WARNING in __device_add_disk

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
87ef12027b9b1dd0e0b12cf311fbcb19f9d92539 (Wed Apr 18 19:48:17 2018 +0000)
Merge tag 'ceph-for-4.17-rc2' of git://github.com/ceph/ceph-client
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=3337db851ace689ceb50

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5356500264419328
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=1808800213120130118
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3337db851ace689ceb50@xxxxxxxxxxxxxxxxxxxxxxxxx
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.

RAX: ffffffffffffffda RBX: 00007fee9fefc6d4 RCX: 0000000000455329
RDX: 0000000020000000 RSI: 0000000000004c80 RDI: 0000000000000014
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000015
R13: 00000000000002a2 R14: 00000000006f6fd0 R15: 0000000000000036
WARNING: CPU: 1 PID: 30128 at block/genhd.c:685  
__device_add_disk+0x104a/0x1340 block/genhd.c:685
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 30128 Comm: syz-executor3 Not tainted 4.17.0-rc1+ #8
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+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:__device_add_disk+0x104a/0x1340 block/genhd.c:685
RSP: 0018:ffff8801b320f8a8 EFLAGS: 00010246
RAX: 0000000000040000 RBX: ffff8801d7fbe880 RCX: ffffc90007714000
RDX: 0000000000040000 RSI: ffffffff83434d6a RDI: 0000000000000005
RBP: ffff8801b320fa78 R08: ffff8801a9364400 R09: ffffed003b5e46c2
R10: ffffed003b5e46c2 R11: ffff8801daf23613 R12: 00000000fffffff4
R13: ffff8801b320fa50 R14: ffff8801d7fbede0 R15: ffff8801d7fbe884
 device_add_disk+0x22/0x30 block/genhd.c:705
 add_disk include/linux/genhd.h:397 [inline]
 loop_add+0x70b/0x9c0 drivers/block/loop.c:1873
 loop_control_ioctl+0x178/0x500 drivers/block/loop.c:1970
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x1cf/0x16a0 fs/ioctl.c:684
 ksys_ioctl+0xa9/0xd0 fs/ioctl.c:701
 __do_sys_ioctl fs/ioctl.c:708 [inline]
 __se_sys_ioctl fs/ioctl.c:706 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:706
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455329
RSP: 002b:00007fee9fefbc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fee9fefc6d4 RCX: 0000000000455329
RDX: 0000000020000000 RSI: 0000000000004c80 RDI: 0000000000000014
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000015
R13: 00000000000002a2 R14: 00000000006f6fd0 R15: 0000000000000036
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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