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

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Message-ID: <20090624064236.GE31415@kernel.dk>
Date:	Wed, 24 Jun 2009 08:42:37 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jesper Dangaard Brouer <hawk@...x.dk>
Cc:	"David S. Miller" <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	dougthompson@...ssion.com, bluesmoke-devel@...ts.sourceforge.net,
	Patrick McHardy <kaber@...sh.net>,
	christine.caulfield@...glemail.com, Trond.Myklebust@...app.com,
	linux-wireless@...r.kernel.org, johannes@...solutions.net,
	yoshfuji@...ux-ipv6.org, shemminger@...ux-foundation.org,
	linux-nfs@...r.kernel.org, bfields@...ldses.org, neilb@...e.de,
	linux-ext4@...r.kernel.org, tytso@....edu, adilger@....com,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 09/10] cfq-iosched: Uses its own open-coded rcu_barrier.

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@...x.dk>
> ---
> 
>  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@...x.dk 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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