netdev - Re: [PATCH RFC 1/2] tun: don't require serialization lock on tx

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Date:	Wed, 13 Apr 2016 11:48:09 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Paolo Abeni <pabeni@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kurz <gkurz@...ux.vnet.ibm.com>,
	Jason Wang <jasowang@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH RFC 1/2] tun: don't require serialization lock on tx

On 13.04.2016 11:41, Michael S. Tsirkin wrote:
> On Wed, Apr 13, 2016 at 11:04:46AM +0200, Paolo Abeni wrote:
>> The current tun_net_xmit() implementation don't need any external
>> lock since it relay on rcu protection for the tun data structure
>> and on socket queue lock for skb queuing.
>>
>> This patch set the NETIF_F_LLTX feature bit in the tun device, so
>> that on xmit, in absence of qdisc, no serialization lock is acquired
>> by the caller.
>>
>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>> ---
>>   drivers/net/tun.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index faf9297..42992dc 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1796,7 +1796,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>>   		dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
>>   				   TUN_USER_FEATURES | NETIF_F_HW_VLAN_CTAG_TX |
>>   				   NETIF_F_HW_VLAN_STAG_TX;
>> -		dev->features = dev->hw_features;
>> +		dev->features = dev->hw_features | NETIF_F_LLTX;
>
> the documentation says:
>          NETIF_F_LLTX_BIT,               /* LockLess TX - deprecated.  Please */
>                                          /* do not use LLTX in new drivers */

In networking/netdev-features.txt

  * LLTX driver (deprecated for hardware drivers)

NETIF_F_LLTX should be set in drivers that implement their own locking in
transmit path or don't need locking at all (e.g. software tunnels).
In ndo_start_xmit, it is recommended to use a try_lock and return
NETDEV_TX_LOCKED when the spin lock fails.  The locking should also properly
protect against other callbacks (the rules you need to find out).

Don't use it for new drivers.

I think this is documentation is correct and it is only deprecated for 
hardware drivers.

Bye,
Hannes