netdev - Re: [PATCH net-next 7/7] net: move skb->dropcount to skb->cb[]

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Date:	Thu, 26 Feb 2015 20:52:30 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Shmulik Ladkani <shmulik.ladkani@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 7/7] net: move skb->dropcount to skb->cb[]

On Thu, Feb 26, 2015 at 5:32 PM, Shmulik Ladkani
<shmulik.ladkani@...il.com> wrote:
> On Thu, 26 Feb 2015 12:48:14 +0200 Eyal Birger <eyal.birger@...il.com> wrote:
>> After giving it some additional thought and research, I don't think
>> this is necessary.
>>
>> The sizeof operator on struct sock_skb_cb would give a padded result
>> which would take care of the structure alignment.
>>
>> This is under the assumption that skb->cb[] size is of a proper multiple (which
>> I think is rather safe).
>
> Yes, you are correct.
>
> With the assumption that skb->cb and 'cb' size are properly aligned and
> sized (compile time asserted, btw?), you can simply:
>
> struct your_thing {     // name it
>         u32 dropcount;
> };
>
> struct sock_skb_cb {
>         /* protocol families specifc CBs */
>         u8 pf_reserved[PAD];
>         struct your_thing sock_cb;
> };
>
> where:
>
> #define PAD (FIELD_SIZEOF(struct sk_buff, cb) - sizeof(struct your_thing))
>

struct sock_skb_cb as I see it represents the sock generic metadata
required in the skb->cb[] and its placement at the end of skb->cb[]
is an implementation detail - so it, in my view, is what you referred to as
"struct your_thing".

Coding-wise I would rather avoid the additional enclosing struct and stick
with the proposed implementation.

Eyal.
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