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out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/average.c b/lib/average.c
index 99a67e662b3c..114d1beae0c7 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;