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

["Peter Dimov" <pdimov at mmltd dot net>]

Paolo Carlini wrote:
> Robert Zeh wrote:
>
> > As a user who reads the mailing list and has experience with
> > multi-threaded applications,
> > my plea is for the first version of libstdc++'s tr1::shared_ptr to
> > use a mutex.
>
> FWIW my opinion (again, I'm not a MT expert, even less a lock-free
> expert) I agree completely with you. Unfortunately, as Peter and
> Alexander are teaching us, even in that case, there are quite a few
> subtle issues...

Robert's suggestion, if I understand correctly, is to use a mutex for all 
operations, including counting (what boost::shared_ptr currently does on 
non-Windows platforms). This is guaranteed to be safe, but may generate some 
bad publicity, as the recent c.l.c++.m thread has shown.

However, if __exchange_and_add is reviewed and found/changed to be 
fully-fenced, this will automatically take care of all memory visibility 
issues in shared_ptr as well.