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

Will Deacon
2015-07-14T11:08:11Z

On Tue, Jul 14, 2015 at 11:58:37AM +0100, Peter Zijlstra wrote:
> 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:
> > > 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 */

If a picture is worth a thousand words, a patch is worth at least a thousand
pictures.

I'll add this for v2.

Will