netdev - Re: [PATCH net-next] tipc: use rcu dereference functions properly

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Date:   Wed, 3 Jul 2019 16:33:44 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     network dev <netdev@...r.kernel.org>,
        Jon Maloy <jon.maloy@...csson.com>,
        Ying Xue <ying.xue@...driver.com>,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net-next] tipc: use rcu dereference functions properly

On Wed, Jul 3, 2019 at 6:08 AM David Miller <davem@...emloft.net> wrote:
>
> From: Xin Long <lucien.xin@...il.com>
> Date: Tue,  2 Jul 2019 00:54:55 +0800
>
> > For these places are protected by rcu_read_lock, we change from
> > rcu_dereference_rtnl to rcu_dereference, as there is no need to
> > check if rtnl lock is held.
> >
> > For these places are protected by rtnl_lock, we change from
> > rcu_dereference_rtnl to rtnl_dereference/rcu_dereference_protected,
> > as no extra memory barriers are needed under rtnl_lock() which also
> > protects tn->bearer_list[] and dev->tipc_ptr/b->media_ptr updating.
> >
> > rcu_dereference_rtnl will be only used in the places where it could
> > be under rcu_read_lock or rtnl_lock.
> >
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
>
> In the cases where RTNL is held, even if rcu_read_lock() is also taken,
> we should use rtnl_dereference() because that avoids the READ_ONCE().
Right, that's what I did in this patch.

But for the places where it's sometimes called under rtnl_lock() only and
sometimes called under rcu_read_lock() only, like tipc_udp_is_known_peer()
and tipc_udp_rcast_add(), I kept rcu_dereference_rtnl(). makes sense?