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From: "vlad at solidsands dot nl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/105673] New: _M_futex_wait_until_steady() always
 falls back to old syscall with FUTEX_WAIT operation even after successful
 FUTEX_WAIT_BITSET
Date: Fri, 20 May 2022 15:12:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105673

            Bug ID: 105673
           Summary: _M_futex_wait_until_steady() always falls back to old
                    syscall with FUTEX_WAIT operation even after
                    successful FUTEX_WAIT_BITSET
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vlad at solidsands dot nl
  Target Milestone: ---

We were doing coverage analysis of GNU libstdc++ library and noticed that in
the function _M_futex_wait_until_steady() (src/c++11/futex.cc) the second p=
art
of the function is always executed even though the FUTEX_WAIT_BITSET syscal=
l is
available on the system.

How to reproduce:
    auto f =3D std::async(std::launch::async, foo);
    const auto start{std::chrono::steady_clock::now()};
    auto status =3D f.wait_until(start + std::chrono::seconds{5});

We suspect the reason might be because the if statement on line 217 is miss=
ing
an else branch.
(https://gcc.gnu.org/git?p=3Dgcc.git;a=3Dblob;f=3Dlibstdc%2B%2B-v3/src/c%2B=
%2B11/futex.cc;h=3D4d56c286ec597c19e4dc20bfb0c82d03ee93da8d;hb=3DHEAD#l217)
The sibling function _M_futex_wait_until() does have an else branch for sim=
ilar
if statement. Without the else branch if futex syscall was successful the
function still falls back to the alternative version and performs another f=
utex
syscall.=