netdev - Re: [PATCH net-next] mld: add missing rtnl_lock() in do_ipv6

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Message-ID: <634ed5a7-eccc-3749-e386-841141a30038@gmail.com>
Date:   Wed, 31 Mar 2021 01:02:34 +0900
From:   Taehee Yoo <ap420073@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] mld: add missing rtnl_lock() in
 do_ipv6_getsockopt()

On 3/31/21 12:40 AM, Eric Dumazet wrote:
 > This seems a serious regression compared to old code (in net tree)
 >
 > Have you added RTNL requirement in all this code ?
 >
 > We would like to use RTNL only if strictly needed.

Yes, I agree with you.
This patchset actually relies on existed RTNL, which is 
setsockopt_needs_rtnl().
And remained RTNL was replaced by mc_lock.
So, this patchset actually doesn't add new RTNL except in this case.

Fortunately, I think It can be replaced by RCU because,
1. ip6_mc_msfget() doesn't need the sleepable functions.
2. It is not the write critical section.
So, RCU can be used instead of RTNL for ip6_mc_msfget().
How do you think about it?