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

From: Jens Axboe <[email-masked]>
Date: Wed, 24 Jun 2009 08:42:37 +0200

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 <[email-masked]>
> ---
> 
>  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
> +	 *
> +	 * [email-masked] 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

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