__ipv6_dev_mc_dec() internally uses sleepable functions so that caller
must not acquire atomic locks. But caller, which is addrconf_verify_rtnl()
acquires rcu_read_lock_bh().
So this warning occurs in the __ipv6_dev_mc_dec().

Test commands:
    ip netns add A
    ip link add veth0 type veth peer name veth1
    ip link set veth1 netns A
    ip link set veth0 up
    ip netns exec A ip link set veth1 up
    ip a a 2001:db8::1/64 dev veth0 valid_lft 2 preferred_lft 1

Splat looks like:
============================
WARNING: suspicious RCU usage
5.12.0-rc6+ #515 Not tainted
-----------------------------
kernel/sched/core.c:8294 Illegal context switch in RCU-bh read-side
critical section!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
4 locks held by kworker/4:0/1997:
 #0: ffff88810bd72d48 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at:
process_one_work+0x761/0x1440
 #1: ffff888105c8fe00 ((addr_chk_work).work){+.+.}-{0:0}, at:
process_one_work+0x795/0x1440
 #2: ffffffffb9279fb0 (rtnl_mutex){+.+.}-{3:3}, at:
addrconf_verify_work+0xa/0x20
 #3: ffffffffb8e30860 (rcu_read_lock_bh){....}-{1:2}, at:
addrconf_verify_rtnl+0x23/0xc60

stack backtrace:
CPU: 4 PID: 1997 Comm: kworker/4:0 Not tainted 5.12.0-rc6+ #515
Workqueue: ipv6_addrconf addrconf_verify_work
Call Trace:
 dump_stack+0xa4/0xe5
 ___might_sleep+0x27d/0x2b0
 __mutex_lock+0xc8/0x13f0
 ? lock_downgrade+0x690/0x690
 ? __ipv6_dev_mc_dec+0x49/0x2a0
 ? mark_held_locks+0xb7/0x120
 ? mutex_lock_io_nested+0x1270/0x1270
 ? lockdep_hardirqs_on_prepare+0x12c/0x3e0
 ? _raw_spin_unlock_irqrestore+0x47/0x50
 ? trace_hardirqs_on+0x41/0x120
 ? __wake_up_common_lock+0xc9/0x100
 ? __wake_up_common+0x620/0x620
 ? memset+0x1f/0x40
 ? netlink_broadcast_filtered+0x2c4/0xa70
 ? __ipv6_dev_mc_dec+0x49/0x2a0
 __ipv6_dev_mc_dec+0x49/0x2a0
 ? netlink_broadcast_filtered+0x2f6/0xa70
 addrconf_leave_solict.part.64+0xad/0xf0
 ? addrconf_join_solict.part.63+0xf0/0xf0
 ? nlmsg_notify+0x63/0x1b0
 __ipv6_ifa_notify+0x22c/0x9c0
 ? inet6_fill_ifaddr+0xbe0/0xbe0
 ? lockdep_hardirqs_on_prepare+0x12c/0x3e0
 ? __local_bh_enable_ip+0xa5/0xf0
 ? ipv6_del_addr+0x347/0x870
 ipv6_del_addr+0x3b1/0x870
 ? addrconf_ifdown+0xfe0/0xfe0
 ? rcu_read_lock_any_held.part.27+0x20/0x20
 addrconf_verify_rtnl+0x8a9/0xc60
 addrconf_verify_work+0xf/0x20
 process_one_work+0x84c/0x1440

In order to avoid this problem, it uses rcu_read_unlock_bh() for
a short time. RCU is used for avoiding freeing
ifp(struct *inet6_ifaddr) while ifp is being used. But this will
not be released even if rcu_read_unlock_bh() is used.
Because before rcu_read_unlock_bh(), it uses in6_ifa_hold(ifp).
So this is safe.

Fixes: 63ed8de4 ("mld: add mc_lock for protecting per-interface mld data")
Suggested-by: Eric Dumazet <edumazet@google.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>