netdev - Re: [PATCH net-next] tun: avoid unnecessary READ_ONCE in tun_net

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Date:   Wed, 6 Dec 2017 11:39:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] tun: avoid unnecessary READ_ONCE in tun_net_xmit



On 2017年12月06日 11:11, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
>
> The statement no longer serves a purpose.
>
> Commit fa35864e0bb7 ("tuntap: Fix for a race in accessing numqueues")
> added the ACCESS_ONCE to avoid a race condition with skb_queue_len.
>
> Commit 436accebb530 ("tuntap: remove unnecessary sk_receive_queue
> length check during xmit") removed the affected skb_queue_len check.
>
> Commit 96f84061620c ("tun: add eBPF based queue selection method")
> split the function, reading the field a second time in the callee.
> The temp variable is now only read once, so just remove it.
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>   drivers/net/tun.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 787cc35ef89b..c2ad8f3858d1 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -990,14 +990,12 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>   	struct tun_struct *tun = netdev_priv(dev);
>   	int txq = skb->queue_mapping;
>   	struct tun_file *tfile;
> -	u32 numqueues = 0;
>   
>   	rcu_read_lock();
>   	tfile = rcu_dereference(tun->tfiles[txq]);
> -	numqueues = READ_ONCE(tun->numqueues);
>   
>   	/* Drop packet if interface is not attached */
> -	if (txq >= numqueues)
> +	if (txq >= tun->numqueues)
>   		goto drop;
>   
>   	if (!rcu_dereference(tun->steering_prog))

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks