Index: lib/sanitizer_common/sanitizer_platform_limits_netbsd.h
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_netbsd.h
+++ lib/sanitizer_common/sanitizer_platform_limits_netbsd.h
@@ -38,6 +38,7 @@
 extern unsigned siginfo_t_sz;
 extern unsigned struct_itimerval_sz;
 extern unsigned pthread_t_sz;
+extern unsigned pthread_mutex_t_sz;
 extern unsigned pthread_cond_t_sz;
 extern unsigned pid_t_sz;
 extern unsigned timeval_sz;
Index: lib/sanitizer_common/sanitizer_platform_limits_netbsd.cc
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_netbsd.cc
+++ lib/sanitizer_common/sanitizer_platform_limits_netbsd.cc
@@ -83,6 +83,7 @@
 unsigned struct_sigaction_sz = sizeof(struct sigaction);
 unsigned struct_itimerval_sz = sizeof(struct itimerval);
 unsigned pthread_t_sz = sizeof(pthread_t);
+unsigned pthread_mutex_t_sz = sizeof(pthread_mutex_t);
 unsigned pthread_cond_t_sz = sizeof(pthread_cond_t);
 unsigned pid_t_sz = sizeof(pid_t);
 unsigned timeval_sz = sizeof(timeval);
Index: lib/sanitizer_common/sanitizer_platform_limits_posix.h
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_posix.h
+++ lib/sanitizer_common/sanitizer_platform_limits_posix.h
@@ -46,6 +46,7 @@
   extern unsigned siginfo_t_sz;
   extern unsigned struct_itimerval_sz;
   extern unsigned pthread_t_sz;
+  extern unsigned pthread_mutex_t_sz;
   extern unsigned pthread_cond_t_sz;
   extern unsigned pid_t_sz;
   extern unsigned timeval_sz;
Index: lib/sanitizer_common/sanitizer_platform_limits_posix.cc
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_posix.cc
+++ lib/sanitizer_common/sanitizer_platform_limits_posix.cc
@@ -210,6 +210,7 @@
   unsigned struct_sigaction_sz = sizeof(struct sigaction);
   unsigned struct_itimerval_sz = sizeof(struct itimerval);
   unsigned pthread_t_sz = sizeof(pthread_t);
+  unsigned pthread_mutex_t_sz = sizeof(pthread_mutex_t);
   unsigned pthread_cond_t_sz = sizeof(pthread_cond_t);
   unsigned pid_t_sz = sizeof(pid_t);
   unsigned timeval_sz = sizeof(timeval);
Index: lib/tsan/rtl/tsan_interceptors.cc
===================================================================
--- lib/tsan/rtl/tsan_interceptors.cc
+++ lib/tsan/rtl/tsan_interceptors.cc
@@ -1354,10 +1354,15 @@
   if (o == 0 || f == 0)
     return errno_EINVAL;
   atomic_uint32_t *a;
-  if (!SANITIZER_MAC)
-    a = static_cast<atomic_uint32_t*>(o);
-  else  // On OS X, pthread_once_t has a header with a long-sized signature.
+
+  if (SANITIZER_MAC)
     a = static_cast<atomic_uint32_t*>((void *)((char *)o + sizeof(long_t)));
+  else if (SANITIZER_NETBSD)
+    a = static_cast<atomic_uint32_t*>
+          ((void *)((char *)o + __sanitizer::pthread_mutex_t_sz));
+  else
+    a = static_cast<atomic_uint32_t*>(o);
+
   u32 v = atomic_load(a, memory_order_acquire);
   if (v == 0 && atomic_compare_exchange_strong(a, &v, 1,
                                                memory_order_relaxed)) {