How to avoid false positives with Helgrind?

The problem is that Helgrind doesn't understand GCC's atomic builtins, so doesn't realise that they are race-free and impose ordering on the program.

There are ways to annotate your code to help Helgrind, see http://valgrind.org/docs/manual/hg-manual.html#hg-manual.effective-use (but I'm not sure how to use them here, I already tried what sbabbi shows and it only solves part of the problem).

I would avoid yielding in a busy loop anyway, it's a poor form of synchronization. It could be done with a condition variable like so:

#include <thread>#include <atomic>#include <iostream>#include <condition_variable>int main(){    bool isReady(false);    std::mutex mx;    std::condition_variable cv;    int i = 1;    std::thread t([&isReady, &i, &mx, &cv]()    {        i = 2;        std::unique_lock<std::mutex> lock(mx);        isReady = true;        cv.notify_one();    });    {        std::unique_lock<std::mutex> lock(mx);        cv.wait(lock, [&] { return isReady; });    }    i = 3;    t.join();    std::cout << i;    return 0;}

Valgrind cannot know that the while (!isReady) loop (along with the implicit memory_order_release and memory_order_consume flags on the store and load), implies that the statement i = 2 is dependency ordered before i = 3.

You have to explicitly state this invariant by using valgrind ANNOTATE_HAPPENS_BEFORE and ANNOTATE_HAPPENS_AFTER macros:

#include <valgrind/drd.h>#include <thread>#include <atomic>#include <iostream>int main(){    std::atomic<bool> isReady(false);    int i = 1;    std::thread t([&isReady, &i]()    {        i = 2;        ANNOTATE_HAPPENS_BEFORE(&isReady);        isReady = true;    });    while (!isReady)        std::this_thread::yield();    ANNOTATE_HAPPENS_AFTER(&isReady);    i = 3;    t.join();    std::cout << i;    return 0;}

Here we are saying that the line at ANNOTATE_HAPPENS_BEFORE always happens before the line at ANNOTATE_HAPPENS_AFTER, we know that due to inspection of the program logic, but valgrind cannot prove that for you.

This program produces:

valgrind --tool=helgrind ./a.out==714== Helgrind, a thread error detector==714== Copyright (C) 2007-2015, and GNU GPL'd, by OpenWorks LLP et al.==714== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info==714== Command: ./val==714== ==714== ---Thread-Announcement------------------------------------------==714== ==714== Thread #1 is the program's root thread==714== ==714== ---Thread-Announcement------------------------------------------==714== ==714== Thread #2 was created==714==    at 0x59E169E: clone (in /usr/lib/libc-2.23.so)==714==    by 0x4E421D9: create_thread (in /usr/lib/libpthread-2.23.so)==714==    by 0x4E43C42: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.23.so)==714==    by 0x4C316F3: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)==714==    by 0x4C327D7: pthread_create@* (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)==714==    by 0x5113DB4: __gthread_create (gthr-default.h:662)==714==    by 0x5113DB4: std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) (thread.cc:163)==714==    by 0x40109C: std::thread::thread<main::{lambda()#1}>(main::{lambda()#1}&&) (in /home/ennio/val)==714==    by 0x400F55: main (in /home/ennio/val)==714== ==714== ----------------------------------------------------------------==714== ==714== Possible data race during read of size 1 at 0xFFF00061F by thread #1==714== Locks held: none==714==    at 0x401585: std::atomic<bool>::operator bool() const (in /home/ennio/val)==714==    by 0x400F61: main (in /home/ennio/val)==714== ==714== This conflicts with a previous write of size 1 by thread #2==714== Locks held: none==714==    at 0x4015D5: std::__atomic_base<bool>::operator=(bool) (in /home/ennio/val)==714==    by 0x401550: std::atomic<bool>::operator=(bool) (in /home/ennio/val)==714==    by 0x400F1B: main::{lambda()#1}::operator()() const (in /home/ennio/val)==714==    by 0x40146F: void std::_Bind_simple<main::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (in /home/ennio/val)==714==    by 0x40140C: std::_Bind_simple<main::{lambda()#1} ()>::operator()() (in /home/ennio/val)==714==    by 0x4013EB: std::thread::_State_impl<std::_Bind_simple<main::{lambda()#1} ()> >::_M_run() (in /home/ennio/val)==714==    by 0x5113A9E: execute_native_thread_routine (thread.cc:83)==714==    by 0x4C318E7: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)==714==  Address 0xfff00061f is on thread #1's stack==714==  in frame #1, created by main (???:)==714== 3==714== ==714== For counts of detected and suppressed errors, rerun with: -v==714== Use --history-level=approx or =none to gain increased speed, at==714== the cost of reduced accuracy of conflicting-access information==714== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

To remove the error on isReady itself, I assume a suppression file on __atomic_base<bool>::operator= would be sufficient.