Re: [PATCH v4 1/7] sched/fair: Provide u64 read for 32-bits arch helper

[Vincent Donnefort]

[...]

> > +#ifdef CONFIG_64BIT
> > +# define u64_u32_load_copy(var, copy)       var
> > +# define u64_u32_store_copy(var, copy, val) (var = val)
> > +#else
> > +# define u64_u32_load_copy(var, copy)					\
> > +({									\
> > +	u64 __val, __val_copy;						\
> > +	do {								\
> > +		__val_copy = copy;					\
> > +		/*							\
> > +		 * paired with u64_u32_store, ordering access		\
> > +		 * to var and copy.					\
> > +		 */							\
> > +		smp_rmb();						\
> > +		__val = var;						\
> > +	} while (__val != __val_copy);					\
> > +	__val;								\
> > +})
> > +# define u64_u32_store_copy(var, copy, val)				\
> > +do {									\
> > +	typeof(val) __val = (val);					\
> > +	var = __val;							\
> > +	/*								\
> > +	 * paired with u64_u32_load, ordering access to var and		\
> > +	 * copy.							\
> > +	 */								\
> > +	smp_wmb();							\
> > +	copy = __val;							\
>
> `copy = __val;` should be `copy = var`.
>
> If var equal to val we do not need to do store. Check this condition
> in the above macro to avoid a redundant store.
>
>   if (var != __val)
>     var = __val;

Judging by the users of this macro, var = val is very much unlikely to
happen. Also, I don't think we want to waste a if here.

[...]