From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Dalton Subject: [PATCH net-next v5 5/6] lib: Ensure EWMA does not store wrong intermediate values Date: Thu, 16 Jan 2014 18:36:15 -0800 Message-ID: <1389926176-6699-6-git-send-email-mwdalton@google.com> References: <1389926176-6699-1-git-send-email-mwdalton@google.com> Cc: netdev@vger.kernel.org, Eric Dumazet , Rusty Russell , "Michael S. Tsirkin" , Jason Wang , Ben Hutchings , virtualization@lists.linux-foundation.org, Michael Dalton To: "David S. Miller" Return-path: Received: from mail-ob0-f202.google.com ([209.85.214.202]:36965 "EHLO mail-ob0-f202.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751557AbaAQCge (ORCPT ); Thu, 16 Jan 2014 21:36:34 -0500 Received: by mail-ob0-f202.google.com with SMTP id vb8so731416obc.5 for ; Thu, 16 Jan 2014 18:36:33 -0800 (PST) In-Reply-To: <1389926176-6699-1-git-send-email-mwdalton@google.com> Sender: netdev-owner@vger.kernel.org List-ID: To ensure ewma_read() without a lock returns a valid but possibly out of date average, modify ewma_add() by using ACCESS_ONCE to prevent intermediate wrong values from being written to avg-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 Acked-by: Michael S. Tsirkin Acked-by: Eric Dumazet Signed-off-by: Michael Dalton --- 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