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Subject: Re: [PATCH] net: 8390: Fix possible data races in __ei_get_stats
To:     Jia-Ju Bai <baijiaju1990@gmail.com>, davem@davemloft.net,
        fthain@telegraphics.com.au, joe@perches.com
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20180507140809.28847-1-baijiaju1990@gmail.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <e48345b4-fe9f-d267-777b-6a51baba0307@gmail.com>
Date:   Mon, 7 May 2018 07:15:07 -0700
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On 05/07/2018 07:08 AM, Jia-Ju Bai wrote:
> The write operations to "dev->stats" are protected by 
> the spinlock on line 862-864, but the read operations to
> this data on line 858 and 867 are not protected by the spinlock.
> Thus, there may exist data races for "dev->stats".
> 
> To fix the data races, the read operations to "dev->stats" are 
> protected by the spinlock, and a local variable is used for return.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
>  drivers/net/ethernet/8390/lib8390.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/8390/lib8390.c b/drivers/net/ethernet/8390/lib8390.c
> index c9c55c9eab9f..198952247d30 100644
> --- a/drivers/net/ethernet/8390/lib8390.c
> +++ b/drivers/net/ethernet/8390/lib8390.c
> @@ -852,19 +852,25 @@ static struct net_device_stats *__ei_get_stats(struct net_device *dev)
>  	unsigned long ioaddr = dev->base_addr;
>  	struct ei_device *ei_local = netdev_priv(dev);
>  	unsigned long flags;
> +	struct net_device_stats *stats;
> +
> +	spin_lock_irqsave(&ei_local->page_lock, flags);
>  
>  	/* If the card is stopped, just return the present stats. */
> -	if (!netif_running(dev))
> -		return &dev->stats;
> +	if (!netif_running(dev)) {
> +		stats = &dev->stats;
> +		spin_unlock_irqrestore(&ei_local->page_lock, flags);
> +		return stats;
> +	}
>  
> -	spin_lock_irqsave(&ei_local->page_lock, flags);
>  	/* Read the counter registers, assuming we are in page 0. */
>  	dev->stats.rx_frame_errors  += ei_inb_p(ioaddr + EN0_COUNTER0);
>  	dev->stats.rx_crc_errors    += ei_inb_p(ioaddr + EN0_COUNTER1);
>  	dev->stats.rx_missed_errors += ei_inb_p(ioaddr + EN0_COUNTER2);
> +	stats = &dev->stats;
>  	spin_unlock_irqrestore(&ei_local->page_lock, flags);
>  
> -	return &dev->stats;
> +	return stats;
>  }
>  
>  /*
> 

dev->stats is not a pointer, it is an array embedded in the 
struct net_device

So this patch is not needed, since dev->stats can not change.