netdev - Re: [PATCH net v2] net/mlx5e: Fix race in mlx5e_sw_stats and mlx5e_vport

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Date:   Thu, 20 Apr 2017 17:00:13 +0300
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Martin KaFai Lau <kafai@...com>, Gal Pressman <galp@...lanox.com>
Cc:     Linux Netdev List <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net v2] net/mlx5e: Fix race in mlx5e_sw_stats and mlx5e_vport_stats

On Thu, Apr 20, 2017 at 2:32 AM, Martin KaFai Lau <kafai@...com> wrote:
> 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.
>
> 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
>
> Cc: Saeed Mahameed <saeedm@...lanox.com>
> Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Martin KaFai Lau <kafai@...com>
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index 66c133757a5e..246786bb861b 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -174,7 +174,7 @@ static void mlx5e_tx_timeout_work(struct work_struct *work)
>
>  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;
> @@ -229,12 +229,14 @@ 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)
>  {
> +       struct mlx5e_vport_stats temp;
>         int outlen = MLX5_ST_SZ_BYTES(query_vport_counter_out);
> -       u32 *out = (u32 *)priv->stats.vport.query_vport_out;
> +       u32 *out = (u32 *)temp.query_vport_out;
>         u32 in[MLX5_ST_SZ_DW(query_vport_counter_in)] = {0};
>         struct mlx5_core_dev *mdev = priv->mdev;
>
> @@ -245,6 +247,7 @@ static void mlx5e_update_vport_counters(struct mlx5e_priv *priv)
>
>         memset(out, 0, outlen);

Actually you don't need any temp here, it is safe to just remove this
redundant memset
and mlx5_cmd_exec will do the copy for you.

>         mlx5_cmd_exec(mdev, in, sizeof(in), out, outlen);
> +       memcpy(priv->stats.vport.query_vport_out, out, outlen);
>  }

Anyway we still need a spin lock here, and also for all the counters
under priv->stats which are affected by this race as well.

If you want I can accept this as a temporary  fix for net and Gal will
work on a spin lock based mechanism to fix the memcpy race for all the
counters.

>
>  static void mlx5e_update_pport_counters(struct mlx5e_priv *priv)
> --
> 2.9.3
>