netdev - Re: [PATCH 4/8] ethoc: prevent overflow of rx counter

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Message-ID: <1290616213.3013.600.camel@localhost>
Date:	Wed, 24 Nov 2010 16:30:13 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jonas Bonn <jonas@...thpole.se>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 4/8] ethoc: prevent overflow of rx counter

On Wed, 2010-11-24 at 14:40 +0100, Jonas Bonn wrote:
> Rewind cur_rx to prevent it from overflowing.
> 
> Signed-off-by: Jonas Bonn <jonas@...thpole.se>
> ---
>  drivers/net/ethoc.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/ethoc.c b/drivers/net/ethoc.c
> index 53c03f2..7d1b5d8 100644
> --- a/drivers/net/ethoc.c
> +++ b/drivers/net/ethoc.c
> @@ -408,6 +408,9 @@ static int ethoc_rx(struct net_device *dev, int limit)
>  	struct ethoc *priv = netdev_priv(dev);
>  	int count;
>  
> +	/* Prevent overflow of priv->cur_rx by rewinding it */	
> +	priv->cur_rx = priv->cur_rx % priv->num_rx;
> +

Division is expensive; you should either use masking (if priv->num_rx is
guaranteed to be a power of 2) or check for overflow whenever you
increment priv->cur_rx:

	if (++priv->cur_rx == priv->num_rx)
		priv->cur_rx = 0;

Ben.

>  	for (count = 0; count < limit; ++count) {
>  		unsigned int entry;
>  		struct ethoc_bd bd;

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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