===============================
[ INFO: suspicious RCU usage. ]
4.9.108-gb7d377b #50 Not tainted
-------------------------------
kernel/rcu/tree_plugin.h:678 Illegal synchronize_rcu() in RCU read-side critical section!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 0
2 locks held by udevd/7614:
 #0:  (rcu_read_lock){......}, at: [<ffffffff815a049d>] path_init+0xf4d/0x1580 include/linux/compiler.h:243
 #1:  (rcu_callback){......}, at: [<ffffffff8128777e>] __rcu_reclaim kernel/rcu/rcu.h:108 [inline]
 #1:  (rcu_callback){......}, at: [<ffffffff8128777e>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 #1:  (rcu_callback){......}, at: [<ffffffff8128777e>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 #1:  (rcu_callback){......}, at: [<ffffffff8128777e>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 #1:  (rcu_callback){......}, at: [<ffffffff8128777e>] rcu_process_callbacks+0x98e/0x12b0 kernel/rcu/tree.c:3037

stack backtrace:
CPU: 1 PID: 7614 Comm: udevd Not tainted 4.9.108-gb7d377b #50
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801db307d60 ffffffff81eb3e29 ffff8801d7253000 0000000000000000
 0000000000000002 ffffffff83c6e0e0 ffff8801d88de658 ffff8801db307d90
 ffffffff81425fc0 ffff8801d88de500 ffff8801d93fcc48 ffff8801d93fc448
Call Trace:
 <IRQ> [  117.972859]  [<ffffffff81eb3e29>] __dump_stack lib/dump_stack.c:15 [inline]
 <IRQ> [  117.972859]  [<ffffffff81eb3e29>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81425fc0>] lockdep_rcu_suspicious.cold.48+0x110/0x141 kernel/locking/lockdep.c:4455
 [<ffffffff81284018>] synchronize_rcu+0x68/0x90 kernel/rcu/tree_plugin.h:675
 [<ffffffff836bcb8a>] __l2tp_session_unhash+0x38a/0x520 net/l2tp/l2tp_core.c:1796
 [<ffffffff836bcf17>] l2tp_tunnel_closeall+0x1f7/0x350 net/l2tp/l2tp_core.c:1364
 [<ffffffff836bd7d2>] l2tp_tunnel_destruct+0x2f2/0x590 net/l2tp/l2tp_core.c:1324
 [<ffffffff83021095>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff8128769e>] __rcu_reclaim kernel/rcu/rcu.h:118 [inline]
 [<ffffffff8128769e>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 [<ffffffff8128769e>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 [<ffffffff8128769e>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 [<ffffffff8128769e>] rcu_process_callbacks+0x8ae/0x12b0 kernel/rcu/tree.c:3037
 [<ffffffff83a005bb>] __do_softirq+0x20b/0x937 kernel/softirq.c:284
 [<ffffffff8114b227>] invoke_softirq kernel/softirq.c:364 [inline]
 [<ffffffff8114b227>] irq_exit+0x147/0x190 kernel/softirq.c:405
 [<ffffffff839ff1a1>] exiting_irq arch/x86/include/asm/apic.h:659 [inline]
 [<ffffffff839ff1a1>] smp_apic_timer_interrupt+0x81/0xa0 arch/x86/kernel/apic/apic.c:960
 [<ffffffff839fb330>] apic_timer_interrupt+0xa0/0xb0 arch/x86/entry/entry_64.S:648
 <EOI> [  118.087857]  [<ffffffff815a0b6e>] ? walk_component+0x9e/0x1bb0 fs/namei.c:1774
 [<ffffffff815a2f09>] link_path_walk+0x889/0x1200 fs/namei.c:2120
 [<ffffffff815a38ce>] path_parentat.isra.38+0x4e/0x140 fs/namei.c:2338
 [<ffffffff815a9223>] filename_parentat.isra.53.part.54+0x173/0x3d0 fs/namei.c:2360
 [<ffffffff815aa783>] filename_parentat fs/namei.c:2357 [inline]
 [<ffffffff815aa783>] user_path_parent fs/namei.c:2597 [inline]
 [<ffffffff815aa783>] do_unlinkat+0x133/0x680 fs/namei.c:4054
 [<ffffffff815ac80a>] SYSC_unlink fs/namei.c:4128 [inline]
 [<ffffffff815ac80a>] SyS_unlink+0x1a/0x20 fs/namei.c:4126
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff839f99d3>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
ODEBUG: object ffff8801db307ce8 is NOT on stack ffff880191ad8000, but annotated
------------[ cut here ]------------
WARNING: CPU: 1 PID: 7614 at lib/debugobjects.c:303 debug_object_is_on_stack lib/debugobjects.c:303 [inline]
WARNING: CPU: 1 PID: 7614 at lib/debugobjects.c:303 __debug_object_init.cold.10+0x6b/0x2b0 lib/debugobjects.c:329