Re: Replace pg_atomic_flag with pg_atomic_bool

On Mon, Jul 6, 2026 at 9:31 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>
> On 06/07/2026 18:05, Ashutosh Bapat wrote:
> > On Mon, Jul 6, 2026 at 3:57 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> >
> >>
> >> I think that's because I think of pg_atomic_flag as an atomic version of
> >> "bool". I'd assume there to be functions similar to the u32/u64
> >> functions, like pg_atomic_read_bool() and pg_atomic_write_bool(). But
> >> alas, neither of those functions exist. To read the value, you have to
> >> use pg_atomic_unlocked_test_flag(), and remember that when the flag is
> >> set, it returns *false*, which feels completely backwards to me. It
> >> makes more sense with pg_atomic_test_set_flag(), which returns 'false'
> >> if the flag was already set and hence was not set again, but it
> >> nevertheless just feels wrong to me.
> >>
> >
> > I am using pg_atomic_test_set_flag() as a non-wait locking, lighter
> > than LWLock mechanism in shared buffer resizing patches. That might
> > change in future. But I think there's use for TAS kind of semantics.
> > But I have also got confused when looking at it as pg_atomic_bool. I
> > think we need both.
>
> The functionality is still there with my patch, as the
> pg_atomic_exchange_bool() function. Would that work for you?

I think I can use it by replacing pg_atomic_test_set_flag(ab) by
!pg_atomic_exchange_bool(ab, true). I haven't tried it. Maybe just
define pg_atomic_test_set_flag(ab) as !pg_atomic_exchange_bool(ab,
true) to be readable? Right now, none of the callers of
pg_atomic_test_set_flag() check its return value (thus use it as TAS).
So maybe we can add pg_atomic_test_set_flag() as a separate patch.

--
Best Wishes,
Ashutosh Bapat