[Bug libstdc++/66146] call_once not C++11-compliant on ppc64le

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66146

--- Comment #47 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:6ee24638ed0ad51e568c799bacf149ba9bd7628b

commit r11-7688-g6ee24638ed0ad51e568c799bacf149ba9bd7628b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 12 11:47:20 2021 +0000

    libstdc++: Revert to old std::call_once implementation [PR 99341]

    The new std::call_once implementation is not backwards compatible,
    contrary to my intention. Because std::once_flag::_M_active() doesn't
    write glibc's "fork generation" into the pthread_once_t object, it's
    possible for glibc and libstdc++ to run two active executions
    concurrently. This violates the primary invariant of the feature!

    This patch reverts std::once_flag and std::call_once to the old
    implementation that uses pthread_once. This means PR 66146 is a problem
    again, but glibc has been changed to solve that. A new API similar to
    pthread_once but supporting failure and resetting the pthread_once_t
    will be proposed for inclusion in glibc and other C libraries.

    This change doesn't simply revert r11-4691 because I want to retain the
    new implementation for non-ghtreads targets (which didn't previously
    support std::call_once at all, so there's no backwards compatibility
    concern). This also leaves the new std::call_once::_M_activate() and
    std::call_once::_M_finish(bool) symbols present in libstdc++.so.6 so
    that code already compiled against GCC 11 can still use them. Those
    symbols will be removed in a subsequent commit (which distros can choose
    to temporarily revert if needed).

    libstdc++-v3/ChangeLog:

            PR libstdc++/99341
            * include/std/mutex [_GLIBCXX_HAVE_LINUX_FUTEX] (once_flag):
            Revert to pthread_once_t implementation.
            [_GLIBCXX_HAVE_LINUX_FUTEX] (call_once): Likewise.
            * src/c++11/mutex.cc [_GLIBCXX_HAVE_LINUX_FUTEX]
            (struct __once_flag_compat): New type matching the reverted
            implementation of once_flag using futexes.
            (once_flag::_M_activate): Remove, replace with ...
            (_ZNSt9once_flag11_M_activateEv): ... alias symbol.
            (once_flag::_M_finish): Remove, replace with ...
            (_ZNSt9once_flag9_M_finishEb): ... alias symbol.
            * testsuite/30_threads/call_once/66146.cc: Removed.