netdev - [IGMP Discuss] Should we use one lock for struct ip_mc

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Date:   Thu, 8 Sep 2022 11:40:21 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Liu Jian <liujian56@...wei.com>,
        Taehee Yoo <ap420073@...il.com>, Jiri Benc <jbenc@...hat.com>
Subject: [IGMP Discuss] Should we use one lock for struct ip_mc_list ?

Hi,

When reviewing commit 23d2b94043ca ("igmp: Add ip_mc_list lock in
ip_check_mc_rcu"). Jiri pointed that struct ip_mc_list is protected by
different lock.

In function ip_check_mc_rcu() and ip_mc_del_src(), the struct ip_mc_list
in in_dev->mc_list is protected by the lock of struct ip_mc_list itself.

But in function igmpv3_send_cr, the ip_mc_list in in_dev->mc_tomb is
protected by in_dev->mc_tomb_lock.

This is no clear doc about what is protected by ip_mc_list->lock.

Is it OK for a single field that be protected by different locks?

Taehee did an update for MLD by using one mc_lock in commit 63ed8de4be81
("mld: add mc_lock for protecting per-interface mld data"). Should we also
do this on IGMP?

Thanks
Hangbin