dpif-netdev: Fix time delta overflow in case of race for meter lock.

There is a race window between getting the time and getting the meter
lock.  This could lead to situation where the thread with larger
current time (this thread called time_{um}sec() later than others)
will acquire meter lock first and update meter->used to the large
value.  Next threads will try to calculate time delta by subtracting
the large meter->used from their lower time getting the negative value
which will be converted to a big unsigned delta.

Fix that by assuming that all these threads received packets in the
same time in this case, i.e. dropping negative delta to 0.

CC: Jarno Rajahalme <jarno@ovn.org>
Fixes: 4b27db6 ("dpif-netdev: Simple DROP meter implementation.")
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2019-September/363126.html
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>

lib/dpif-netdev.c

@@ -5066,6 +5066,14 @@ dp_netdev_run_meter(struct dp_netdev *dp, struct dp_packet_batch *packets_,
     /* All packets will hit the meter at the same time. */
     long_delta_t = now / 1000 - meter->used / 1000; /* msec */
 
+    if (long_delta_t < 0) {
+        /* This condition means that we have several threads fighting for a
+           meter lock, and the one who received the packets a bit later wins.
+           Assuming that all racing threads received packets at the same time
+           to avoid overflow. */
+        long_delta_t = 0;
+    }
+
     /* Make sure delta_t will not be too large, so that bucket will not
      * wrap around below. */
     delta_t = (long_delta_t > (long long int)meter->max_delta_t)