syzbot reminded me that rt6_probe() can run from
atomic contexts.

stack backtrace:

CPU: 1 PID: 7461 Comm: syz-executor.2 Not tainted 5.16.0-rc4-next-20211210-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_usage_bug kernel/locking/lockdep.c:203 [inline]
 valid_state kernel/locking/lockdep.c:3945 [inline]
 mark_lock_irq kernel/locking/lockdep.c:4148 [inline]
 mark_lock.cold+0x61/0x8e kernel/locking/lockdep.c:4605
 mark_usage kernel/locking/lockdep.c:4500 [inline]
 __lock_acquire+0x11d5/0x54a0 kernel/locking/lockdep.c:4981
 lock_acquire kernel/locking/lockdep.c:5639 [inline]
 lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5604
 __fs_reclaim_acquire mm/page_alloc.c:4550 [inline]
 fs_reclaim_acquire+0x115/0x160 mm/page_alloc.c:4564
 might_alloc include/linux/sched/mm.h:253 [inline]
 slab_pre_alloc_hook mm/slab.h:739 [inline]
 slab_alloc_node mm/slub.c:3145 [inline]
 slab_alloc mm/slub.c:3239 [inline]
 kmem_cache_alloc_trace+0x3b/0x2c0 mm/slub.c:3256
 kmalloc include/linux/slab.h:581 [inline]
 kzalloc include/linux/slab.h:715 [inline]
 ref_tracker_alloc+0xe1/0x430 lib/ref_tracker.c:74
 netdev_tracker_alloc include/linux/netdevice.h:3860 [inline]
 dev_hold_track include/linux/netdevice.h:3877 [inline]
 rt6_probe net/ipv6/route.c:661 [inline]
 find_match.part.0+0xac9/0xd00 net/ipv6/route.c:752
 find_match net/ipv6/route.c:825 [inline]
 __find_rr_leaf+0x17f/0xd20 net/ipv6/route.c:826
 find_rr_leaf net/ipv6/route.c:847 [inline]
 rt6_select net/ipv6/route.c:891 [inline]
 fib6_table_lookup+0x649/0xa20 net/ipv6/route.c:2185
 ip6_pol_route+0x1c5/0x11e0 net/ipv6/route.c:2221
 pol_lookup_func include/net/ip6_fib.h:580 [inline]
 fib6_rule_lookup+0x52a/0x6f0 net/ipv6/fib6_rules.c:120
 ip6_route_output_flags_noref+0x2e2/0x380 net/ipv6/route.c:2629
 ip6_route_output_flags+0x72/0x320 net/ipv6/route.c:2642
 ip6_route_output include/net/ip6_route.h:98 [inline]
 ip6_dst_lookup_tail+0x5ab/0x1620 net/ipv6/ip6_output.c:1070
 ip6_dst_lookup_flow+0x8c/0x1d0 net/ipv6/ip6_output.c:1200
 geneve_get_v6_dst+0x46f/0x9a0 drivers/net/geneve.c:858
 geneve6_xmit_skb drivers/net/geneve.c:991 [inline]
 geneve_xmit+0x520/0x3530 drivers/net/geneve.c:1074
 __netdev_start_xmit include/linux/netdevice.h:4685 [inline]
 netdev_start_xmit include/linux/netdevice.h:4699 [inline]
 xmit_one net/core/dev.c:3473 [inline]
 dev_hard_start_xmit+0x1eb/0x920 net/core/dev.c:3489
 __dev_queue_xmit+0x2983/0x3640 net/core/dev.c:4112
 neigh_resolve_output net/core/neighbour.c:1522 [inline]
 neigh_resolve_output+0x50e/0x820 net/core/neighbour.c:1502
 neigh_output include/net/neighbour.h:541 [inline]
 ip6_finish_output2+0x56e/0x14f0 net/ipv6/ip6_output.c:126
 __ip6_finish_output net/ipv6/ip6_output.c:191 [inline]
 __ip6_finish_output+0x61e/0xe80 net/ipv6/ip6_output.c:170
 ip6_finish_output+0x32/0x200 net/ipv6/ip6_output.c:201
 NF_HOOK_COND include/linux/netfilter.h:296 [inline]
 ip6_output+0x1e4/0x530 net/ipv6/ip6_output.c:224
 dst_output include/net/dst.h:451 [inline]
 NF_HOOK include/linux/netfilter.h:307 [inline]
 ndisc_send_skb+0xa99/0x17f0 net/ipv6/ndisc.c:508
 ndisc_send_rs+0x12e/0x6f0 net/ipv6/ndisc.c:702
 addrconf_rs_timer+0x3f2/0x820 net/ipv6/addrconf.c:3898
 call_timer_fn+0x1a5/0x6b0 kernel/time/timer.c:1421
 expire_timers kernel/time/timer.c:1466 [inline]
 __run_timers.part.0+0x675/0xa20 kernel/time/timer.c:1734
 __run_timers kernel/time/timer.c:1715 [inline]
 run_timer_softirq+0xb3/0x1d0 kernel/time/timer.c:1747
 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x123/0x180 kernel/softirq.c:637
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:649
 sysvec_apic_timer_interrupt+0x93/0xc0 arch/x86/kernel/apic/apic.c:1097
 </IRQ>

Fixes: fb67510b ("ipv6: add net device refcount tracker to rt6_probe_deferred()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Link: https://lore.kernel.org/r/20211214025806.3456382-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>