Re: [committed] libstdc++: Use __libc_single_threaded for locale initialization

[Stephan Bergmann <sbergman@redhat.com>]

On 24/11/2020 15:59, Jonathan Wakely via Gcc-patches wrote:
> Most initialization of locales and facets happens before main() during
> startup, when the program is likely to only have one thread. By using
> the new __gnu_cxx::__is_single_threaded() function instead of checking
> __gthread_active_p() we can avoid using pthread_once or atomics for the
> common case.
> 
> That said, I'm not sure why we don't just use a local static variable
> instead, as __cxa_guard_acquire() already optimizes for the
> single-threaded case:
> 
>    static const bool init = (_S_initialize_once(), true);
> 
> I'll revisit that for GCC 12.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* src/c++98/locale.cc (locale::facet::_S_get_c_locale())
> 	(locale::id::_M_id() const): Use __is_single_threaded.
> 	* src/c++98/locale_init.cc (locale::_S_initialize()):
> 	Likewise.
> 
> Tested powerpc64le-linux. Committed to trunk.

I now started to get weird crashes when running LibreOffice test code at 
least with Clang -fsanitize=address and latest libstdc++, where the 
Clang ASan machinery SEGVs while it wants to report some malloc/free 
issue.  That goes away when reverting this commit, and I think the root 
cause is that locale::facet::_S_initialize_once() now gets called twice: 
  First during __cxx_global_var_init when the process is still single 
threaded (so

   if (!__gnu_cxx::__is_single_threaded())

in locale::facet::_S_get_c_locale, reading __libc_single_threaded, is 
false, whereas

   if (__gthread_active_p())

would have been true even if the process still only had a single 
thread).  And again after the process has spawned further threads via 
pthread_create (flipping __libc_single_threaded) and calls 
std::ostringstream() -> ... std::locale() -> ..., at which point

   if (!__gnu_cxx::__is_single_threaded())

in locale::facet::_S_get_c_locale is true now.