[patch] Make std::tr1::shared_ptr thread-safe.

[Paolo Carlini]

Alexander Terekhov wrote:

>[...  __release/acquire_memory_barrier() ...]
>
>>The only reliable implementation of these barriers that I see 
>>is an empty pthread_mutex_lock/pthread_mutex_unlock pair. 
>>    
>>
>Nope. That won't work.
>
Sorry about the trivial question (I'm not MT expert, at least not 'til
today: thanks for the helpful messages!): won't work in principle or
won't work *given the classes of optimizations* implemented in the
current GCC generation?

This punctualization makes a huge difference from our point of view.

Paolo.