INFO: task hung in fib6_rules_net_exit

[syzbot <syzbot+69bf48842d5035daa7a6@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on net-next commit
18845557fd6fc1998f2d0d8c30467f86db587529 (Thu Mar 29 20:24:06 2018 +0000)
Merge tag 'wireless-drivers-next-for-davem-2018-03-29' of  
git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/wireless-drivers-next
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=69bf48842d5035daa7a6

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5281628851011584
Kernel config: https://syzkaller.appspot.com/x/.config?id=-37309782588693906
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+69bf48842d5035daa7a6@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

netlink: 8 bytes leftover after parsing attributes in process  
`syz-executor5'.
IPVS: stopping backup sync thread 7398 ...
INFO: task kworker/u4:3:163 blocked for more than 120 seconds.
       Not tainted 4.16.0-rc6+ #286
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:3    D19440   163      2 0x80000000
Workqueue: netns cleanup_net
Call Trace:
  context_switch kernel/sched/core.c:2862 [inline]
  __schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
  schedule+0xf5/0x430 kernel/sched/core.c:3499
  schedule_preempt_disabled+0x10/0x20 kernel/sched/core.c:3557
  __mutex_lock_common kernel/locking/mutex.c:833 [inline]
  __mutex_lock+0xaad/0x1a80 kernel/locking/mutex.c:893
  mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
  rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
  fib6_rules_net_exit+0x12/0x50 net/ipv6/fib6_rules.c:392
  ops_exit_list.isra.6+0xae/0x150 net/core/net_namespace.c:151
  cleanup_net+0x4d8/0xa90 net/core/net_namespace.c:522
  process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
  worker_thread+0x223/0x1990 kernel/workqueue.c:2247
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
INFO: lockdep is turned off.
NMI backtrace for cpu 1
CPU: 1 PID: 868 Comm: khungtaskd Not tainted 4.16.0-rc6+ #286
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x24d lib/dump_stack.c:53
  nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
  nmi_trigger_cpumask_backtrace+0x123/0x180 lib/nmi_backtrace.c:62
  arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
  trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
  check_hung_task kernel/hung_task.c:132 [inline]
  check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
  watchdog+0x90c/0xd60 kernel/hung_task.c:249
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0x6/0x10  
arch/x86/include/asm/irqflags.h:54
Kernel panic - not syncing: hung_task: blocked tasks
CPU: 1 PID: 868 Comm: khungtaskd Not tainted 4.16.0-rc6+ #286
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x24d lib/dump_stack.c:53
  panic+0x1e4/0x41c kernel/panic.c:183
  check_hung_task kernel/hung_task.c:133 [inline]
  check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
  watchdog+0x91d/0xd60 kernel/hung_task.c:249
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzkaller@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is  
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug  
report.
Note: all commands must start from beginning of the line in the email body.