Re: [PATCH v5 1/3] refcount: Create unchecked atomic_t implementation

[Peter Zijlstra]

On Wed, May 31, 2017 at 10:45:09AM +0000, Reshetova, Elena wrote:

> > +static inline __must_check bool refcount_add_not_zero(unsigned int i,
> > +
> > 	      refcount_t *r)
> > +{
> > +	return atomic_add_return(i, &r->refs) != 0;
> > +}
> 
> Maybe atomic_add_unless(&r->refs, i, 0) in order to be consistent with the below inc_not_zero implementation?

Yes, atomic_add_return() is strictly incorrect here since the add is
unconditional.

> > +static inline __must_check bool refcount_sub_and_test(unsigned int i,
> > +
> > 	      refcount_t *r)
> > +{
> > +	return atomic_sub_return(i, &r->refs) == 0;
> > +}
> 
> Any reason for not using atomic_sub_and_test() here?

> > +static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> > +{
> > +	return atomic_dec_return(&r->refs) == 0;
> > +}
> 
> Same here: atomic_dec_and_test()?

Both those are better because they return condition codes generated from
the operand itself.