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 out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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@xxxxxxxxx>
> Signed-off-by: Michael Dalton <mwdalton@xxxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> 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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