netdev - [drivers/net/vxlan]Why rcu_read_lock is not obtained before rculist travelling

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Message-ID: <199cc09e.de3a.15a8508c1f7.Coremail.yanxb123@163.com>
Date:   Tue, 28 Feb 2017 22:03:48 +0800 (CST)
From:   颜小波 <yanxb123@....com>
To:     "Stephen Hemminger" <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org
Subject: [drivers/net/vxlan]Why rcu_read_lock is not obtained before rculist
 travelling

Hi Stephen,

I am studying vxlan device driver in 4.10 kernel. I see that vxlan_fdb in fdb_head list is rcu protected. call_rcu is invoked to free vxlan fdb, which will defer the vxlan_fdb_free until all rcu reads exist the race condition. 

But I don’t find any rcu_read_lock invoked before travelling fdb_head list.  In vxlan_xmit and vxlan_snoop function, vxlan_find_mac function is called to search the vxlan_fdb of the dst_mac or src_mac. Then information in vxlan_fdb  is used for further process.  But as no rcu_read_lock is obtained before the list travelling, I am wondering if it is possible that vxlan_fdb is freed when it is being used. 

static void vxlan_fdb_destroy(struct vxlan_dev *vxlan, struct vxlan_fdb *f)
{
       netdev_dbg(vxlan->dev,
                  "delete %pM\n", f->eth_addr);

       --vxlan->addrcnt;
       vxlan_fdb_notify(vxlan, f, first_remote_rtnl(f), RTM_DELNEIGH);

       hlist_del_rcu(&f->hlist);
       call_rcu(&f->rcu, vxlan_fdb_free);
}

Thanks

Xiaobo