From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id C5DC73858D39; Fri, 31 Dec 2021 15:28:12 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org C5DC73858D39
From: "fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/103878] New: ThreadSanitizer: false report about data race
Date: Fri, 31 Dec 2021 15:28:12 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: fchelnokov at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-103878-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 31 Dec 2021 15:28:12 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103878

            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 =3D 0;
std::atomic_int a =3D 0;

void foo(int from, int to)=20
{
    for (int i =3D 0; i < 10; i++)
    {
        while (a.load(std::memory_order_relaxed) !=3D from) {}
        std::atomic_thread_fence(std::memory_order_acquire);
        state++;
        a.store(to, std::memory_order_release);
    }
}

int main()
{=20=20=20=20
    auto x =3D std::async(std::launch::async, foo, 0, 1);
    auto y =3D std::async(std::launch::async, foo, 1, 0);
}
```
The access to `state` variable is not a data race, because each thread befo=
re
the modification executes `atomic_thread_fence` to see the results of the o=
ther
thread, and after the modification executes atomic store with
`memory_order_release`. But the sanitizer erroneously reports data race. De=
mo:
https://gcc.godbolt.org/z/9cY3aM3cz

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