$ uname -a
NetBSD chieftec 8.99.6 NetBSD 8.99.6 (GENERIC) #7: Thu Nov  9 20:58:03 CET 2017  root@chieftec:/public/netbsd-root/sys/arch/amd64/compile/GENERIC amd64

$ ./bin/clang++ mutex.cc -o mutex -g -o mutex -O0 -fsanitize=thread -pthread

$ cat mutex.cc                                                                                                                                                                       
#include <pthread.h>
#include <stdio.h>

#include <sanitizer/tsan_interface.h>

typedef unsigned long long invisible_barrier_t;

// Default instance of the barrier, but a test can declare more manually.
invisible_barrier_t barrier;

#ifdef __cplusplus
extern "C" {
void __tsan_testonly_barrier_wait(invisible_barrier_t *barrier);
unsigned long __tsan_testonly_shadow_stack_current_size();
#ifdef __cplusplus
}
#endif

static inline void barrier_init(invisible_barrier_t *barrier, unsigned count) {
  __tsan_testonly_barrier_init(barrier, count);
}

static inline void barrier_wait(invisible_barrier_t *barrier) {
  __tsan_testonly_barrier_wait(barrier);
}

int Global;
pthread_mutex_t mtx;

void *Thread1(void *x) {
  barrier_wait(&barrier);
  pthread_mutex_lock(&mtx);
  Global++;
  pthread_mutex_unlock(&mtx);
  return NULL;
}

void *Thread2(void *x) {
  Global--;
  barrier_wait(&barrier);
  return NULL;
}

void
fn(void)
{
        printf("%s\n", __func__);
        return;
}

pthread_once_t once_control = PTHREAD_ONCE_INIT;


int main() {
  barrier_init(&barrier, 2);
  // CHECK: WARNING: ThreadSanitizer: data race
  // CHECK:   Write of size 4 at {{.*}} by thread T1
  // CHECK:                         (mutexes: write [[M1:M[0-9]+]]):
  // CHECK:   Previous write of size 4 at {{.*}} by thread T2:
  // CHECK:   Mutex [[M1]] (0x{{.*}}) created at:
  // CHECK:     #0 pthread_mutex_init
  // CHECK:     #1 main {{.*}}mutexset1.cc:[[@LINE+1]]
  pthread_mutex_init(&mtx, 0);
  pthread_t t[2];

//  pthread_once(&once_control, fn);

  pthread_create(&t[0], NULL, Thread1, NULL);
  pthread_create(&t[1], NULL, Thread2, NULL);
  pthread_join(t[0], NULL);                                                                                                           
  pthread_join(t[1], NULL);
  pthread_mutex_destroy(&mtx);
}

$ ./mutex                                                                                                                                                                            
==================
WARNING: ThreadSanitizer: data race (pid=2142)
  Write of size 4 at 0x0000014fab00 by thread T1 (mutexes: write M76):
    #0 Thread1(void*) /public/llvm-build/mutex.cc:36:9 (mutex+0x46ebbc)

  Previous write of size 4 at 0x0000014fab00 by thread T2:
    #0 Thread2(void*) /public/llvm-build/mutex.cc:42:9 (mutex+0x46ec87)

  Location is global 'Global' of size 4 at 0x0000014fab00 (mutex+0x0000014fab00)

  Mutex M76 (0x0000014fab08) created at:
    #0 pthread_mutex_init /public/llvm/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:1147:3 (mutex+0x413ef3)
    #1 main /public/llvm-build/mutex.cc:66:3 (mutex+0x46ed66)

  Thread T1 (tid=2, running) created by main thread at:
    #0 pthread_create /public/llvm/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:931:3 (mutex+0x412cf0)
    #1 main /public/llvm-build/mutex.cc:71:3 (mutex+0x46ed85)

  Thread T2 (tid=3, finished) created by main thread at:
    #0 pthread_create /public/llvm/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:931:3 (mutex+0x412cf0)
    #1 main /public/llvm-build/mutex.cc:72:3 (mutex+0x46eda4)

SUMMARY: ThreadSanitizer: data race /public/llvm-build/mutex.cc:36:9 in Thread1(void*)
==================
ThreadSanitizer: reported 1 warnings