Subj : Re: A question about atomic_ptr
To   : comp.programming.threads
From : Chris Thomasson
Date : Fri Apr 15 2005 08:12 pm

> A non-tricky copy constructor cannot be made
> race-resistant, no matter how I fiddle with the assignment.

Here is a "tricky" way to increment a reference count wrt strong thread 
safety:


< snippet from my library >


static AC_INLINE void AC_APIDECL
ac_i686_lfgc_refcount_load
( ac_thread_t *_tls,
  ac_i686_lfgc_refcount_t **pdest,
  ac_i686_lfgc_refcount_t **psrc )
{
  register ac_intword_t refs, refs_cmp;
  register ac_lfgc_smr_hazard_t hazard;
  register ac_i686_lfgc_refcount_t *src, *old = *pdest;


  /* grab tls and hazard pointer */
  if ( ! _tls ) { _tls = ac_thread_self(); };
  hazard = ac_lfgc_smr_get( _tls, 0 );


  for( ;; )
  {
    /* expensive smr load */
    src = (ac_i686_lfgc_refcount_t*)
    ac_i686_lfgc_smr_activate
    ( hazard,
      (ac_i686_node_t**)psrc );
    if ( ! src ) { break; }



    /* addref if greater than zero */
    refs = src->refs;

    while( ( refs & 0x7FFFFFFF ) > 0 )
    {
      refs_cmp = refs;

      refs = ac_atomic_cas_acquire
              ( &src->refs,
                refs_cmp,
                refs_cmp + 1 );

      if ( refs == refs_cmp )
      {
        /* we now own a reference to src */
        ac_i686_lfgc_smr_deactivate( hazard );
        goto ac_i686_lfgc_refcount_load_ok;
      }
    }
  }


  /* store src and release the old */
  ac_i686_lfgc_refcount_load_ok:
    ac_mb_storeptr_release( pdest, src );
    ac_i686_lfgc_refcount_release( old );
}




If you want full source check out the following files my AppCore library 
uses:

ac_i686.h
ac_i686_lfgc_smr.h
ac_i686_lfgc_refcount.h
ac_smr.hpp


ac_i686.c
ac_i686_lfgc_smr.c
ac_i686_gcc.asm


http://appcore.home.comcast.net/appcore_4_1_2005.zip
( full source to AppCore. )




Any questions regarding my implementation of SMR and/or single-word atomic 
reference counting are welcome and encouraged...

:)

.