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Subject: [Bug libstdc++/106183] std::atomic::wait might fail to be unblocked
 by notify_one/all on platforms without platform_wait()
Date: Thu, 04 Aug 2022 12:30:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106183

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:af98cb88eb4be6a1668ddf966e975149bf8610b1

commit r13-1957-gaf98cb88eb4be6a1668ddf966e975149bf8610b1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jul 28 16:15:58 2022 +0100

    libstdc++: Unblock atomic wait on non-futex platforms [PR106183]

    When using a mutex and condition variable, the notifying thread needs to
    increment _M_ver while holding the mutex lock, and the waiting thread
    needs to re-check after locking the mutex. This avoids a missed
    notification as described in the PR.

    By moving the increment of _M_ver to the base _M_notify we can make the
    use of the mutex local to the use of the condition variable, and
    simplify the code a little. We can use a relaxed store because the mutex
    already provides sequential consistency. Also we don't need to check
    whether __addr =3D=3D &_M_ver because we know that's always true for
    platforms that use a condition variable, and so we also know that we
    always need to use notify_all() not notify_one().

    Reviewed-by: Thomas Rodgers <trodgers@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/106183
            * include/bits/atomic_wait.h (__waiter_pool_base::_M_notify):
            Move increment of _M_ver here.
            [!_GLIBCXX_HAVE_PLATFORM_WAIT]: Lock mutex around increment.
            Use relaxed memory order and always notify all waiters.
            (__waiter_base::_M_do_wait) [!_GLIBCXX_HAVE_PLATFORM_WAIT]:
            Check value again after locking mutex.
            (__waiter_base::_M_notify): Remove increment of _M_ver.=