netdev - Re: [RFC PATCH v2] ptr_ring: linked list fallback

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Date:   Wed, 28 Feb 2018 06:11:00 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH v2] ptr_ring: linked list fallback

On Wed, Feb 28, 2018 at 11:39:15AM +0800, Jason Wang wrote:
> 
> 
> On 2018年02月28日 11:28, Jason Wang wrote:
> > > > Well I believe the main user for this is qdisc, which use skb
> > > > array. And we
> > > > can not use what implemented in this patch directly for sk_buff
> > > > without some
> > > > changes on the data structure.
> > > Why not? skb has next and prev pointers at 1st two fields:
> > > 
> > > struct sk_buff {
> > >          union {
> > >                  struct {
> > >                          /* These two members must be first. */
> > >                          struct sk_buff          *next;
> > >                          struct sk_buff          *prev;
> > > ...
> > > }
> > > 
> > > so it's just a question of casting to struct plist.
> > 
> > Well, then the casting can only be done in skb_array implementation?
> 
> Ok, could be done in ptr ring. But still looks tricky, because of the
> different meaning of prev and last.
> 
> Thanks

It doesn't matter much - the meaning is up to whoever has the skb
queued. But sure - adding struct plist into the union is cleaner
and just as easy.