[ovs-dev] [PATCH net] openvswitch: meter: fix race when getting now_ms.

[Tao Liu <thomas.liu@ucloud.cn>]

We have observed meters working unexpected if traffic is 3+Gbit/s
with multiple connections.

now_ms is not pretected by meter->lock, we may get a negative
long_delta_ms when another cpu updated meter->used, then:
    delta_ms = (u32)long_delta_ms;
which will be a large value.

    band->bucket += delta_ms * band->rate;
then we get a wrong band->bucket.

Fixes: 96fbc13d7e77 ("openvswitch: Add meter infrastructure")
Signed-off-by: Tao Liu <thomas.liu@ucloud.cn>
---
 net/openvswitch/meter.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/openvswitch/meter.c b/net/openvswitch/meter.c
index 96b524c..c50ab7f 100644
--- a/net/openvswitch/meter.c
+++ b/net/openvswitch/meter.c
@@ -593,7 +593,7 @@ static int ovs_meter_cmd_del(struct sk_buff *skb, struct genl_info *info)
 bool ovs_meter_execute(struct datapath *dp, struct sk_buff *skb,
 		       struct sw_flow_key *key, u32 meter_id)
 {
-	long long int now_ms = div_u64(ktime_get_ns(), 1000 * 1000);
+	long long int now_ms;
 	long long int long_delta_ms;
 	struct dp_meter_band *band;
 	struct dp_meter *meter;
@@ -610,6 +610,7 @@ bool ovs_meter_execute(struct datapath *dp, struct sk_buff *skb,
 	/* Lock the meter while using it. */
 	spin_lock(&meter->lock);
 
+	now_ms = div_u64(ktime_get_ns(), 1000 * 1000);
 	long_delta_ms = (now_ms - meter->used); /* ms */
 
 	/* Make sure delta_ms will not be too large, so that bucket will not
-- 
1.8.3.1