Re: [PATCH] tools/memory-model: document the "one-time init" pattern

[Alan Stern]

On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote:
> On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:
...
> > +		/* on success, pairs with smp_load_acquire() above and below */
> > +		if (cmpxchg_release(&foo, NULL, p) != NULL) {
> 
> Why do we have cmpxchg_release() anyway?  Under what circumstances is
> cmpxchg() useful _without_ having release semantics?

To answer just the last question: cmpxchg() is useful for lock 
acquisition, in which case it needs to have acquire semantics rather 
than release semantics.

Alan Stern