Re: [PATCH net v3] net/mlx5e: Fix race in mlx5e_sw_stats and mlx5e_vport_stats

[David Miller <davem@davemloft.net>]

From: Martin KaFai Lau <kafai@fb.com>
Date: Thu, 20 Apr 2017 21:40:12 -0700

> 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>

Applied, thanks Martin.