WARNING: bad unlock balance in mptcp_shutdown

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    f5e94d10 Merge tag 'drm-next-2020-04-08' of git://anongit...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17a5dbfbe00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ca75979eeebf06c2
dashboard link: https://syzkaller.appspot.com/bug?extid=6ebb6d4830e8f8815623
compiler:       gcc (GCC) 9.0.0 20181231 (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:
Reported-by: syzbot+6ebb6d4830e8f8815623@xxxxxxxxxxxxxxxxxxxxxxxxx

=====================================
WARNING: bad unlock balance detected!
5.6.0-syzkaller #0 Not tainted
-------------------------------------
syz-executor.5/2215 is trying to release lock (sk_lock-AF_INET6) at:
[<ffffffff87c5203b>] mptcp_shutdown+0x38b/0x550 net/mptcp/protocol.c:1889
but there are no more locks to release!

other info that might help us debug this:
1 lock held by syz-executor.5/2215:
 #0: ffff88804a22eda0 (slock-AF_INET6){+.-.}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:358 [inline]
 #0: ffff88804a22eda0 (slock-AF_INET6){+.-.}-{2:2}, at: release_sock+0x1b/0x1b0 net/core/sock.c:2974

stack backtrace:
CPU: 0 PID: 2215 Comm: syz-executor.5 Not tainted 5.6.0-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+0x188/0x20d lib/dump_stack.c:118
 __lock_release kernel/locking/lockdep.c:4633 [inline]
 lock_release+0x586/0x800 kernel/locking/lockdep.c:4941
 sock_release_ownership include/net/sock.h:1539 [inline]
 release_sock+0x177/0x1b0 net/core/sock.c:2984
 mptcp_shutdown+0x38b/0x550 net/mptcp/protocol.c:1889
 __sys_shutdown+0xf3/0x1a0 net/socket.c:2208
 __do_sys_shutdown net/socket.c:2216 [inline]
 __se_sys_shutdown net/socket.c:2214 [inline]
 __x64_sys_shutdown+0x50/0x70 net/socket.c:2214
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x49/0xb3
RIP: 0033:0x45c889
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fa67df32c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000030
RAX: ffffffffffffffda RBX: 00007fa67df336d4 RCX: 000000000045c889
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 000000000076bfa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000b5e R14: 00000000004cd960 R15: 000000000076bfac


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