Hello,

syzbot hit the following crash on net-next commit
f1c02cfb7b30f5c9d9f4e383260abf89d89c3341 (Wed Feb 28 15:40:08 2018 +0000)
ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses

So far this crash happened 3 times on net-next, upstream.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+e51e2a7401ab556ffde1@syzkaller.appspotmail.com
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IPv6: ADDRCONF(NETDEV_UP): veth0: link is not ready
IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
TCP: request_sock_TCP: Possible SYN flooding on port 20003. Sending  
cookies.  Check SNMP counters.

============================================
WARNING: possible recursive locking detected
4.16.0-rc2+ #247 Not tainted
--------------------------------------------
syzkaller232072/4280 is trying to acquire lock:
  (sk_lock-AF_SMC){+.+.}, at: [<000000003c81ac0a>] lock_sock  
include/net/sock.h:1464 [inline]
  (sk_lock-AF_SMC){+.+.}, at: [<000000003c81ac0a>]  
smc_close_non_accepted+0x1d/0x370 net/smc/af_smc.c:664

but task is already holding lock:
  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>] lock_sock  
include/net/sock.h:1464 [inline]
  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>]  
smc_close_wait_listen_clcsock net/smc/smc_close.c:47 [inline]
  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>]  
smc_close_active+0x8d9/0x11f0 net/smc/smc_close.c:207

other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(sk_lock-AF_SMC);
   lock(sk_lock-AF_SMC);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

1 lock held by syzkaller232072/4280:
  #0:  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>] lock_sock  
include/net/sock.h:1464 [inline]
  #0:  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>]  
smc_close_wait_listen_clcsock net/smc/smc_close.c:47 [inline]
  #0:  (sk_lock-AF_SMC){+.+.}, at: [<00000000952086ed>]  
smc_close_active+0x8d9/0x11f0 net/smc/smc_close.c:207

stack backtrace:
CPU: 0 PID: 4280 Comm: syzkaller232072 Not tainted 4.16.0-rc2+ #247
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
  print_deadlock_bug kernel/locking/lockdep.c:1761 [inline]
  check_deadlock kernel/locking/lockdep.c:1805 [inline]
  validate_chain kernel/locking/lockdep.c:2401 [inline]
  __lock_acquire+0xe8f/0x3e00 kernel/locking/lockdep.c:3431
  lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
  lock_sock_nested+0xc2/0x110 net/core/sock.c:2781
  lock_sock include/net/sock.h:1464 [inline]
  smc_close_non_accepted+0x1d/0x370 net/smc/af_smc.c:664
  smc_close_cleanup_listen net/smc/smc_close.c:30 [inline]
  smc_close_active+0x9aa/0x11f0 net/smc/smc_close.c:209
  smc_release+0x379/0x580 net/smc/af_smc.c:125
  sock_release+0x8d/0x1e0 net/socket.c:594
  sock_close+0x16/0x20 net/socket.c:1149
  __fput+0x327/0x7e0 fs/file_table.c:209
  ____fput+0x15/0x20 fs/file_table.c:243
  task_work_run+0x199/0x270 kernel/task_work.c:113
  exit_task_work include/linux/task_work.h:22 [inline]
  do_exit+0x9bb/0x1ad0 kernel/exit.c:865
  do_group_exit+0x149/0x400 kernel/exit.c:968
  get_signal+0x73a/0x16d0 kernel/signal.c:2469
  do_signal+0x90/0x1e90 arch/x86/kernel/signal.c:809
  exit_to_usermode_loop+0x258/0x2f0 arch/x86/entry/common.c:162
  prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
  syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
  do_syscall_64+0x6e5/0x940 arch/x86/entry/common.c:292
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x446bc9
RSP: 002b:00007fb04b3eed58 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00000000006dcc24 RCX: 0000000000446bc9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000006dcc24
RBP: 0000000000000000 R08: 0000000020000080 R09: 0000000020000080
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc20
R13: 00007ffd2d6a6f2f R14: 00007fb04b3ef9c0 R15: 0000000000000005


---
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