On Wed, 5 Apr 2023 12:56:41 -0400, Alan Huang wrote:
> The original memory barriers in count_stat_eventual.c ensure
> writing to global_count happens before writing to stopflag and
> reading from stopflag happens before later reading from global_count.
>
> Thus, smp_load_acquire and smp_store_release will suffice.
>
> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
> ---
> CodeSamples/count/count_stat_eventual.c | 6 ++----
> count/count.tex | 6 +++---
> 2 files changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/CodeSamples/count/count_stat_eventual.c b/CodeSamples/count/count_stat_eventual.c
> index 967644de..7157ee0e 100644
> --- a/CodeSamples/count/count_stat_eventual.c
> +++ b/CodeSamples/count/count_stat_eventual.c
> @@ -51,8 +51,7 @@ void *eventual(void *arg) //\lnlbl{eventual:b}
> WRITE_ONCE(global_count, sum);
> poll(NULL, 0, 1);
> if (READ_ONCE(stopflag)) {
> - smp_mb();
> - WRITE_ONCE(stopflag, stopflag + 1);
> + smp_store_release(&stopflag, stopflag + 1);
> }
> }
> return NULL;
> @@ -73,9 +72,8 @@ void count_init(void) //\lnlbl{init:b}
> void count_cleanup(void) //\lnlbl{cleanup:b}
> {
> WRITE_ONCE(stopflag, 1);
> - while (READ_ONCE(stopflag) < 3)
> + while (smp_load_acquire(&stopflag) < 3)
> poll(NULL, 0, 1);
> - smp_mb();
> } //\lnlbl{cleanup:e}
> //\end{snippet}
>
> diff --git a/count/count.tex b/count/count.tex
> index 80ada104..8ab67e2e 100644
> --- a/count/count.tex
> +++ b/count/count.tex
> @@ -1013,9 +1013,9 @@ between passes.
>
> The \co{count_cleanup()} function on
> \clnrefrange{cleanup:b}{cleanup:e} coordinates termination.
> -The calls to \co{smp_mb()} here and in \co{eventual()} ensure
> -that all updates to \co{global_count} are visible to code following
> -the call to \co{count_cleanup()}.
> +The call to \co{smp_load_acquire()} here and the call to \co{smp_store_release()}
> +in \co{eventual()} ensure that all updates to \co{global_count} are visible
> +to code following the call to \co{count_cleanup()}.
>
> This approach gives extremely fast counter read-out while still
> supporting linear counter-update scalability.
Looks good to me! Thanks.
Reviewed-by: Akira Yokosawa <akiyks@xxxxxxxxx>
