netdev - Re: [PATCH net-next 3/5] be2net: fix erx->rx_drops_no

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Date:	Mon, 22 Aug 2011 13:43:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sathya Perla <sathya.perla@...lex.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next 3/5] be2net: fix erx->rx_drops_no_frags wrap
 around

Le lundi 22 août 2011 à 16:43 +0530, Sathya Perla a écrit :
> The rx_drops_no_frags HW counter for RSS rings is 16bits in HW and can
> wraparound often. Maintain a 32-bit accumulator in the driver to prevent
> frequent wraparound.
> 
> Signed-off-by: Sathya Perla <sathya.perla@...lex.com>
> ---
>  drivers/net/ethernet/emulex/benet/be_main.c |   21 ++++++++++++++++++---
>  1 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/emulex/benet/be_main.c b/drivers/net/ethernet/emulex/benet/be_main.c
> index 2375c0c..4e588d8 100644
> --- a/drivers/net/ethernet/emulex/benet/be_main.c
> +++ b/f
> @@ -378,6 +378,17 @@ static void populate_lancer_stats(struct be_adapter *adapter)
>  				pport_stats->rx_drops_too_many_frags_lo;
>  }
>  
> +static void accumulate_16bit_val(u32 *acc, u16 val)
> +{
> +#define lo(x)			(x & 0xFFFF)
> +#define hi(x)			(x & 0xFFFF0000)
> +	bool wrapped = val < lo(*acc);
> +
> +	*acc = hi(*acc) + val;
> +	if (wrapped)
> +		*acc += 65536;
> +}
> +
>  

This adds a race for lockless SNMP readers (in be_get_stats64())

They can see the 32bit value going backward.

You have to be very careful to read the *acc once, and write it once.




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