lib: Ensure EWMA does not store wrong intermediate values
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out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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 @@
*/
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;