Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Gianni Mariani
Date : Fri Jan 21 2005 09:19 am

Joseph Seigh wrote:
....
> 
> In the sense that you can order memory accesses.  They may differ in
> which types of memory accesses they order, e.g. Sun sparc has more
> types of memory barriers than Intel.  It's not usually a problem
> since you can use the stronger memory barrier if the more precise
> one isn't available, e.g. an MFENCE on intel for an #LoadStore on
> sparc, or #StoreStore on sparc for no membar on intel pentium.  This
> doesn't solve the compiler ordering problem though.
> 

How about this scheme ?  Would this be faster than the
memory barrier example ?  I suppose that depends on how
fast the RW lock is.

#define MTSafeStatic( Type, Name, Initializer )             \
     static ReadWriteMutex           xmutex_ ## Name;        \
     Type * xlocal_ ## Name;                                 \
                                                             \
     {                                                       \
         // take a read lock                                 \
         ReadLock<ReadWriteMutex>        l_rlock;            \
         static Type * volatile x_marker;                    \
                                                             \
         xlocal_ ## Name = x_marker;                         \
                                                             \
         if ( ! xlocal_ ## Name )                            \
         {                                                   \
             // unlock the read lock                         \
             ReadUnLock<ReadWriteMutex>  l_runlock;          \
             // take the static initializer lock             \
             Lock<Mutex> l_lock( StaticInitializerMutex );   \
             // take the write lock                          \
             WriteLock<ReadWriteMutex>   l_wlock;            \
                                                             \
             xlocal_ ## Name = x_marker;                     \
             if ( ! xlocal_ ## Name )                        \
             {                                               \
                 static Type s_Obj Initializer;              \
                                                             \
                 x_local ## Name = & s_Obj ;                 \
                 x_marker = & s_Obj;                         \
             }                                               \
         }                                                   \
     }                                                       \
                                                             \
     Type & Name = * xlocal_ ## Name;                        \
// End macro

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