Re: making GStaticMutexes faster

[Sebastian Wilhelmi <wilhelmi ira uka de>]

Hi Sander,

> > I think "cache coherence" is a topic, not a particular property...
> >
> > The particular property you are proposing relying on is if processor
> > #1 writes location A and then location B, then another processor will
> > never see a write to location B then a write to location A.
> >
> > (Since the writes are here separated by a function return it is
> > pretty unlikely that instruction reordering would cause problems,
> > so it probably is mostly a question of cache behavior, yes.)
> >
> 
> Well, modern RISC processors aren't really under pretty much any
> constraints to re-ordering writes. Unless you use memory barriers 
> and/or special setup in page tables.

Write reordering isn't a problem here, because when creating the mutex this is
protected by another mutex, which also acts an a memory barrier:

GMutex *
g_static_mutex_get_mutex_impl (GMutex** mutex)
{
  GMutex *new_mutex;

  if (*mutex)
    return *mutex;

  g_mutex_lock (g_mutex_protect_static_mutex_allocation);

  if (!(*mutex)) 
    new_mutex = g_mutex_new (); 

  g_mutex_unlock (g_mutex_protect_static_mutex_allocation);

  /* No problems with write reordering, as between here and the creation
     of new_mutex is a memory barrier */

  *mutex = new_mutex;
  
  return *mutex;
}
 
So the only issue is indeed cache coherency. See my other reply as well.

Bye,
Sebastian
-- 
Sebastian Wilhelmi
mailto:wilhelmi ira uka de
http://goethe.ira.uka.de/~wilhelmi