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SubjectWARNING: refcount bug in smc_tcp_listen_work

syzbot found the following crash on:

HEAD commit: 75d4e704fa8d netdev-FAQ: clarify DaveM's position for stab..
git tree: net-next
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:

A link change request failed with some changes committed already. Interface
bridge0 may have been left with an inconsistent configuration, please check.
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 23870 at lib/refcount.c:187
refcount_sub_and_test+0x2d3/0x330 lib/refcount.c:187
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 23870 Comm: kworker/0:4 Not tainted 4.17.0-rc7+ #79
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: events smc_tcp_listen_work
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1b9/0x294 lib/dump_stack.c:113
panic+0x22f/0x4de kernel/panic.c:184
__warn.cold.8+0x163/0x1b3 kernel/panic.c:536
report_bug+0x252/0x2d0 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:178 [inline]
do_error_trap+0x1de/0x490 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:refcount_sub_and_test+0x2d3/0x330 lib/refcount.c:187
RSP: 0018:ffff880194aaf4f8 EFLAGS: 00010286
RAX: 0000000000000026 RBX: 0000000000000000 RCX: ffffffff8160b8ad
RDX: 0000000000000000 RSI: ffffffff81610561 RDI: ffff880194aaf058
RBP: ffff880194aaf5e0 R08: ffff88014148e500 R09: 0000000000000006
R10: ffff88014148e500 R11: 0000000000000000 R12: 00000000ffffffff
R13: ffff880194aaf5b8 R14: 0000000000000001 R15: ffff880194aaf728
refcount_dec_and_test+0x1a/0x20 lib/refcount.c:212
sock_put include/net/sock.h:1668 [inline]
smc_tcp_listen_work+0xb94/0xec0 net/smc/af_smc.c:1073
process_one_work+0xc1e/0x1b50 kernel/workqueue.c:2145
worker_thread+0x1cc/0x1440 kernel/workqueue.c:2279
kthread+0x345/0x410 kernel/kthread.c:240
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

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