[Bug libstdc++/68921] New: [5/6 Regression] std::future::wait() makes invalid futex calls and spins

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68921

            Bug ID: 68921
           Summary: [5/6 Regression] std::future::wait() makes invalid
                    futex calls and spins
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
                CC: torvald at gcc dot gnu.org
  Target Milestone: ---
            Target: i?86-*linux*

On 32-bit linux the following spins in a tight loop until it times out:

#include <future>
#include <thread>

int main() {
  std::promise<void> p;
  auto f = p.get_future();

  std::thread t([&p](){
    std::this_thread::sleep_for(std::chrono::seconds(10));
    p.set_value();
  });

  f.wait();

  t.join();
}

strace shows thousands of invalid calls:

futex(0x8cf2a24, FUTEX_WAIT, 2147483648, {4289120584, 134527555}) = -1 EINVAL
(Invalid argument)

It's called from the infinite loop in
__atomic_futex_unsigned::_M_load_and_test_until in <bits/atomic_futex.h>