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From: "anthony.ajw at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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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: Mon, 25 Jul 2022 13:51:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106183

Anthony Williams <anthony.ajw at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |anthony.ajw at gmail dot c=
om

--- Comment #3 from Anthony Williams <anthony.ajw at gmail dot com> ---
This is one of the common mistakes I mention when teaching people about
condition variables. Just because the data being waited for is atomic, does=
n't
guarantee that the condition variable state is updated: you need the mutex =
to
synchronize that.

In current libstdc++ trunk libstdc++-v3/include/bits/atomic_wait.h insert a
line in _M_notify at line 235:

https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dblob;f=3Dlibstdc%2B%2B-v3/include/=
bits/atomic_wait.h;h=3D125b1cad88682384737c048ac236af9c4deab957;hb=3Drefs/h=
eads/trunk

      void
      _M_notify(const __platform_wait_t* __addr, bool __all, bool __bare)
noexcept
      {
        if (!(__bare || _M_waiting()))
          return;

#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
        __platform_notify(__addr, __all);
#else
/// INSERT HERE
        { std::lock_guard<mutex> __lock(_M_mtx); }
/// END INSERT
        if (__all)
          _M_cv.notify_all();
        else
          _M_cv.notify_one();
#endif
      }

The lock/unlock here ensures that the notify is correctly synchronized with=
 the
wait.=