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

["Michael S. Tsirkin" <mst@redhat.com>]

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 out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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@gmail.com>
> Signed-off-by: Michael Dalton <mwdalton@google.com>

Acked-by: Michael S. Tsirkin <mst@redhat.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