С++ 11 (g++ thread sanitized) Заказ неатомических операций с атомикой (ложный положительный?)

Я экспериментирую с g++ и дезинфицирующим средством для потоков, и я думаю, что получаю ложные срабатывания. Это правда, или я делаю какую-то большую ошибку?

Программа (вырезать и вставить из Энтони Уильямса: С++ Concurrency в действии, стр. 145, список 5.13)

#include <atomic>
#include <thread>
#include <assert.h>
bool x=false;
std::atomic<bool> y;
std::atomic<int> z;
void write_x_then_y()
{
  x=true;
  std::atomic_thread_fence(std::memory_order_release);
  y.store(true,std::memory_order_relaxed);
}
void read_y_then_x()
{
  while(!y.load(std::memory_order_relaxed));
  std::atomic_thread_fence(std::memory_order_acquire);
  if(x)
    ++z;
}
int main()
{
  x=false;
  y=false;
  z=0;
  std::thread a(write_x_then_y);
  std::thread b(read_y_then_x);
  a.join();
  b.join();
  assert(z.load()!=0);
}

Скомпилировано с помощью

g++ -o a -g -Og -pthread a.cpp -fsanitize=thread

g++ версия

~/build/px> g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/6.1.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --disable-libgcj --with-isl --enable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 6.1.1 20160621 (Red Hat 6.1.1-3) (GCC)

Я получаю:

~/build/px> ./a
==================
WARNING: ThreadSanitizer: data race (pid=13794)
  Read of size 1 at 0x000000602151 by thread T2:
    #0 read_y_then_x() /home/ostri/build/px/a.cpp:17 (a+0x000000401014)
    #1 void std::_Bind_simple<void (*())()>::_M_invoke<>(std::_Index_tuple<>) /usr/include/c++/6.1.1/functional:1400 (a+0x000000401179)
    #2 std::_Bind_simple<void (*())()>::operator()() /usr/include/c++/6.1.1/functional:1389 (a+0x000000401179)
    #3 std::thread::_State_impl<std::_Bind_simple<void (*())()> >::_M_run() /usr/include/c++/6.1.1/thread:196 (a+0x000000401179)
    #4 <null> <null> (libstdc++.so.6+0x0000000baaae)

  Previous write of size 1 at 0x000000602151 by thread T1:
    #0 write_x_then_y() /home/ostri/build/px/a.cpp:9 (a+0x000000400fbd)
    #1 void std::_Bind_simple<void (*())()>::_M_invoke<>(std::_Index_tuple<>) /usr/include/c++/6.1.1/functional:1400 (a+0x000000401179)
    #2 std::_Bind_simple<void (*())()>::operator()() /usr/include/c++/6.1.1/functional:1389 (a+0x000000401179)
    #3 std::thread::_State_impl<std::_Bind_simple<void (*())()> >::_M_run() /usr/include/c++/6.1.1/thread:196 (a+0x000000401179)
    #4 <null> <null> (libstdc++.so.6+0x0000000baaae)

  Location is global 'x' of size 1 at 0x000000602151 (a+0x000000602151)

  Thread T2 (tid=13797, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x000000028380)
    #1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0x0000000badc4)
    #2 main /home/ostri/build/px/a.cpp:26 (a+0x000000401097)

  Thread T1 (tid=13796, finished) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x000000028380)
    #1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0x0000000badc4)
    #2 main /home/ostri/build/px/a.cpp:25 (a+0x00000040108a)

SUMMARY: ThreadSanitizer: data race /home/ostri/build/px/a.cpp:17 in read_y_then_x()
==================
ThreadSanitizer: reported 1 warnings

Я получил это предупреждение в более сложной программе, и я подумал, что это моя ошибка, но теперь даже "программа школьных книг" демонстрирует такое же поведение. Это (т.е. Какой-то компилятор отсутствует) me или g++?

ОБНОВЛЕНО Взято из ссылка

#if defined(__SANITIZE_THREAD__)
#define TSAN_ENABLED
#elif defined(__has_feature)
#if __has_feature(thread_sanitizer)
#define TSAN_ENABLED
#endif
#endif

#ifdef TSAN_ENABLED
#define TSAN_ANNOTATE_HAPPENS_BEFORE(addr) \
    AnnotateHappensBefore(__FILE__, __LINE__, (void*)(addr))
#define TSAN_ANNOTATE_HAPPENS_AFTER(addr) \
    AnnotateHappensAfter(__FILE__, __LINE__, (void*)(addr))
extern "C" void AnnotateHappensBefore(const char* f, int l, void* addr);
extern "C" void AnnotateHappensAfter(const char* f, int l, void* addr);
#else
#define TSAN_ANNOTATE_HAPPENS_BEFORE(addr)
#define TSAN_ANNOTATE_HAPPENS_AFTER(addr)
#endif

#include <atomic>
#include <thread>
#include <assert.h>
bool x=false;
std::atomic<bool> y;
std::atomic<int> z;
void write_x_then_y()
{
  x=true;
  std::atomic_thread_fence(std::memory_order_release);
  TSAN_ANNOTATE_HAPPENS_BEFORE(&x);
  y.store(true,std::memory_order_relaxed);
}
void read_y_then_x()
{
  while(!y.load(std::memory_order_relaxed));
  std::atomic_thread_fence(std::memory_order_acquire);
  TSAN_ANNOTATE_HAPPENS_AFTER(&x);
  if(x)
    ++z;
}
{
  x=false;
  y=false;
  z=0;
  std::thread a(write_x_then_y);
  std::thread b(read_y_then_x);
  a.join();
  b.join();
  assert(z.load()!=0);
}

Скомпилировать команду

g++ -o a -g -Og -pthread a.cpp -fsanitize=thread -D__SANITIZE_THREAD__

Ответы

  x=true;                                              // W
  std::atomic_thread_fence(std::memory_order_release); // A
  y.store(true,std::memory_order_relaxed);             // X

  while(!y.load(std::memory_order_relaxed));           // Y
  std::atomic_thread_fence(std::memory_order_acquire); // B
  if(x)                                                // R
     ++z;

std::atomic_thread_fence(std::memory_order_acquire);
if(x)
  ++z;
while(!y.load(std::memory_order_relaxed));