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

Jonathan Wakely jwakely@redhat.com
Fri Nov 27 12:26:29 GMT 2020


On 27/11/20 11:15 +0000, Jonathan Wakely wrote:
>On 27/11/20 09:43 +0100, Stephan Bergmann via Libstdc++ wrote:
>>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.
>
>Gah, yes, that's broken. Sorry. I'm reverting that part and testing
>it now.
>
>I still think we should just use local static variables here and let
>the runtime do the initialization and optimize it for single-threaded,
>but I'll leave that for GCC 12. There might be some reason that wasn't
>done originally, maybe so the library could be built with
>-fno-threadsafe-statics (which I don't think we should support) or
>maybe just because it predates the threadsafe-static code working
>well.

Fixed with this patch, tested powerp64le-linux (glibc 2.32).

Pushed to trunk.

Thanks for the testing and bug report, as always.


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