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Date:   Thu, 3 Dec 2020 22:42:21 +0000
From:   "Huang, Joseph" <Joseph.Huang@...min.com>
To:     Nikolay Aleksandrov <nikolay@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>
CC:     Roopa Prabhu <roopa@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Lüssing <linus.luessing@...3.blue>
Subject: Re: [PATCH] bridge: Fix a deadlock when enabling multicast snooping

> From: Huang, Joseph
> Sent: Thursday, December 3, 2020 4:53 PM
> To: Nikolay Aleksandrov <nikolay@...dia.com>; Jakub Kicinski
> <kuba@...nel.org>
> Cc: Roopa Prabhu <roopa@...dia.com>; David S. Miller
> <davem@...emloft.net>; bridge@...ts.linux-foundation.org;
> netdev@...r.kernel.org; linux-kernel@...r.kernel.org; Linus Lüssing
> <linus.luessing@...3.blue>
> Subject: RE: [PATCH] bridge: Fix a deadlock when enabling multicast snooping
>
> > From: Nikolay Aleksandrov <nikolay@...dia.com>
> > Sent: Thursday, December 3, 2020 3:47 PM
> > To: Jakub Kicinski <kuba@...nel.org>; Huang, Joseph
> > <Joseph.Huang@...min.com>
> > Cc: Roopa Prabhu <roopa@...dia.com>; David S. Miller
> > <davem@...emloft.net>; bridge@...ts.linux-foundation.org;
> > netdev@...r.kernel.org; linux-kernel@...r.kernel.org; Linus Lüssing
> > <linus.luessing@...3.blue>
> > Subject: Re: [PATCH] bridge: Fix a deadlock when enabling multicast
> > snooping
> >
> > On 03/12/2020 20:28, Jakub Kicinski wrote:
> > > 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 <Joseph.Huang@...min.com>
> > >
> > > Nik, Linus - how does this one look?
> > >
> >
> > Hi,
> > Thanks, somehow I missed this one too. Need to check my email config. :) I
> > believe I see how it can happen, although it's not straight-forward to follow.
> > A selftest for this case would be great, and any traces (e.g. hung task)
> would
> > help a lot as well.
> > Correct me if I'm wrong but the sequence is something like:
> > br_multicast_join_snoopers -> ipv6_dev_mc_inc -> __ipv6_dev_mc_inc ->
> > igmp6_group_added
> > -> MLDv1 (mode) igmp6_join_group() -> Again MLDv1 mode
> > -> igmp6_join_group() -> igmp6_join_group
> > -> igmp6_send() on the bridge device -> br_dev_xmit and onto the bridge
> > -> mcast processing code
> > which uses the multicast_lock spinlock. Right?
>
> That is correct.
>
> Here's a stack trace from a typical run:
>
> echo -n 1 > /sys/devices/virtual/net/gmn0/bridge/multicast_snooping
> [  936.146754] rcu: INFO: rcu_preempt self-detected stall on CPU
> [  936.152534] rcu:   0-....: (5594 ticks this GP)
> idle=75a/1/0x4000000000000002 softirq=2787/2789 fqs=2625
> [  936.162026]        (t=5253 jiffies g=4205 q=12)
> [  936.166041] Task dump for CPU 0:
> [  936.169272] sh              R  running task        0  1315   1295 0x00000002
> [  936.176332] Call trace:
> [  936.178797]  dump_backtrace+0x0/0x140
> [  936.182469]  show_stack+0x14/0x20
> [  936.185793]  sched_show_task+0x108/0x138
> [  936.189727]  dump_cpu_task+0x40/0x50
> [  936.193313]  rcu_dump_cpu_stacks+0x94/0xd0
> [  936.197420]  rcu_sched_clock_irq+0x75c/0x9c0
> [  936.201698]  update_process_times+0x2c/0x68
> [  936.205893]  tick_sched_handle.isra.0+0x30/0x50
> [  936.210432]  tick_sched_timer+0x48/0x98
> [  936.214272]  __hrtimer_run_queues+0x110/0x1b0
> [  936.218635]  hrtimer_interrupt+0xe4/0x240
> [  936.222656]  arch_timer_handler_phys+0x30/0x40
> [  936.227106]  handle_percpu_devid_irq+0x80/0x140
> [  936.231654]  generic_handle_irq+0x24/0x38
> [  936.235669]  __handle_domain_irq+0x60/0xb8
> [  936.239774]  gic_handle_irq+0x5c/0x148
> [  936.243535]  el1_irq+0xb8/0x180
> [  936.246689]  queued_spin_lock_slowpath+0x118/0x3b0
> [  936.251495]  _raw_spin_lock+0x5c/0x68
> [  936.255221]  br_multicast_add_group+0x40/0x170 [bridge]
> [  936.260491]  br_multicast_rcv+0x7ac/0xe30 [bridge]
> [  936.265322]  br_dev_xmit+0x140/0x368 [bridge]
> [  936.269689]  dev_hard_start_xmit+0x94/0x158
> [  936.273876]  __dev_queue_xmit+0x5ac/0x7f8
> [  936.277890]  dev_queue_xmit+0x10/0x18
> [  936.281563]  neigh_resolve_output+0xec/0x198
> [  936.285845]  ip6_finish_output2+0x240/0x710
> [  936.290039]  __ip6_finish_output+0x130/0x170
> [  936.294318]  ip6_output+0x6c/0x1c8
> [  936.297731]  NF_HOOK.constprop.0+0xd8/0xe8
> [  936.301834]  igmp6_send+0x358/0x558
> [  936.305326]  igmp6_join_group.part.0+0x30/0xf0
> [  936.309774]  igmp6_group_added+0xfc/0x110
> [  936.313787]  __ipv6_dev_mc_inc+0x1a4/0x290
> [  936.317885]  ipv6_dev_mc_inc+0x10/0x18
> [  936.321677]  br_multicast_open+0xbc/0x110 [bridge]
> [  936.326506]  br_multicast_toggle+0xec/0x140 [bridge]
>
>
> >
> > One question - shouldn't leaving have the same problem? I.e.
> > br_multicast_toggle -> br_multicast_leave_snoopers
> > -> br_ip6_multicast_leave_snoopers -> ipv6_dev_mc_dec ->
> > -> igmp6_group_dropped -> igmp6_leave_group ->
> > MLDv1 mode && last reporter -> igmp6_send() ?
> >
> > I think it was saved by the fact that !br_opt_get(br,
> > BROPT_MULTICAST_ENABLED) would be true and the multicast lock won't
> be
> > acquired in the br_dev_xmit path? If so, I'd appreciate a comment about
> that
> > because it's not really trivial to find out. :)
>
> That's a really good point. Leave should have deadlocked as well, but when I
> tested the patch, I was able to turn on/off multicast snooping multiple times
> without any problem.
>
> Is it because this line in igmp6_leave_group?
>
>               if (ma->mca_flags & MAF_LAST_REPORTER)
>                       igmp6_send(&ma->mca_addr, ma->idev->dev,
>                               ICMPV6_MGM_REDUCTION);
>
> Perhaps MAF_LAST_REPORTER was not set, so igmp6_send was not called?
>
> >
> > Anyhow, the patch is fine as-is too:
> > Acked-by: Nikolay Aleksandrov <nikolay@...dia.com>
> >
> > Thanks,
> >  Nik
>
> Thanks,
> Joseph

Would it be advisable if we move br_multicast_leave_snoopers out of the critical
section as well? Even though I can't really verify that if this is helpful since I haven't
seen it deadlock when disabling multicast snooping.

Thanks,
Joseph

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