netdev - Re: [PATCH net-next 02/13] sk_buff: add skb extension infrastructure

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Message-ID: <20181213114431.5m33avk5eqn7s2yp@breakpoint.cc>
Date:   Thu, 13 Dec 2018 12:44:31 +0100
From:   Florian Westphal <fw@...len.de>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        cpaasch@...le.com, peter.krystad@...el.com,
        mathew.j.martineau@...ux.intel.com
Subject: Re: [PATCH net-next 02/13] sk_buff: add skb extension infrastructure

Eric Dumazet <eric.dumazet@...il.com> wrote:
> On 12/13/2018 03:03 AM, Florian Westphal wrote:
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> >> So, cloning would do an refcount_inc(), and deleting the extension would do an refcount_dec_and_test() ?
> >>
> >> That is what I called an extra pair of atomic operations.
> > 
> > If it replaces 1:1 current mptcp skb->private out-of-tree storage, then
> > yes.
> > 
> 
> One day I will write a book on the number of atomic operations done on a TCP sendmsg() system call :/

Just to clarify: mptcp skb->private out-of-tree storage is maintened
using atomic_ops too, so if its converted 1:1 nothing changes in the
*mptcp tree*.

If its possible to re-arrange the out-of-tree so that it doesn't need
that (only used while skb is not cloned), then the same applies to the
extension conversion.

So this, as far as *this series* is concerened, a "future problem" that
may or may not exist.