Subj : Re: A question about atomic_ptr
To   : comp.programming.threads
From : Chris Thomasson
Date : Sun Apr 17 2005 01:48 am

>> But there's one other thing: shared_ptr is two pointers, not one. Without
>> DWCAS, or at least double-wide atomic loads and stores, even hazards 
>> aren't
>> enough to make it atomic. :-(
>
> Humm... There should be a way. I will try to apply my SMR implementation 
> to your shared_ptr< T > and see what I can come up with.

I think I may be getting somewhere. I made the following additions to 
shared_ptr<T> API:




// *** detail::sp_counted_base ***


1. *Add this to private section:
-----------------

static void AC_CDECL lfgc_dtor( void *s )
{
  ac_cpu_node_t *node = (ac_cpu_node_t*)s;
  delete (sp_counted_base*)node->state;
  ac_thread_cpu_node_cache_push( ac_thread_self(), node );
}


struct ac_lfgc_node_t
{
  ac_cpu_node_t *m_node;


  void init( sp_counted_base *_base )
  {
    m_node = ac_thread_cpu_node_cache_pop
              ( ac_thread_self(),
                _base );
  }


  void collect()
  {
    if ( m_node )
    {
      ac_cpu_node_t *node = m_node;

      m_node = 0;

      ac_i686_lfgc_smr_collect
       ( ac_thread_self(),
         lfgc_dtor,
         node );
    }
  }

};


ac_lfgc_node_t m_node;




2. *Add this to public section:
-----------------

bool add_ref_lock_if_greater_than_zero()
{
  #if defined(BOOST_HAS_THREADS)
    mutex_type::scoped_lock lock(mtx_);
  #endif

  if(use_count_ == 0) { return false; }
  ++use_count_;
  return true;
}




3. *Change the ctor to:
-----------------

sp_counted_base(): use_count_(1), weak_count_(1)
{
  m_node.init( this );
}




4. *Change the destruct to:
-----------------

virtual void destruct()
{
  m_node.collect();
}








// *** detail::shared_count ***


1. *Add this to public section:
-----------------

void tricky_copy( shared_count &r ) // nothrow
{
  sp_counted_base *src, *old = pi_;

  ac_thread_t *_tls = ac_thread_self();

  ac_i686_lfgc_smr_hazard_t hazard =
    ac_lfgc_smr_get( _tls, 0 );

  do
  {
    src = (sp_counted_base*)
      ac_lfgc_smr_activate
        ( hazard,
          (ac_cpu_node_t**)&r.pi_ );

    if ( ! src ) { break; }

  } while ( ! src->add_ref_lock_if_greater_than_zero() );

  pi_ = src;
  if ( old ) { old->release(); }
  if ( src ) { ac_lfgc_smr_deactivate( hazard ); }
}








// *** shared_ptr ***


1. *Add this to public section:
-----------------

void tricky_copy( shared_ptr &r )
{
  pn.tricky_copy( r.pn );
  px = r.px;
}






I believe shared_ptr can work well with SMR using these ad-hoc tweaks... I 
am going to run some tests and study the shared_ptr impl a bit more to be 
totally sure...

:)

.