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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out of date average, modify ewma_add() by using ACCESS_ONCE to prevent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>
This commit is contained in:
Michael Dalton 2014-01-16 22:23:29 -08:00 committed by David S. Miller
parent a953be53ce
commit 03144b5869
1 changed files with 4 additions and 2 deletions
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6
lib/average.c
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@ -53,8 +53,10 @@ EXPORT_SYMBOL(ewma_init);
*/ */
struct ewma *ewma_add(struct ewma *avg, unsigned long val) struct ewma *ewma_add(struct ewma *avg, unsigned long val)
{ {
avg->internal = avg->internal ? unsigned long internal = ACCESS_ONCE(avg->internal);
(((avg->internal << avg->weight) - avg->internal) +
ACCESS_ONCE(avg->internal) = internal ?
(((internal << avg->weight) - internal) +
(val << avg->factor)) >> avg->weight : (val << avg->factor)) >> avg->weight :
(val << avg->factor); (val << avg->factor);
return avg; return avg;