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

Peter Zijlstra
2015-07-14T10:58:42Z

On Tue, Jul 14, 2015 at 11:32:20AM +0100, Will Deacon wrote:
> 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.

#ifndef atomic_add_return_relaxed

#define atomic_add_return_relaxed atomic_add_return
/*
* If one cannot define a more relaxed version,
* acquire/release are out the window too.
*/
#define atomic_add_return_acquire atomic_add_return
#define atomic_add_return_release atomic_add_return

#else /* relaxed */

#ifndef atomic_add_return_acquire
#define atomic_add_return_acquire(args...) \
do { \
atomic_add_return_relaxed(args); \
smp_mb__after_atomic(); \
} while (0)
#endif

#ifndef atomic_add_return_release
#define atomic_add_return_release(args...) \
do { \
smp_mb__before_atomic(); \
atomic_add_return_relaxed(args); \
} while (0)
#endif

#endif /* relaxed */