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Date:   Thu, 3 Dec 2020 10:28:02 -0800
From:   Jakub Kicinski <>
To:     Joseph Huang <>
Cc:     Roopa Prabhu <>,
        Nikolay Aleksandrov <>,
        "David S. Miller" <>,
        <>, <>,
        Linus L├╝ssing <>
Subject: Re: [PATCH] bridge: Fix a deadlock when enabling multicast snooping

On Tue, 1 Dec 2020 16:40:47 -0500 Joseph Huang wrote:
> When enabling multicast snooping, bridge module deadlocks on multicast_lock
> if 1) IPv6 is enabled, and 2) there is an existing querier on the same L2
> network.
> The deadlock was caused by the following sequence: While holding the lock,
> br_multicast_open calls br_multicast_join_snoopers, which eventually causes
> IP stack to (attempt to) send out a Listener Report (in igmp6_join_group).
> Since the destination Ethernet address is a multicast address, br_dev_xmit
> feeds the packet back to the bridge via br_multicast_rcv, which in turn
> calls br_multicast_add_group, which then deadlocks on multicast_lock.
> The fix is to move the call br_multicast_join_snoopers outside of the
> critical section. This works since br_multicast_join_snoopers only deals
> with IP and does not modify any multicast data structures of the bridge,
> so there's no need to hold the lock.
> Fixes: 4effd28c1245 ("bridge: join all-snoopers multicast address")
> Signed-off-by: Joseph Huang <>

Nik, Linus - how does this one look?

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