BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: __list_del_entry_valid+0x29/0x90 lib/list_debug.c:48
PGD 0 P4D 0 
Oops: 0000 [#1] SMP
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 21 Comm: kworker/u4:1 Not tainted 4.15.0-rc3-next-20171214+ #67
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
RIP: 0010:__list_del_entry_valid+0x29/0x90 lib/list_debug.c:48
RSP: 0018:ffffc90000d17608 EFLAGS: 00010007
RAX: 0000000000000000 RBX: ffff8801fa282400 RCX: dead000000000200
RDX: 0000000000000000 RSI: ffff88021534ae10 RDI: ffff8801fa282400
RBP: ffffc90000d17608 R08: 0000000d1b5f97d5 R09: 000000000000000e
R10: ffffc90000d175d8 R11: 0000000000000000 R12: ffff880215690180
R13: ffff88021534ae10 R14: ffff88021534ae10 R15: ffff8802152b9000
FS:  0000000000000000(0000) GS:ffff88021fd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000000301e003 CR4: 00000000001606e0
Call Trace:
 __list_del_entry include/linux/list.h:117 [inline]
 list_del_init include/linux/list.h:159 [inline]
 cfq_remove_request+0x33/0x200 block/cfq-iosched.c:2501
 cfq_dispatch_insert+0x5a/0x150 block/cfq-iosched.c:2993
 cfq_dispatch_request block/cfq-iosched.c:3510 [inline]
 cfq_dispatch_requests+0x4a9/0x1130 block/cfq-iosched.c:3544
 elv_next_request block/blk-core.c:2697 [inline]
 blk_peek_request+0x298/0x500 block/blk-core.c:2723
 scsi_request_fn+0x3b/0x7a0 drivers/scsi/scsi_lib.c:1796
 __blk_run_queue_uncond block/blk-core.c:409 [inline]
 __blk_run_queue+0x83/0x120 block/blk-core.c:429
 queue_unplugged+0x76/0x230 block/blk-core.c:3488
 blk_flush_plug_list+0x2fb/0x3a0 block/blk-core.c:3600
 blk_queue_bio+0x2bb/0x900 block/blk-core.c:1990
 generic_make_request+0x13e/0x500 block/blk-core.c:2303
 submit_bio+0x9a/0x180 block/blk-core.c:2411
 ext4_io_submit+0x5e/0x80 fs/ext4/page-io.c:356
 ext4_writepages+0x672/0x1760 fs/ext4/inode.c:2829
 do_writepages+0x42/0x110 mm/page-writeback.c:2340
 __writeback_single_inode+0x75/0x950 fs/fs-writeback.c:1322
 writeback_sb_inodes+0x289/0x660 fs/fs-writeback.c:1586
 __writeback_inodes_wb+0x77/0xe0 fs/fs-writeback.c:1655
 wb_writeback+0x44e/0x690 fs/fs-writeback.c:1764
 wb_check_start_all fs/fs-writeback.c:1888 [inline]
 wb_do_writeback fs/fs-writeback.c:1914 [inline]
 wb_workfn+0x4f9/0x820 fs/fs-writeback.c:1948
 process_one_work+0x288/0x7a0 kernel/workqueue.c:2112
 worker_thread+0x43/0x4d0 kernel/workqueue.c:2246
 kthread+0x149/0x170 kernel/kthread.c:238
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:524
Code: 00 00 55 48 8b 07 48 b9 00 01 00 00 00 00 ad de 48 8b 57 08 48 89 e5 48 39 c8 74 27 48 b9 00 02 00 00 00 00 ad de 48 39 ca 74 2c <48> 8b 32 48 39 fe 75 35 48 8b 50 08 48 39 f2 75 40 b8 01 00 00 
RIP: __list_del_entry_valid+0x29/0x90 lib/list_debug.c:48 RSP: ffffc90000d17608
CR2: 0000000000000000
---[ end trace 0b7a577353770dbb ]---