skb free patches

skb free patches

[Martin Townsend]

Hi Alex,

I've prepared the patches for freeing skb outside of the decompress 
routines but now I'm unsure whether this is the best thing to do.

Plus side
it makes the decompression routine neater, no goto's, just return an 
error code.
Centralises the freeing of the skb.
Easier to maintain, future decompression routines don't have to worry 
about making sure all error paths have freed the skb.

Negative side
Decompression routines musn't reallocate the skb, ie make a copy. The 
skb pointer that is passed in must still be valid on return otherwise we 
get a double free.  I think skb_cow is ok as it calls pskb_expand_head 
which just makes more room in the data but if a routine was to use 
skb_copy then I think we would be in trouble.

Maybe decompression routines should only be using routines like skb_cow 
as they are more effecient.  What are your thoughts before I send the 
series?


- Martin.

Re: skb free patches

[Alexander Aring]

Hi Martin,

sorry, I didn't notice this mail.

On Tue, Nov 04, 2014 at 01:47:55PM +0000, Martin Townsend wrote:
> Hi Alex,
> 
> Any thoughts on this?
> 

go ahead and just send patches. For the future I want to change the
802.15.4 6LoWPAN to a common solution. We should do it like 802.15.4
parsing. Not like current mainline, we should do it like frame parsing
in the rework branch [0]. I stole this kind of frame parsing mechanism
from mac80211. For adaptation in 6LoWPAN just replace the 802.15.4 frame
type to the 6LoWPAN dispatch value and you will get something like this
[1].

Nevertheless your solution to have a kfree_skb outside of
iphc_decompress is welcome here. Because the CALL_RXH function returns
on failure 'RX_DROP_UNUSABLE' and hits the handler result function [3]
which runs kfree_skb there.


For now to prepare this outside I will ack this patch. But we are not
alone anymore this needs also an ack from Jukka Rissanen. Check out:

./scripts/get_maintainer.pl -f net/6lowpan

This architecture change need to be acked from both L2 branches which
use 6LoWPAN. Please tag your patch with "6lowpan: $MY_GREAT_CHANGE" and
send it to linux-wpan and linux-bluetooth mailinglist, you can get all
cc members with:

./scripts/get_maintainer.pl -f net/6lowpan

or

./scripts/get_maintainer.pl $MY_GREAT_PACH_FILE

- Alex

[0] https://github.com/linux-wpan/linux-wpan-next/blob/wpan_rework_rfc/net/mac802154/rx.c
[1] https://github.com/linux-wpan/linux-wpan-next/blob/wpan_rework_rfc/net/ieee802154/6lowpan/rx.c#L174
[3] https://github.com/linux-wpan/linux-wpan-next/blob/wpan_rework_rfc/net/ieee802154/6lowpan/rx.c#L27