WARNING: suspicious RCU usage in fib6

Kernel	Title	Repro	Cause bisect	Fix bisect	Count	Last	Reported	Patched	Status
android-414	WARNING: suspicious RCU usage in fib6_del	C			35	1753d	1984d	0/1	public: reported C repro on 2019/04/11 00:00
upstream	WARNING: suspicious RCU usage in fib6_del net				1	1784d	1784d	0/28	closed as invalid on 2019/11/19 23:24

============================= WARNING: suspicious RCU usage 5.9.0-rc4-syzkaller #0 Not tainted ----------------------------- net/ipv6/ip6_fib.c:1996 suspicious rcu_dereference_protected() usage! other info that might help us debug this: rcu_scheduler_active = 2, debug_locks = 1 4 locks held by syz-executor.5/8095: #0: ffffffff8a7ea708 (rtnl_mutex){+.+.}-{3:3}, at: ppp_release+0x178/0x240 drivers/net/ppp/ppp_generic.c:401 #1: ffff88804c422dd8 (&net->ipv6.fib6_gc_lock){+.-.}-{2:2}, at: spin_trylock_bh include/linux/spinlock.h:414 [inline] #1: ffff88804c422dd8 (&net->ipv6.fib6_gc_lock){+.-.}-{2:2}, at: fib6_run_gc+0x21b/0x2d0 net/ipv6/ip6_fib.c:2312 #2: ffffffff89bd6a40 (rcu_read_lock){....}-{1:2}, at: __fib6_clean_all+0x0/0x290 net/ipv6/ip6_fib.c:2613 #3: ffff8880a82e6430 (&tb->tb6_lock){+.-.}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:359 [inline] #3: ffff8880a82e6430 (&tb->tb6_lock){+.-.}-{2:2}, at: __fib6_clean_all+0x107/0x290 net/ipv6/ip6_fib.c:2245 stack backtrace: CPU: 1 PID: 8095 Comm: syz-executor.5 Not tainted 5.9.0-rc4-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:77 [inline] dump_stack+0x198/0x1fd lib/dump_stack.c:118 fib6_del+0x12b4/0x1630 net/ipv6/ip6_fib.c:1996 fib6_clean_node+0x39b/0x570 net/ipv6/ip6_fib.c:2180 fib6_walk_continue+0x4aa/0x8e0 net/ipv6/ip6_fib.c:2102 fib6_walk+0x182/0x370 net/ipv6/ip6_fib.c:2150 fib6_clean_tree+0xdb/0x120 net/ipv6/ip6_fib.c:2230 __fib6_clean_all+0x120/0x290 net/ipv6/ip6_fib.c:2246 fib6_clean_all net/ipv6/ip6_fib.c:2257 [inline] fib6_run_gc+0x113/0x2d0 net/ipv6/ip6_fib.c:2320 ndisc_netdev_event+0x217/0x350 net/ipv6/ndisc.c:1805 notifier_call_chain+0xb5/0x200 kernel/notifier.c:83 call_netdevice_notifiers_info+0xb5/0x130 net/core/dev.c:2033 call_netdevice_notifiers_extack net/core/dev.c:2045 [inline] call_netdevice_notifiers net/core/dev.c:2059 [inline] dev_close_many+0x30b/0x650 net/core/dev.c:1634 rollback_registered_many+0x3a8/0x1210 net/core/dev.c:9261 rollback_registered net/core/dev.c:9329 [inline] unregister_netdevice_queue+0x2dd/0x570 net/core/dev.c:10410 unregister_netdevice include/linux/netdevice.h:2774 [inline] ppp_release+0x216/0x240 drivers/net/ppp/ppp_generic.c:403 __fput+0x285/0x920 fs/file_table.c:281 task_work_run+0xdd/0x190 kernel/task_work.c:141 tracehook_notify_resume include/linux/tracehook.h:188 [inline] exit_to_user_mode_loop kernel/entry/common.c:163 [inline] exit_to_user_mode_prepare+0x1e1/0x200 kernel/entry/common.c:190 syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:265 entry_SYSCALL_64_after_hwframe+0x44/0xa9 RIP: 0033:0x416f01 Code: 75 14 b8 03 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 04 1b 00 00 c3 48 83 ec 08 e8 0a fc ff ff 48 89 04 24 b8 03 00 00 00 0f 05 <48> 8b 3c 24 48 89 c2 e8 53 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01 RSP: 002b:000000000169fbe0 EFLAGS: 00000293 ORIG_RAX: 0000000000000003 RAX: 0000000000000000 RBX: 0000000000000006 RCX: 0000000000416f01 RDX: 0000000000000000 RSI: 000000000000068f RDI: 0000000000000005 RBP: 0000000000000001 R08: 00000000e63be68f R09: 00000000e63be693 R10: 000000000169fcd0 R11: 0000000000000293 R12: 0000000001190fd8 R13: 00000000002a4495 R14: ffffffffffffffff R15: 000000000118cf4c