WARNING in __flush_work

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    39ad1c1b6bb8 Add linux-next specific files for 20190129
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=135e382f400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a2b2e9c0bc43c14d
dashboard link: https://syzkaller.appspot.com/bug?extid=ba2a929dcf8e704c180e
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17cc2298c00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=176c96ef400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ba2a929dcf8e704c180e@xxxxxxxxxxxxxxxxxxxxxxxxx

WARNING: CPU: 1 PID: 3472 at kernel/workqueue.c:2911  
__flush_work+0x880/0xa60 kernel/workqueue.c:2911
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 3472 Comm: kworker/1:2 Not tainted 5.0.0-rc4-next-20190129 #21
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: md_misc mddev_delayed_delete
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1db/0x2d0 lib/dump_stack.c:113
 panic+0x2cb/0x65c kernel/panic.c:214
 __warn.cold+0x20/0x48 kernel/panic.c:571
 report_bug+0x263/0x2b0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 fixup_bug arch/x86/kernel/traps.c:173 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:290
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
RIP: 0010:__flush_work+0x880/0xa60 kernel/workqueue.c:2911
Code: 74 58 e8 93 54 2e 00 fb 66 0f 1f 44 00 00 45 31 e4 e9 7d fd ff ff e8  
7f 54 2e 00 0f 0b 45 31 e4 e9 6e fd ff ff e8 70 54 2e 00 <0f> 0b 45 31 e4  
e9 5f fd ff ff e8 61 54 2e 00 4c 89 ff 45 31 e4 e8
RSP: 0018:ffff88809acb7558 EFLAGS: 00010293
RAX: ffff88809acc4580 RBX: ffff88809acb76e8 RCX: ffffffff8153bde9
RDX: 0000000000000000 RSI: ffffffff8153c5c0 RDI: 0000000000000001
RBP: ffff88809acb7710 R08: ffff88809acc4580 R09: ffff88809acc4e48
R10: ffff88809acc4580 R11: 0000000000000000 R12: ffff88809dd24628
R13: dffffc0000000000 R14: 0000000000000001 R15: 0000000000000001
 __cancel_work_timer+0x48d/0x810 kernel/workqueue.c:3007
 cancel_work_sync+0x18/0x20 kernel/workqueue.c:3043
 blk_sync_queue+0x33/0x190 block/blk-core.c:234
 blk_cleanup_queue+0x100/0x200 block/blk-core.c:367
 md_free+0xcb/0x1b0 drivers/md/md.c:5215
 kobject_cleanup lib/kobject.c:662 [inline]
 kobject_release lib/kobject.c:691 [inline]
 kref_put include/linux/kref.h:67 [inline]
 kobject_put.cold+0x28f/0x2ec lib/kobject.c:708
 mddev_delayed_delete+0x34/0x40 drivers/md/md.c:5243
 process_one_work+0xd0c/0x1ce0 kernel/workqueue.c:2153
 worker_thread+0x143/0x14a0 kernel/workqueue.c:2296
 kthread+0x357/0x430 kernel/kthread.c:247
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches