[PATCH net-next] tun: avoid unnecessary READ_ONCE in tun_net_xmit

[PATCH net-next] tun: avoid unnecessary READ_ONCE in tun_net_xmit

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.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@google.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))
-- 
2.15.1.424.g9478a66081-goog

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

[Jason Wang]

On 2017年12月06日 11:11, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.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@google.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@redhat.com>

Thanks

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

[David Miller]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Tue,  5 Dec 2017 22:11:17 -0500

> From: Willem de Bruijn <willemb@google.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@google.com>

Applied, thanks Willem.