Re: [PATCH 1/5] atomics: add acquire/release/relaxed variants of some atomic operations

Will Deacon
2015-07-14T10:32:24Z

On Tue, Jul 14, 2015 at 11:25:11AM +0100, Peter Zijlstra wrote:
> On Mon, Jul 13, 2015 at 01:31:23PM +0100, Will Deacon wrote:
> > +
> > +/*
> > + * Relaxed variants of xchg, cmpxchg and some atomic operations.
> > + *
> > + * We support four variants:
> > + *
> > + * - Fully ordered: The default implementation, no suffix required.
> > + * - Acquire: Provides ACQUIRE semantics, _acquire suffix.
> > + * - Release: Provides RELEASE semantics, _release suffix.
> > + * - Relaxed: No ordering guarantees, _relaxed suffix.
> > + *
> > + * See Documentation/memory-barriers.txt for ACQUIRE/RELEASE definitions.
> > + */
> > +
> > +#ifndef atomic_read_acquire
> > +#define atomic_read_acquire(v) smp_load_acquire(&(v)->counter)
> > +#endif
> > +
> > +#ifndef atomic_set_release
> > +#define atomic_set_release(v, i) smp_store_release(&(v)->counter, (i))
> > +#endif
> > +
> > +#ifndef atomic_add_return_relaxed
> > +#define atomic_add_return_relaxed atomic_add_return
> > +#endif
> > +#ifndef atomic_add_return_acquire
> > +#define atomic_add_return_acquire atomic_add_return
> > +#endif
> > +#ifndef atomic_add_return_release
> > +#define atomic_add_return_release atomic_add_return
> > +#endif
>
> Could we not define _{acquire,release} in terms of _relaxed and
> smp_mb__{after,before}_atomic() ?

I actually started out with that, but it penalises architectures that
don't have _relaxed implementations of some routines.

For example, cmpxchg_acquire would likely expand to:

cmpxchg_relaxed(); smp_mb__after_atomic();
-> cmpxchg(); smp_mb__after_atomic();
-> smp_mb(); __cmpxchg(); __smp_mb(); smp_mb();

(where __cmpxchg() is some arch-internal cmpxchg implementation)

Will