Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Gianni Mariani
Date : Fri Jan 21 2005 07:52 pm

David Schwartz wrote:
> "Gianni Mariani" <gi2nospam@mariani.ws> wrote in message 
> news:iaudnW0LQIcam2zcRVn-ig@speakeasy.net...
....

>>#define MTSafeStatic( Type, Name, Initializer )             \
>>    static Type * x_ ## Name;                               \
>>    Type * xlocal_ ## Name =  x_ ## Name;                   \
>>                                                            \
>>    ThreadLocal<bool, false>   m_val;                       \
>>                                                            \
>>    if ( ! m_val )                                          \
>>    {                                                       \
>>        MemoryBarrierSyncReads();                           \
>>    }                                                       \
>>                                                            \
>>    if ( ! xlocal_ ## Name )                                \
>>    {                                                       \
>>        Lock<Mutex>     l_lock( StaticInitializerMutex );   \
>>                                                            \
>>        if ( ! x_ ## Name )                                 \
>>        {                                                   \
>>            static Type v_ ## Name Initializer;             \
>>                                                            \
>>            MemoryBarrierSyncWrites();                      \
>>            x_ ## Name = & v_ ## Name ;                     \
>>            xlocal_ ## Name = & v_ ## Name;                 \
>>        }                                                   \
>>    }                                                       \
>>                                                            \
>>    m_val = true;                                           \
>>                                                            \
>>    Type & Name = * xlocal_ ## Name;                        \
>>// End macro
> 
> 
>     This isn't quite right. Another thread could see 'm_val' as true, and 
> the 'xlocal' as non-NULL and still not see the writes inside 'x'. You need a 
> memory barrier in every possible case, unfortunately. Not all architectures 
> provide a way for one thread to force another thread to see something.

Just a minute - how can that be.

On the first time through, a memory barrier sync is done.  Worst case, 
you can't come out of this code normally without the construction being 
completed. Once you decide that you're locking the mutex, I assume 
barriers are looked after.  The m_val value is thread local, meaning 
each thread executed the memory barrier at least once - once the memory 
barrier is synced once, why would it need to be synced again ?  If it 
did, somthing much more serious is broken.

.