netdev - Re: [PATCH net-next 01/12] net: sched: flower: don't check for rtnl on head dereference

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Message-ID: <CAM_iQpXkjp3JVtY63kKvrmEkgBR7oUcO7vsDZhxMSUdF+CPONA@mail.gmail.com>
Date:   Fri, 1 Mar 2019 16:51:33 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 01/12] net: sched: flower: don't check for rtnl
 on head dereference

On Thu, Feb 28, 2019 at 10:35 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> On Thu 28 Feb 2019 at 00:49, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Tue, Feb 26, 2019 at 6:57 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >>
> >>
> >> On Mon 25 Feb 2019 at 22:39, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >> > On Mon, Feb 25, 2019 at 8:11 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >> >>
> >> >>
> >> >> On Fri 22 Feb 2019 at 19:32, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >> >> >
> >> >> > So if it is no longer RCU any more, why do you still use
> >> >> > rcu_dereference_protected()? That is, why not just deref it as a raw
> >> >> > pointer?
> >> >
> >> >
> >> > Any answer for this question?
> >>
> >> I decided that since there is neither possibility of concurrent pointer
> >> assignment nor deallocation of object that it points to, most performant
> >> solution would be using rcu_dereference_protected() which is the only
> >> RCU dereference helper that doesn't use READ_ONCE. I now understand that
> >> this is confusing (and most likely doesn't provide any noticeable
> >> performance improvement anyway!) and will change this patch to use
> >> rcu_dereference_raw() as you suggest.
> >
> > Yeah, please make sure sparse is happy with that. :)
>
> I checked my flower change with sparse. It produced a lot of warnings,
> some of which are several years old. None in the code I changed though:

If so, we can address this later, it is not urgent.