[Bug c++/103878] New: ThreadSanitizer: false report about data race

[Bug c++/103878] New: ThreadSanitizer: false report about data race

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103878

            Bug ID: 103878
           Summary: ThreadSanitizer: false report about data race
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The program as follows is valid:
```
#include <atomic>
#include <iostream>
#include <future>

int state = 0;
std::atomic_int a = 0;

void foo(int from, int to) 
{
    for (int i = 0; i < 10; i++)
    {
        while (a.load(std::memory_order_relaxed) != from) {}
        std::atomic_thread_fence(std::memory_order_acquire);
        state++;
        a.store(to, std::memory_order_release);
    }
}

int main()
{    
    auto x = std::async(std::launch::async, foo, 0, 1);
    auto y = std::async(std::launch::async, foo, 1, 0);
}
```
The access to `state` variable is not a data race, because each thread before
the modification executes `atomic_thread_fence` to see the results of the other
thread, and after the modification executes atomic store with
`memory_order_release`. But the sanitizer erroneously reports data race. Demo:
https://gcc.godbolt.org/z/9cY3aM3cz

Related discussion: https://stackoverflow.com/q/70542993/7325599

[Bug c++/103878] ThreadSanitizer: false report about data race

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103878

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Isn't this simply the documented limitation of tsan?
https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html#debug.races

[Bug sanitizer/103878] ThreadSanitizer: false report about data race

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103878

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Since it was reported that clang/LLVM have the same issue, is there a bug
report about that?

[Bug sanitizer/103878] ThreadSanitizer: false report about data race

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103878

--- Comment #3 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
Bugreport for Clang/LLVM: https://github.com/llvm/llvm-project/issues/52942