https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106183
Anthony Williams <anthony.ajw at gmail dot com> changed:
What |Removed |Added
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CC| |anthony.ajw at gmail dot com
--- 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, doesn'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=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/bits/atomic_wait.h;h=125b1cad88682384737c048ac236af9c4deab957;hb=refs/heads/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.
