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

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Date:   Tue, 02 Jul 2019 15:08:11 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     lucien.xin@...il.com
Cc:     netdev@...r.kernel.org, jon.maloy@...csson.com,
        ying.xue@...driver.com, tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net-next] tipc: use rcu dereference functions properly

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().