netdev - Re: [patch net-next-2.6] bonding: remove skb_share_check in handle

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Date:	Wed, 2 Mar 2011 13:29:58 +0100
From:	Jiri Pirko <jpirko@...hat.com>
To:	Andy Gospodarek <andy@...yhouse.net>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, fubar@...ibm.com,
	eric.dumazet@...il.com, nicolas.2p.debian@...il.com
Subject: Re: [patch net-next-2.6] bonding: remove skb_share_check in
 handle_frame

Wed, Mar 02, 2011 at 11:03:55AM CET, jpirko@...hat.com wrote:
>Tue, Mar 01, 2011 at 09:38:43PM CET, andy@...yhouse.net wrote:
>>On Tue, Mar 01, 2011 at 10:29:07AM +0100, Jiri Pirko wrote:
>>> Unapplicable, sorry (wrong branch :(). Here's corrected patch:
>>> 
>>> Subject: [PATCH net-next-2.6 v2] bonding: remove skb_share_check in handle_frame
>>> 
>>> No need to do share check here.
>>> 
>>> Signed-off-by: Jiri Pirko <jpirko@...hat.com>
>>> ---
>>>  drivers/net/bonding/bond_main.c |    3 ---
>>>  1 files changed, 0 insertions(+), 3 deletions(-)
>>> 
>>> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>>> index 584f97b..367ea60 100644
>>> --- a/drivers/net/bonding/bond_main.c
>>> +++ b/drivers/net/bonding/bond_main.c
>>> @@ -1498,9 +1498,6 @@ static struct sk_buff *bond_handle_frame(struct sk_buff *skb)
>>>  	struct net_device *slave_dev;
>>>  	struct net_device *bond_dev;
>>>  
>>> -	skb = skb_share_check(skb, GFP_ATOMIC);
>>> -	if (unlikely(!skb))
>>> -		return NULL;
>>>  	slave_dev = skb->dev;
>>>  	bond_dev = ACCESS_ONCE(slave_dev->master);
>>>  	if (unlikely(!bond_dev))
>>> -- 
>>> 1.7.3.4
>>> 
>>
>>Why did you decide to get rid of it here rather than the 3 places in the
>>bonding driver where it is currently needed?  I think this can cover
>>those cases since bond_handle_frame will be called after the ptype_all
>>handlers before any of the ptype handlers.
>
>I have already a patch prepared which converts bond ptype handlers into
>being called from bond_handle_frame. You are propably right that this
>should probably stay here.
>
>So please Dave, drop this patch for now. Thanks.

Thinking about this more I'm pretty convinced that skb_share_check is
not needed here.

If I got that correctly, skb_share_check is neede when user acually
modifies skb for his needs only. On the other hand, the only change
to skb is setting skb->dev and this change needs to be visible later on.
And given that skb is returned at the end of the function, changes are
never local (makes sense).

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