[Bug libstdc++/96817] __cxa_guard_acquire unsafe against dynamically loaded pthread

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96817

--- Comment #17 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:e6923541fae5081b646f240d54de2a32e17a0382

commit r11-3484-ge6923541fae5081b646f240d54de2a32e17a0382
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Sep 26 20:32:36 2020 +0100

    libstdc++: Use __libc_single_threaded to optimise atomics [PR 96817]

    Glibc 2.32 adds a global variable that says whether the process is
    single-threaded. We can use this to decide whether to elide atomic
    operations, as a more precise and reliable indicator than
    __gthread_active_p.

    This means that guard variables for statics and reference counting in
    shared_ptr can use less expensive, non-atomic ops even in processes that
    are linked to libpthread, as long as no threads have been created yet.
    It also means that we switch to using atomics if libpthread gets loaded
    later via dlopen (this still isn't supported in general, for other
    reasons).

    We can't use __libc_single_threaded to replace __gthread_active_p
    everywhere. If we replaced the uses of __gthread_active_p in std::mutex
    then we would elide the pthread_mutex_lock in the code below, but not
    the pthread_mutex_unlock:

      std::mutex m;
      m.lock();            // pthread_mutex_lock
      std::thread t([]{}); // __libc_single_threaded = false
      t.join();
      m.unlock();          // pthread_mutex_unlock

    We need the lock and unlock to use the same "is threading enabled"
    predicate, and similarly for init/destroy pairs for mutexes and
    condition variables, so that we don't try to release resources that were
    never acquired.

    There are other places that could use __libc_single_threaded, such as
    _Sp_locker in src/c++11/shared_ptr.cc and locale init functions, but
    they can be changed later.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96817
            * include/ext/atomicity.h (__gnu_cxx::__is_single_threaded()):
            New function wrapping __libc_single_threaded if available.
            (__exchange_and_add_dispatch, __atomic_add_dispatch): Use it.
            * libsupc++/guard.cc (__cxa_guard_acquire, __cxa_guard_abort)
            (__cxa_guard_release): Likewise.
            * testsuite/18_support/96817.cc: New test.