netdev - Re: Understanding mutual exclusion between rtnl_lock and rcu_read

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Date:   Fri, 3 Feb 2017 13:40:23 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Joel Cunningham <joel.cunningham@...com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Understanding mutual exclusion between rtnl_lock and rcu_read_lock

On Thu, Feb 2, 2017 at 6:05 PM, Joel Cunningham <joel.cunningham@...com> wrote:
>
> In the case of SIOCSIFHWADDR, we get a pointer to the net_device through __dev_get_by_name() and then pass it to dev_set_mac_address() to modify through ndo_set_mac_address().  I didn’t see any uses of RCU APIs on the writer side and that’s why I figured there was something going on with rtnl_lock() that I didn’t understand or that the dev_ioctl function wasn’t re-entrant from another CPU
>

You are right, that RCU read lock could merely protect the netdevice from
being unregistered concurrently, can't prevent a concurrent dev_ifsioc().

I don't know why Eric changed it to RCU read lock, it is not a hot path, using
rtnl lock is fine and can guarantee a atomic read.