Re: [PATCH 09/10] cfq-iosched: Uses its own open-coded rcu_barrier.

Jens Axboe
2009-06-24T06:42:35Z

On Tue, Jun 23 2009, Jesper Dangaard Brouer wrote:
> This module cfq-iosched, has discovered the value of waiting for
> call_rcu() completion, but its has its own open-coded implementation
> of rcu_barrier(), which I don't think is 'strong' enough.
>
> This patch only leaves a comment for the maintainers to consider.

We need a stronger primitive and rcu_barrier(), since we also need to
wait for the rcu calls to even be scheduled. So I don't think the below
can be improved, it's already fine.

>
> Signed-off-by: Jesper Dangaard Brouer <hawk-4UpuNZONu4c@public.gmane.org>
> ---
>
> block/cfq-iosched.c | 6 ++++++
> 1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 833ec18..c15555b 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2657,6 +2657,12 @@ static void __exit cfq_exit(void)
> /*
> * this also protects us from entering cfq_slab_kill() with
> * pending RCU callbacks
> + *
> + * hawk-4UpuNZONu4c@public.gmane.org 2009-06-18: Maintainer please consider using
> + * rcu_barrier() instead of this open-coded wait for
> + * completion implementation. I think it provides a better
> + * guarantee that all CPUs are finished, although
> + * elv_ioc_count_read() do consider all CPUs.
> */
> if (elv_ioc_count_read(ioc_count))
> wait_for_completion(&all_gone);
>

--
Jens Axboe