linux-kernel - Re: [PATCH] rtnetlink: Synchronze net in rtnl

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Message-ID: <8070c296-5767-6783-c34e-14112b2b412e@arista.com>
Date:   Mon, 25 Feb 2019 23:27:42 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, linux-kernel@...r.kernel.org
Cc:     0x7f454c46@...il.com, "David S. Miller" <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        netdev@...r.kernel.org
Subject: Re: [PATCH] rtnetlink: Synchronze net in rtnl_unregister()



On 2/25/19 11:21 PM, Dmitry Safonov wrote:
> Hi Eric,
> 
> On 2/25/19 11:09 PM, Eric Dumazet wrote:
>> On 02/25/2019 01:27 PM, Dmitry Safonov wrote:
>>> While it's possible to document that rtnl_unregister() requires
>>> synchronize_net() afterwards - unlike rtnl_unregister_all(), I believe
>>> the module exit is very much slow-path.
>>
>> rtnl_unregister_all() needs the sychronize_rcu() at this moment
>> because of the kfree(tab), not because of the kfree_rcu(link, rcu);
> 
> I may be wrong here, but shouldn't we wait for grace period to elapse by
> the reason that rtnl_msg_handlers are protected by RCU, not only by rtnl?
> Like, without synchronize_net() in rtnl_unregister() - what prevents
> module exit race to say, rtnetlink_rcv_msg()=>rtnl_get_link()?

There is synchronize_rcu() in free_module() - but I believe it's a bit
too far in unloading. Maybe, I'm missing another call on the way.

Thanks,
          Dmitry