Atomic swap

[Marcel Moolenaar]

On Thu, Aug 07, 2003 at 03:06:33PM -0400, Portante, Peter wrote:
> Marcel,
> 
> > atomic_swap_long(volatile long *dst, long val, long *res)
> > {
> > 	__asm (	"1: ldq_l   t0,%0\n"
> > 		"   mov     %1,t1\n"
> > 		"   stq_c   t1,%0\n"
> > 		"   beq     t1,1b\n"
> > 		"   stq     t0,%3\n"
		^^^^^^^^^^^^^^^^^^^^

Whoops, typo: %3 should be %2.

> > 	    :: "m"(*dst), "r"(val), "m"(*res) : "memory");
> > }
> > 
> A word of caution on performing that stq without an MB before it:
> another processor cannot read that location and the destination
> location and assume anything about their contents based on what
> they read unless an MB is between them.

Good point. I don't think we have to worry about it, though. This
function has a specific usage (see src/lib/libpthread/sys/lock.c).
If I understand the code correctly, *res is never used other than
the caller of atomic_swap_long that writes to *res.

-- 
 Marcel Moolenaar	  USPA: A-39004		 marcel at xcllnt.net