netdev - Re: [PATCH net-next] tun: replace get_cpu_ptr with this_cpu

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Date:   Mon, 10 Dec 2018 13:04:47 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Prashant Bhole <bhole_prashant_q7@....ntt.co.jp>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Subject: Re: [PATCH net-next] tun: replace get_cpu_ptr with this_cpu_ptr when
 bh disabled

On Mon, Dec 10, 2018 at 02:06:18PM +0900, Prashant Bhole wrote:
> tun_xdp_one() runs with local bh disabled. So there is no need to
> disable preemption by calling get_cpu_ptr while updating stats. This
> patch replaces the use of get_cpu_ptr() with this_cpu_ptr() as a
> micro-optimization. Also removes related put_cpu_ptr call.
> 
> Signed-off-by: Prashant Bhole <bhole_prashant_q7@....ntt.co.jp>

Seems to make sense but how about a comment here explaining
by it's safe?

> ---
>  drivers/net/tun.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 6760b86547df..abb750a9c9ca 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -2494,12 +2494,11 @@ static int tun_xdp_one(struct tun_struct *tun,
>  	skb_record_rx_queue(skb, tfile->queue_index);
>  	netif_receive_skb(skb);
>  
> -	stats = get_cpu_ptr(tun->pcpu_stats);
> +	stats = this_cpu_ptr(tun->pcpu_stats);
>  	u64_stats_update_begin(&stats->syncp);
>  	stats->rx_packets++;
>  	stats->rx_bytes += skb->len;
>  	u64_stats_update_end(&stats->syncp);
> -	put_cpu_ptr(stats);
>  
>  	if (rxhash)
>  		tun_flow_update(tun, rxhash, tfile);
> -- 
> 2.17.2
>