netdev - Re: [PATCH net-next v4 5/6] lib: Ensure EWMA does not store wrong intermediate values

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Date:	Thu, 16 Jan 2014 22:25:22 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Michael Dalton <mwdalton@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jason Wang <jasowang@...hat.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next v4 5/6] lib: Ensure EWMA does not store wrong
 intermediate values

On Thu, Jan 16, 2014 at 11:52:29AM -0800, Michael Dalton wrote:
> To ensure ewma_read() without a lock returns a valid but possibly
> out of date average, modify ewma_add() by using ACCESS_ONCE to prevent
> intermediate wrong values from being written to avg->internal.
> 
> Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Michael Dalton <mwdalton@...gle.com>

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  lib/average.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/average.c b/lib/average.c
> index 99a67e6..114d1be 100644
> --- a/lib/average.c
> +++ b/lib/average.c
> @@ -53,8 +53,10 @@ EXPORT_SYMBOL(ewma_init);
>   */
>  struct ewma *ewma_add(struct ewma *avg, unsigned long val)
>  {
> -	avg->internal = avg->internal  ?
> -		(((avg->internal << avg->weight) - avg->internal) +
> +	unsigned long internal = ACCESS_ONCE(avg->internal);
> +
> +	ACCESS_ONCE(avg->internal) = internal ?
> +		(((internal << avg->weight) - internal) +
>  			(val << avg->factor)) >> avg->weight :
>  		(val << avg->factor);
>  	return avg;
> -- 
> 1.8.5.2
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