I am using Clang-8 with thread sanitizer enabled to compile the code below
std::atomic<std::string*> ptr {nullptr};
int data {0};
void producer() {
std::string* p = new std::string("Hello");
data = 42;
ptr.store(p, std::memory_order_release);
}
void consumer() {
std::string* p2;
if(!(p2 = ptr.load(std::memory_order_relaxed))) {
// Data is not ready, just return
return ;
}
std::atomic_thread_fence(std::memory_order_seq_cst);
assert(data == 42); // Never fired
}
int main() {
std::thread t1(producer);
std::thread t2(consumer);
t1.join(); t2.join();
}
Thread sanitizer reports a data race warning on the global variable data. Why this is considered a data race?
==================
WARNING: ThreadSanitizer: data race (pid=20388)
Read of size 4 at 0x000001191718 by thread T2:
#0 consumer() <null> (a.out+0x4afe32)
#1 void std::__invoke_impl<void, void (*)()>(std::__invoke_other, void (*&&)()) <null> (a.out+0x4b1fad)
#2 std::__invoke_result<void (*)()>::type std::__invoke<void (*)()>(void (*&&)()) <null> (a.out+0x4b1ee0)
#3 decltype(std::__invoke(_S_declval<0ul>())) std::thread::_Invoker<std::tuple<void (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) <null> (a.out+0x4b1e88)
#4 std::thread::_Invoker<std::tuple<void (*)()> >::operator()() <null> (a.out+0x4b1e28)
#5 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)()> > >::_M_run() <null> (a.out+0x4b1c1c)
#6 <null> <null> (libstdc++.so.6+0xbd66e)
Previous write of size 4 at 0x000001191718 by thread T1:
#0 producer() <null> (a.out+0x4afcca)
#1 void std::__invoke_impl<void, void (*)()>(std::__invoke_other, void (*&&)()) <null> (a.out+0x4b1fad)
#2 std::__invoke_result<void (*)()>::type std::__invoke<void (*)()>(void (*&&)()) <null> (a.out+0x4b1ee0)
#3 decltype(std::__invoke(_S_declval<0ul>())) std::thread::_Invoker<std::tuple<void (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) <null> (a.out+0x4b1e88)
#4 std::thread::_Invoker<std::tuple<void (*)()> >::operator()() <null> (a.out+0x4b1e28)
#5 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)()> > >::_M_run() <null> (a.out+0x4b1c1c)
#6 <null> <null> (libstdc++.so.6+0xbd66e)
Location is global 'data' of size 4 at 0x000001191718 (a.out+0x000001191718)
Thread T2 (tid=20391, running) created by main thread at:
#0 pthread_create <null> (a.out+0x424775)
#1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0xbd924)
#2 main <null> (a.out+0x4afec1)
Thread T1 (tid=20390, finished) created by main thread at:
#0 pthread_create <null> (a.out+0x424775)
#1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0xbd924)
#2 main <null> (a.out+0x4afeae)
SUMMARY: ThreadSanitizer: data race in consumer()
==================
ThreadSanitizer: reported 1 warnings
Update
I have read the Why does ThreadSanitizer report a race with this lock-free example? and compile the code using Clang-8. It does not show any data race warning. So I think my case is different.
