On Mon, Jul 27, 2020 at 09:49:25PM +0800, Boqun Feng wrote:
[...]
> So C11 features are available to gnu89 as extensions, also I tried to
> compile the following code with -std=gnu89:
>
> #include <stdio.h>
>
> typedef struct {
> int a;
> } atomic_t;
>
> void g(void) {
> printf("this is g\n");
> }
>
> void h(void) {
> printf("this is f\n");
> }
>
> #define gen(x) _Generic((x), atomic_t : h(), int : g())
>
> int main(void) {
> int a;
> atomic_t b;
> gen(a);
> gen(b);
> gen(b);
> }
>
> , and it worked.
>
> Besides, please note that in include/linux/compiler_types.h, _Generic is
> already used.
> > > With these primitives introduced, you can avoid () to add those
> > > rcu_segcblist_*_seglen() which have #ifdefs in them. Of course, an
> > > alternative would be that we implement rcu_segcblist_*_seglen() using
> > > _Generic, but I think someone else may have the similar problems or
> > > requirement (already or in the future), so it might be worthwhile to
> > > introduce the gen_ primitives for broader usage.
> >
> > One issue is code using memory barriers around the operation, such as
> > in rcu_segcblist_add_len() where you use smp_mb__before_atomic() for
> > the atomic version, and regular smp_mb() for the non-atomic version.
> > So ifdef will still exist to some degree.
> >
>
> Right, I think we can have two functions: long_add_mb() and
> atomic_long_add_mb(), this part is similar to ifdef approach, but we can
> make a gen_long_add_mb() based on these two functions, and
> gen_long_add_mb() simply switches between those functions according to
> the actual type of the field, which I think is better than ifdef
> approach at readability and maintenance.
Thanks for clarification. I agree with your idea, would you be able to write
a patch to add the helpers my patch can use?
If others are Ok with your idea, I can go ahead and use your helpers.
(I could write the helpers myself as well, next time I send the patch).
thanks,
- Joel
