Re: [PATCH v1 2/4] nptl: Continue use arch prefered atomic exchange in spinlock loop

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Thu, Sep 29, 2022 at 9:35 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
> Hi Noah,
>
> Did you try building it both ways? I don't think this could ever compile:
>
> +  if (__glibc_likely (pthread_spin_lock_grab_lock (lock, &val, 1)))
>
> and:
>
> +#  define pthread_spin_lock_grab_lock(mem, val, c) \
> +    atomic_compare_exchange_weak_acquire (lock, &val, 1))
>
> The define uses 'lock' and 'mem' inconsistently and the use of the macro
> expands into &&val...
>
> Apart from that there is the question whether we should keep the weird
> ATOMIC_EXCHANGE_USES_CAS setting - I have removed it in my atomic
> patch series since most targets appear confused as to what it means (so
> are likely to have the wrong setting). Also there is no evidence it is actually
> faster. So using exchange in both cases is easier (and less error prone!).
>
> Also you do realize that no matter how much you change this code, it
> won't make a difference on x86, right?

Why's that?
>
> Cheers,
> Wilco