Comment 5 for bug 1837788

Tested with bionic-proposed (problem exists)
---

# uname -rv
4.15.0-56-generic #62-Ubuntu SMP Wed Jul 24 20:18:55 UTC 2019

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
[ 39.096888] bcache: register_bdev() registered backing device loop0/null 2>&1
[ 39.109511] bcache: run_cache_set() invalidating existing data
[ 39.121256] bcache: register_cache() registered cache device loop1
[ 43.083569] WARNING: CPU: 3 PID: 1207 at /build/linux-dHA1gi/linux-4.15.0/drivers/md/bcache/writeback.c:624 bch_cached_dev_writeback_sta$
t+0x13f/0x1b0 [bcache]
[ 43.092279] Modules linked in: bcache isofs nls_iso8859_1 kvm_intel kvm irqbypass input_leds joydev serio_raw sch_fq_codel ib_iser rdma_$
m iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 as$
nc_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear psmouse virtio_blk virtio_net f$
oppy
[ 43.107715] CPU: 3 PID: 1207 Comm: setup-bcache-wb Not tainted 4.15.0-56-generic #62-Ubuntu
[ 43.111108] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 43.114797] RIP: 0010:bch_cached_dev_writeback_start+0x13f/0x1b0 [bcache]
[ 43.117589] RSP: 0018:ffffbcb4c13a3d28 EFLAGS: 00010283
[ 43.119762] RAX: ffff9e8171f74080 RBX: ffff9e8172610000 RCX: 0000000000000000
[ 43.122664] RDX: 0000000000000000 RSI: ffff9e817fda70a0 RDI: ffff9e8177003800
[ 43.125572] RBP: ffffbcb4c13a3d30 R08: 00000000000270a0 R09: ffffffff826afc1d
[ 43.128590] R10: 0000000000000000 R11: 0000000000000224 R12: ffff9e8172610010
[ 43.131574] R13: ffff9e8172610000 R14: ffff9e8172060000 R15: ffff9e8172610ac8
[ 43.134538] FS: 00007f2bb42dc540(0000) GS:ffff9e817fd80000(0000) knlGS:0000000000000000
[ 43.137983] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 43.140430] CR2: 00005608572738f0 CR3: 0000000235020000 CR4: 00000000000006e0
[ 43.143389] Call Trace:
[ 43.144575] bch_cached_dev_attach+0x335/0x4f0 [bcache]
[ 43.146835] __cached_dev_store+0x749/0x7e0 [bcache]
[ 43.148950] ? __cached_dev_store+0x749/0x7e0 [bcache]
[ 43.151144] bch_cached_dev_store+0x39/0xc0 [bcache]
[ 43.153266] sysfs_kf_write+0x3c/0x50
[ 43.154892] kernfs_fop_write+0x125/0x1a0
[ 43.156682] __vfs_write+0x1b/0x40
[ 43.158211] vfs_write+0xb1/0x1a0
[ 43.159710] SyS_write+0x55/0xc0
[ 43.161167] do_syscall_64+0x73/0x130
[ 43.162772] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 43.164891] RIP: 0033:0x7f2bb3ddc154
[ 43.166471] RSP: 002b:00007fffaa0a1628 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 43.169675] RAX: ffffffffffffffda RBX: 00005608572717f0 RCX: 00007f2bb3ddc154
[ 43.172573] RDX: 0000000000000025 RSI: 00005608572717f0 RDI: 0000000000000001
[ 43.175461] RBP: 0000000000000025 R08: 000000000000f000 R09: 000056085727394d
[ 43.178356] R10: 00000000000001b6 R11: 0000000000000246 R12: 0000000000000001
[ 43.180809] R13: 0000000000000025 R14: 7fffffffffffffff R15: 00007fffaa0a2f39
[ 43.182978] Code: 0a 00 00 0f 88 85 00 00 00 75 18 48 8b 35 22 8f 4d c3 48 8d 93 a0 0a 00 00 bf 00 20 00 00 e8 49 16 f2 c1 8b 83 80 0b 00
 00 eb a1 <0f> 0b e9 5c ff ff ff b8 f4 ff ff ff eb 93 49 8d 8c 24 88 0a 00
[ 43.188541] ---[ end trace 6c8b3c91f79be271 ]---
[ 43.191546] bcache: bch_cached_dev_attach() Caching loop0 as bcache0 on set 0bd9f71d-b21f-4b5b-b4e0-dbb4e1126138

# reboot
# # comment last line in script.

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
#
[ 62.048472] bcache: register_bdev() registered backing device loop0
[ 62.081965] bcache: run_cache_set() invalidating existing data
[ 62.101912] bcache: register_cache() registered cache device loop1
(wait a few seconds)
[ 70.108124] BUG: unable to handle kernel NULL pointer dereference at 0000000000000340
[ 70.112057] IP: update_writeback_rate+0x36/0x2d0 [bcache]
[ 70.114648] PGD 0 P4D 0
[ 70.116007] Oops: 0000 [#1] SMP PTI
[ 70.117768] Modules linked in: bcache isofs nls_iso8859_1 kvm_intel kvm irqbypass joydev input_leds serio_raw sch_fq_codel ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear psmouse virtio_blk virtio_net fl
oppy
[ 70.128731] CPU: 3 PID: 52 Comm: kworker/3:1 Not tainted 4.15.0-56-generic #62-Ubuntu
[ 70.131046] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 70.133671] Workqueue: events update_writeback_rate [bcache]
[ 70.135369] RIP: 0010:update_writeback_rate+0x36/0x2d0 [bcache]
[ 70.137113] RSP: 0018:ffffb88440e17e48 EFLAGS: 00010202
[ 70.138792] RAX: 0000000000000018 RBX: ffff9c50b2650b08 RCX: ffff9c50bfda2160
[ 70.140988] RDX: ffff9c50bfda2160 RSI: 00000000000000c0 RDI: ffff9c50b2650b08
[ 70.143194] RBP: ffffb88440e17e78 R08: 0000000000000129 R09: 0000000000000000
[ 70.145367] R10: 0000000000000000 R11: 00000000003d0900 R12: ffff9c50b63d76c0
[ 70.147543] R13: ffff9c50bfda2140 R14: 0000000000000000 R15: 0000000000000000
[ 70.149708] FS: 0000000000000000(0000) GS:ffff9c50bfd80000(0000) knlGS:0000000000000000
[ 70.152267] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 70.154081] CR2: 0000000000000340 CR3: 0000000226e0a000 CR4: 00000000000006e0
[ 70.156247] Call Trace:
[ 70.157159] process_one_work+0x1de/0x420
[ 70.158494] worker_thread+0x32/0x410
[ 70.159735] kthread+0x121/0x140
[ 70.160853] ? process_one_work+0x420/0x420
[ 70.162218] ? kthread_create_worker_on_cpu+0x70/0x70
[ 70.163839] ret_from_fork+0x35/0x40
[ 70.165036] Code: 41 56 41 55 41 54 53 48 83 ec 08 4c 8b b7 78 f5 ff ff f0 80 8f 98 f5 ff ff 10 f0 83 44 24 fc 00 48 8b 87 98 f5 ff ff a8
 08 74 0b <49> 8b 86 40 03 00 00 a8 08 74 1d f0 80 a7 98 f5 ff ff ef f0 83
[ 70.170597] RIP: update_writeback_rate+0x36/0x2d0 [bcache] RSP: ffffb88440e17e48
[ 70.173842] CR2: 0000000000000340
[ 70.175672] ---[ end trace d16a2c8bfba43bf7 ]---