffer);
   // This may call libc -> needs initialized shadow.
diff --git a/compiler-rt/lib/hwasan/hwasan_fuchsia.cpp b/compiler-rt/lib/hwasan/hwasan_fuchsia.cpp
--- a/compiler-rt/lib/hwasan/hwasan_fuchsia.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_fuchsia.cpp
@@ -130,7 +130,7 @@
 static void ThreadStartHook(void *hook, thrd_t self) {
   Thread *thread = static_cast<Thread *>(hook);
   FinishThreadInitialization(thread);
-  thread->InitRandomState();
+  thread->EnsureRandomStateInited();
 }
 
 // This is the function that sets up the stack ring buffer and enables us to use
diff --git a/compiler-rt/lib/hwasan/hwasan_linux.cpp b/compiler-rt/lib/hwasan/hwasan_linux.cpp
--- a/compiler-rt/lib/hwasan/hwasan_linux.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_linux.cpp
@@ -250,7 +250,7 @@
 // ---------------------- TSD ---------------- {{{1
 
 extern "C" void __hwasan_thread_enter() {
-  hwasanThreadList().CreateCurrentThread()->InitRandomState();
+  hwasanThreadList().CreateCurrentThread()->EnsureRandomStateInited();
 }
 
 extern "C" void __hwasan_thread_exit() {
diff --git a/compiler-rt/lib/hwasan/hwasan_thread.h b/compiler-rt/lib/hwasan/hwasan_thread.h
--- a/compiler-rt/lib/hwasan/hwasan_thread.h
+++ b/compiler-rt/lib/hwasan/hwasan_thread.h
@@ -28,12 +28,17 @@
 
   void Init(uptr stack_buffer_start, uptr stack_buffer_size,
             const InitState *state = nullptr);
-  void InitRandomState();
+
   void InitStackAndTls(const InitState *state = nullptr);
 
   // Must be called from the thread itself.
   void InitStackRingBuffer(uptr stack_buffer_start, uptr stack_buffer_size);
 
+  inline void EnsureRandomStateInited() {
+    if (UNLIKELY(!random_state_inited_))
+      InitRandomState();
+  }
+
   void Destroy();
 
   uptr stack_top() { return stack_top_; }
@@ -70,6 +75,7 @@
   // via mmap() and *must* be valid in zero-initialized state.
   void ClearShadowForThreadStackAndTLS();
   void Print(const char *prefix);
+  void InitRandomState();
   uptr vfork_spill_;
   uptr stack_top_;
   uptr stack_bottom_;
@@ -89,6 +95,8 @@
 
   bool announced_;
 
+  bool random_state_inited_;  // whether random_state_ has been initialized.
+
   friend struct ThreadListHead;
 };
 
diff --git a/compiler-rt/lib/hwasan/hwasan_thread.cpp b/compiler-rt/lib/hwasan/hwasan_thread.cpp
--- a/compiler-rt/lib/hwasan/hwasan_thread.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_thread.cpp
@@ -27,6 +27,7 @@
 
 void Thread::InitRandomState() {
   random_state_ = flags()->random_tags ? RandomSeed() : unique_id_;
+  random_state_inited_ = true;
 
   // Push a random number of zeros onto the ring buffer so that the first stack
   // tag base will be random.
@@ -125,6 +126,7 @@
 tag_t Thread::GenerateRandomTag(uptr num_bits) {
   DCHECK_GT(num_bits, 0);
   if (tagging_disabled_) return 0;
+  EnsureRandomStateInited();
   tag_t tag;
   const uptr tag_mask = (1ULL << num_bits) - 1;
   do {
diff --git a/compiler-rt/test/hwasan/TestCases/pthread_create.c b/compiler-rt/test/hwasan/TestCases/pthread_create.c
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/hwasan/TestCases/pthread_create.c
@@ -0,0 +1,22 @@
+// Tests that our thread initialization hooks work properly with random_tags=1.
+// RUN: %clang_hwasan %s -o %t
+// RUN: %env_hwasan_opts=random_tags=1 %run %t
+// REQUIRES: stable-runtime
+
+#include <pthread.h>
+
+#include <sanitizer/hwasan_interface.h>
+
+volatile int state;
+
+void *Increment(void *arg) {
+  ++state;
+  return NULL;
+}
+
+int main() {
+  __hwasan_enable_allocator_tagging();
+  pthread_t t1;
+  pthread_create(&t1, NULL, Increment, NULL);
+  pthread_join(t1, NULL);
+}
diff --git a/compiler-rt/test/hwasan/TestCases/thread-uaf.c b/compiler-rt/test/hwasan/TestCases/thread-uaf.c
--- a/compiler-rt/test/hwasan/TestCases/thread-uaf.c
+++ b/compiler-rt/test/hwasan/TestCases/thread-uaf.c
@@ -1,8 +1,6 @@
 // Tests UAF detection where Allocate/Deallocate/Use
 // happen in separate threads.
-// RUN: %clang_hwasan %s -o %t && not %run %t > %t.out 2>&1
-// RUN: cat %t.out | FileCheck %s
-// RUN: cat %t.out | FileCheck --check-prefix=CHECK-THREAD %s
+// RUN: %clang_hwasan %s -o %t && not %run %t 2>&1 | FileCheck %s
 // REQUIRES: stable-runtime
 
 #include <pthread.h>
@@ -37,10 +35,10 @@
   // CHECK: in Deallocate
   // CHECK: previously allocated here:
   // CHECK: in Allocate
-  // CHECK-THREAD-DAG: Thread: T2 0x
-  // CHECK-THREAD-DAG: Thread: T3 0x
-  // CHECK-THREAD-DAG: Thread: T0 0x
-  // CHECK-THREAD-DAG: Thread: T1 0x
+  // CHECK-DAG: Thread: T2 0x
+  // CHECK-DAG: Thread: T3 0x
+  // CHECK-DAG: Thread: T0 0x
+  // CHECK-DAG: Thread: T1 0x
   __sync_fetch_and_add(&state, 1);
   return NULL;
 }