net/mlx5e: Fix race in mlx5e_sw_stats and mlx5e_vport_stats

We have observed a sudden spike in rx/tx_packets and rx/tx_bytes
reported under /proc/net/dev.  There is a race in mlx5e_update_stats()
and some of the get-stats functions (the one that we hit is the
mlx5e_get_stats() which is called by ndo_get_stats64()).

In particular, the very first thing mlx5e_update_sw_counters()
does is 'memset(s, 0, sizeof(*s))'.  For example, if mlx5e_get_stats()
is unlucky at one point, rx_bytes and rx_packets could be 0.  One second
later, a normal (and much bigger than 0) value will be reported.

This patch is to use a 'struct mlx5e_sw_stats temp' to avoid
a direct memset zero on priv->stats.sw.

mlx5e_update_vport_counters() has a similar race.  Hence, addressed
together.  However, memset zero is removed instead because
it is not needed.

I am lucky enough to catch this 0-reset in rx multicast:
eth0: 41457665   76804   70    0    0    70          0     47085 15586634   87502    3    0    0     0       3          0
eth0: 41459860   76815   70    0    0    70          0     47094 15588376   87516    3    0    0     0       3          0
eth0: 41460577   76822   70    0    0    70          0         0 15589083   87521    3    0    0     0       3          0
eth0: 41463293   76838   70    0    0    70          0     47108 15595872   87538    3    0    0     0       3          0
eth0: 41463379   76839   70    0    0    70          0     47116 15596138   87539    3    0    0     0       3          0

v2: Remove memset zero from mlx5e_update_vport_counters()
v1: Use temp and memcpy

Fixes: 9218b44dcc05 ("net/mlx5e: Statistics handling refactoring")
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Suggested-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx5/core/en_main.c

@@ -174,7 +174,7 @@ unlock:
 
 static void mlx5e_update_sw_counters(struct mlx5e_priv *priv)
 {
-	struct mlx5e_sw_stats *s = &priv->stats.sw;
+	struct mlx5e_sw_stats temp, *s = &temp;
 	struct mlx5e_rq_stats *rq_stats;
 	struct mlx5e_sq_stats *sq_stats;
 	u64 tx_offload_none = 0;
@@ -229,6 +229,7 @@ static void mlx5e_update_sw_counters(struct mlx5e_priv *priv)
 	s->link_down_events_phy = MLX5_GET(ppcnt_reg,
 				priv->stats.pport.phy_counters,
 				counter_set.phys_layer_cntrs.link_down_events);
+	memcpy(&priv->stats.sw, s, sizeof(*s));
 }
 
 static void mlx5e_update_vport_counters(struct mlx5e_priv *priv)
@@ -243,7 +244,6 @@ static void mlx5e_update_vport_counters(struct mlx5e_priv *priv)
 	MLX5_SET(query_vport_counter_in, in, op_mod, 0);
 	MLX5_SET(query_vport_counter_in, in, other_vport, 0);
 
-	memset(out, 0, outlen);
 	mlx5_cmd_exec(mdev, in, sizeof(in), out, outlen);
 }