Re: [dpdk-dev] [PATCH] net: do not insert VLAN tag to shared mbufs

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Apr 16, 2019 at 04:51:26PM +0100, Ferruh Yigit wrote:
> The vlan_insert() is buggy when it tires to handle the shared mbufs,
> instead don't support inserting VLAN tag into shared mbufs and return
> an error for that case.
> 
> Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
> ---
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: Chas Williams <chas3@att.com>
> 
> This is another approach to RFC to fix the vlan_insert:
> https://patches.dpdk.org/patch/51870/
> 
> vlan_insert() mostly used by drivers to insert VLAN tag into packet
> data in Tx path, drivers creating new copies of mbufs in Tx path may
> result unexpected behavior, like not freed or double freed mbufs.
> ---
>  lib/librte_net/rte_ether.h | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
>
So what is the API to be used if one does want to insert a vlan tag into a
shared mbuf?

Also, why is it such a problem to create new copies of data inside the
driver if that is necessary? You create a copy and use that, freeing the
original (i.e. in all likelyhood decrememting the ref-count since you no
longer use it). You already have the pointer to the mbuf pool from the
original buffer so you can get a copy from the same place. I'm curious to
know why it would be impossible to do a functionally correct
implementation?

/Bruce