Re: libstdc++/7445: poor performance of std::locale::classic() inmulti-threaded applications

[Andrew Pollard <Andrew dot Pollard at brooks-pri dot com>]

Benjamin Kosnik wrote:
> > I still think the two problems I mentioned are still there even with
> > this version of the patch, namely
> >
> > a) Two threads can enter the method at once, the first will get the
> >    lock, set up _S_classic, etc, and release the lock, then the 2nd
> >    will do exactly the same things again.
> >    This can be fixed by checking _S_classic again after the auto lock
> >
> > b) If a second thread enters while another is in the process of
> > setting
> >    up _S_classic, ie it has assigned _S_classic, but not initialised
> >    c_locale, the 2nd thread will return the unconstructed c_locale
> >    object.
> >    This can be fixed by setting _S_classic as the last thing in the
> >    method
> >
> > I think the location of the static mutex doesn't matter.
> >
> > So, again, I think the entire routine needs to be changed to something
> > like this... (comments included to indicate reasoning...)
>
> I'm not excited by this patch. There has got to be a better way. Loren,
> can you commet on this? I should have just waited for you before I
> checked this in anyway. 
Fair enough. It was just a quick patch to fix the problems introduced by
moving the location of the locking. I agree that there is probably a better
way to do it though.

> >   const locale&
> >   locale::classic()
> >   {
> >     // libstdc++/7445 - Optimisation to avoid the auto-lock once the
> >     // Initial creation has been done
> >     if (_S_classic)
> >       return c_locale;
> weak
> >     static _STL_mutex_lock __lock __STL_MUTEX_INITIALIZER;
> >     _STL_auto_lock __auto(__lock);
> >
> >     // In case multiple threads call locale::classic() at the same
> >     // time with _S_classic unset, check if another thread might
>>     // have already done the work
> >     if (_S_classic)
> >       return c_locale;
> really weak
> >     locale::_Impl* __tmp_S_classic = 0;
> >     try
> >       {
> >         // 26 Standard facets, 2 references.
> >         // One reference for _M_classic, one for _M_global
> >         facet** f = new(&facet_vec) facet*[_GLIBCPP_NUM_FACETS];
> >         for (size_t __i = 0; __i < _GLIBCPP_NUM_FACETS; ++__i)
> >           f[__i] = 0;
> >
> >         __tmp_S_classic = new (&c_locale_impl) _Impl(f, 2, true);
> >         new (&c_locale) locale(__tmp_S_classic);
> >
> >         // Set _S_classic last when we know we have done all the
> >         // initialisation
> >         _S_classic = _S_global = __tmp_S_classic;
> >       }
> >     catch(...)
> >       {
> >         // Just call destructor, so that locale_impl_c's memory is
> >         // not deallocated via a call to delete.
> >         if (tmp_S_classic)
> >           tmp_S_classic->~_Impl();
> >         _S_classic = _S_global = 0;
> >         __throw_exception_again;
> >       }
> >     return c_locale;
> >   }
Might be "weak", should do the job though :-)

Andrew.
--
       Andrew Pollard - Senior Software Engineer (APF)
   Brooks-PRI Automation - Planning and Logistics Solutions
Email: Andrew.Pollard@brooks-pri.com - Tel: +44 (0)118 9215603