More on memory barriers

[Richard Henderson]

On Wed, Sep 15, 2004 at 02:22:03AM -0400, Jason Merrill wrote:
> if (!initialized)
>   {
>     acquire_lock();
>     if (!initialized)
>       {
>         initialize ();
>         memory_barrier(); // force serialization on all CPUs
>         initialized = true;
>       }
>     release_lock();
>   }
> 
> On Alpha we need a second memory barrier, i.e.
> 
> else
>   read_memory_barrier ();

Well, no, Alpha needs

  read_memory_barrier ();
  if (!initialized)
    {
      acquire_lock ();
      if (!initialized)
	{
	  initialize ();
	  write_memory_barrier ();
	  initialized = true;
	}
      release_lock ();
    }

> On ia64, does a release barrier have the desired semantics, or do we need a
> full barrier?

Release is sufficient.  In particular, the barriers that must be
present for the lock release are sufficient to ensure that the
read of initialized happens after the write.

We do still need a write_memory_barrier here.  For that,
__sync_synchronize is still the best implementation available.


r~