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From Liviu Nicoara <nikko...@hates.ms>
Subject Re: STDCXX-1056 [was: Re: STDCXX forks]
Date Wed, 12 Sep 2012 13:04:09 GMT
On 09/12/12 00:21, Stefan Teleman wrote:
> On Tue, Sep 11, 2012 at 10:18 PM, Liviu Nicoara <nikkoara@hates.ms> wrote:
>
>> AFAICT, there are two cases to consider:
>>
>> 1. Using STDCXX locale database initializes the __rw_punct_t data in the
>> first, properly synchronized pass through __rw_get_numpunct. All subsequent
>> calls use the __rw_punct_t data to construct returned objects.
>> 2. Using the C library locales does the same in the first pass, via
>> setlocale and localeconv, but setlocale synchronization is via a per-process
>> lock. The facet data, once initialized is used just like above.
>>
>> I probably missed this in the previous conversation, but did you detect a
>> race condition in the tests if the facets are simply forwarding to the
>> private virtual interface? I.e., did you detect that the facet
>> initialization code is unsafe? I think the facet __rw_punct_t data is safely
>> initialized in both cases, it's the caching that is done incorrectly.
>
> I originally thought so too, but now I'm having doubts. :-) And I
> haven't tracked it down with 100% accuracy yet. I saw today this
> comment in src/facet.cpp, line 358:
>
> // a per-process array of facet pointers sufficiently large
> // to hold (pointers to) all standard facets for 8 locales
> static __rw_facet*  std_facet_buf [__rw_facet::_C_last_type * 8];

The localization code is cleverly dense and low-level. There are a few patterns that are used
throughout, though. One is the function-local static buffers, e.g., the locales buffer and
the facets buffer, in locale_body.cpp and facet.cpp, respectively. They both start from a
reasonable size and are grown and shrunk according to the needs, but neither locales nor facets
are ever evicted from them. See:

locale_body:936  -- reduction of locale buffer
locale_body:1006 -- expansion of locale buffer

facet.cpp:397    -- reduction of facet buffer
facet.cpp:462    -- expansion of facet buffer

Facets buffer is guarded by a "static" guard in:

__rw_facet::_C_manage:366

The locales buffer is protected by a "static" guard in:

__rw_locale::_C_manage:880


>
> [...] this is all investigative stuff for tomorrow. :-)
>
> and I agree with Martin that breaking ABI in a minor release is really
> not an option. I'm trying to find the best way of making these facets
> thread-safe while inflicting the least horrible performance hit.

We are getting close to the point where we can summarize all these findings and gauge an appropriate
course of action.

>
> I will run your tests tomorrow and let you know. :-)


That would be very helpful!

Liviu

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