Re: [PATCH v5 bluetooth-next] 6lowpan: Use pskb_expand_head in IPHC decompression.

[Alexander Aring <alex.aring@gmail.com>]

On Mon, Oct 13, 2014 at 10:28:44AM +0100, Martin Townsend wrote:
...
> >> If it's ok with everyone I'll change pskb_expand_head to skb_cow or skb_cow_head and leave the bluetooth code as it is with this v5 patch.
> > For me it's confusing, you do that on skb, but we never used skb
> > afterwards (maybe on freeing) but we use local_skb, the local_skb should
> > always be cloned because we running skb_clone before.
> skb_clone does ensure that the skb isn't shared anymore so I think it will be safe to remove the skb_share_check.  Or I can move it to the start of the function like we do for 802.15.4?

We need to clarify the following points:

skb_is_shared:

It's used twice in some packet handler functions or elsewhere.

now a skb_clone:

A skb clone is only a copy of struct sk_buff, so we can safely run
skb_pull and skb_push, skb->dev = foo. Simple for modify skb attributes.

now a skb_copy:

It's a complete private copy with private data buffer which allow us to
make skb->data[#] = foobar;



Now skb_share_check do:

if skb_is_shared true, then make a clone. This doesn't allow to
manipulate the data, but skb_cow make a copy of the private data so this
is okay for IPHC call. Means after skb_cow, we have something like
skb_copy, but skb_cow should be faster. ;-)

Also skb_share_check checks if skb_is_shared is false, then we don't
need a skb_clone, because it's not shared, then we are allow to
manipulate skb attributes, because nobody else use it.



I would add skb_share_check at beginning of this function like 802.15.4
but the IPV6 DISPATCH of bluetooth do a complete copy of the data buffer
which seems not necessary, but this is another issue and maybe I think
that the IPV6 DISPATCH value isn't needed by bluetooth 6lowpan. <-- But I
am not sure about this. I also can't see that bluetooth 6lowpan runs
skb_pull for the one byte dispatch value. Jukka need to check that, if
he has time and feel like to doing it.

- Alex