netdev - Re: [PATCH net-next 2/7] rds: hold a sock ref from rds_message to the rds

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Date:   Sun, 28 Jan 2018 14:51:48 +0100
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        santosh.shilimkar@...cle.com
Subject: Re: [PATCH net-next 2/7] rds: hold a sock ref from rds_message to the rds_sock

On Thu, Jan 25, 2018 at 4:35 PM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> On (01/25/18 15:44), Willem de Bruijn wrote:
>> > This patch manages the m_rs assignment in the rds_message with
>> > the necessary refcount book-keeping.
>>
>> You may alos be able to do the same as tcp zerocopy and
>> hold an sk reference on the notification skb.
>
> We tether the notification skb to the rds socket after the
> refcount on the rds_message goes to zero, so we already have a
> ref from the sk to the notification skb,
>
> If we kept a refcount of all notification skb's (even the ones that
> are not ready to be unpinned yet) on the sk, then we have additional
> complexity trying to figure out which skb's are ready for notification
> at any point, so not sure it would make things simpler..

I don't quite follow. Every notification skb is created when pages refcount
is increased. It persists until at least rds_rm_zerocopy_callback, after data
skb has been freed and pages refcount has been decreased.

In this callback, skb is consumed if another skb is already queued on
the error queue, otherwise it is queued itself. It needs to hold a sock ref
until it can be queued.