netdev - [net-next 5/6] igb: fix timecompare

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Date:	Thu, 13 Oct 2011 23:21:27 -0700
From:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:	davem@...emloft.net
Cc:	Jacob Keller <jacob.e.keller@...el.com>, netdev@...r.kernel.org,
	gospo@...hat.com, sassmann@...hat.com,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: [net-next 5/6] igb: fix timecompare_upate race condition

From: Jacob Keller <jacob.e.keller@...el.com>

This patch closes a possible race condition when timestamping using
the timecompare_update function as a method to detect clock skew of
the internal cycle counter. Because timecompare_update usually allows
skew detection no more than once a second, if ptpd or other software
performs a clock offset (for example, using the "date" command), there
is a small window of time where the clock skew will not match the
current kernel wall time. This patch forces the timecompare_update to
calculate skew every time we timestamp a packet, which removes the
possibility of this race condition.

Signed-off-by: Jacob E Keller <jacob.e.keller@...el.com>
Tested-by: Aaron Brown <aaron.f.brown@...el.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
---
 drivers/net/ethernet/intel/igb/igb_main.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index c10cc71..8f3296d 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -5581,7 +5581,15 @@ static void igb_systim_to_hwtstamp(struct igb_adapter *adapter,
 		regval <<= IGB_82580_TSYNC_SHIFT;
 
 	ns = timecounter_cyc2time(&adapter->clock, regval);
-	timecompare_update(&adapter->compare, ns);
+
+	/*
+	 * force timecompare_update to calculate the skew (even if
+	 * less than one second has passed since the last update) in
+	 * order to prevent the possibility that an offset has been
+	 * applied to the wall time. this ensures valid timestamps are
+	 * passed to the network stack.
+	 */
+	timecompare_update(&adapter->compare, 0);
 	memset(shhwtstamps, 0, sizeof(struct skb_shared_hwtstamps));
 	shhwtstamps->hwtstamp = ns_to_ktime(ns);
 	shhwtstamps->syststamp = timecompare_transform(&adapter->compare, ns);
-- 
1.7.6.4

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