[Bug libstdc++/97949] New: Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[Bug libstdc++/97949] New: Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[fiesh at zefix dot tv]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

            Bug ID: 97949
           Summary: Recursive calls of std::call_once together with cout
                    leads to deadlock under mingw64
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fiesh at zefix dot tv
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-w64-mingw32.static

The following code locks under Windows 10:

-----------------------------
#include <chrono>
#include <iostream>
#include <mutex>
#include <thread>

std::once_flag flag0;
std::once_flag flag1;

void innerDoOnce()
{
        std::call_once(flag0, []() {
                using namespace std::chrono_literals;
                std::cout << "innerDoOnce() called...\n";
                std::this_thread::sleep_for(1s);
        });
}

void outerDoOnce()
{
        std::call_once(flag1, []() {
                std::cout << "outerDoOnce() called...\n";
                innerDoOnce();
        });
}

int main()
{
        std::thread t0(innerDoOnce);
        std::thread t1(innerDoOnce);
        std::thread t2(outerDoOnce);
        std::thread t3(outerDoOnce);
        t0.join();
        t1.join();
        t2.join();
        t3.join();
        std::cout << "aggi\n";
}
---------------------

Removing the `std::cout` calls seems to make the issue go away.

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-11-23
     Ever confirmed|0                           |1

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This isn't specific to mingw, it's a bug in the std::call_once implementation
for non-TLS targets. t2 runs outerDoOnce() and tries to acquire a mutex lock
before running innerDoOnce(), but that mutex is held by t3 while it waits to
try to run outerDoOnce(). t3 won't proceed until t2 finishes, which is blocked
by t3.

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh, and I think the cout calls just slow things down and introduce some
serialization (in stdio) so that the threads run concurrently.

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.0                        |12.2

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 12.1 is being released, retargeting bugs to GCC 12.2.

[Bug libstdc++/97949] Recursive calls of std::call_once together with cout leads to deadlock under mingw64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97949

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.2                        |---

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This won't be fixed for 12.x