Re: [boost] [intrusive_ptr] adding thread safety

From: Dave Compton (abcdave_at_[hidden])
Date: 2006-08-01 16:31:47

 > JOAQUIN LOPEZ MU?Z wrote:
 > > Hello,
 > >
 > > A naive usage of intrusive_ptr involves having a
 > > refcount embedded in the pointed to object and
 > > providing add_red and release functions like:
 > >
 > > void intrusive_ptr_add_ref(const my_class* p)
 > > {
 > > ++(p->ref);
 > > }
 > >
 > > template<typename T>
 > > void intrusive_ptr_release(const my_class* p)
 > > {
 > > if(--(p->ref)==0){
 > > // whatever
 > > }
 > > }
 > >
 > > But, AFAICS, this is not thread safe, as intrusive_ptr
 > > itself does not add any thread safety layer. So my
 > > question is how to efficiently provide add_ref and release
 > > functions which are thread safe (wrt to refcounting)? I can
 > > use a class-level boost::detail::lightweight_mutex in the
 > > obvious way, but I understand shared_ptr uses much more
 > > advanced mechanisms and maybe these are available for
 > > (semi)public consuption at boost/detail.
 >
 > An atomic counter should suffice. boost::detail::atomic_count.

Is it enough to use an atomic counter?  It seems like the entire
decrement *and test* operation in intrusive_ptr_release
needs to be atomic.  Otherwise you run the risk of
the "// whatever" code being executed twice in
different threads.

Sorry for responding to a topic that is a little dated.  I ran across
this thread yesterday when I was dealing with the same issue.