Commit 03144b58 authored Jan 16, 2014 by Michael Dalton Committed by David S. Miller Jan 16, 2014

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out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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

parent a953be53

lib/average.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -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;