[PATCH net-next v5 5/6] lib: Ensure EWMA does not store wrong intermediate values

[Michael Dalton <mwdalton@google.com>]

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out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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>
---
 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