Comment 6 for bug 1837788

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

# uname -rv
5.0.0-22-generic #23-Ubuntu SMP Tue Jul 23 17:23:54 UTC 2019

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
[ 57.992319] bcache: register_bdev() registered backing device loop0/null 2>&1
[ 57.997331] bcache: run_cache_set() invalidating existing data
[ 58.009201] bcache: register_cache() registered cache device loop1
[ 58.009232] bcache: register_bcache() error /dev/loop0: device already registered (emitting change event)
[ 62.008954] WARNING: CPU: 2 PID: 1294 at drivers/md/bcache/writeback.c:837 bch_cached_dev_writeback_start+0x156/0x164 [bcache]
[ 62.014280] Modules linked in: bcache crc64 isofs nls_iso8859_1 kvm_intel kvm irqbypass input_leds joydev 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 raid$
56 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear psmouse virtio_blk virtio$
net net_failover failover floppy
[ 62.027201] CPU: 2 PID: 1294 Comm: setup-bcache-wb Not tainted 5.0.0-22-generic #23-Ubuntu
[ 62.029611] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 62.032236] RIP: 0010:bch_cached_dev_writeback_start+0x156/0x164 [bcache]
[ 62.034227] Code: 88 4c 0d 00 00 74 09 8b 83 e0 0c 00 00 5b 5d c3 48 8b 35 35 ee b6 e0 48 8d 93 08 0c 00 00 bf 00 20 00 00 e8 7c 81 3e d$
 eb dd <0f> 0b e9 55 ff ff ff b8 f4 ff ff ff eb 8c 48 c7 c7 e0 e1 8c c0 e8
[ 62.039605] RSP: 0018:ffffae25012a3d38 EFLAGS: 00010207
[ 62.042135] RAX: ffff8ae1b1e915c0 RBX: ffff8ae1b20d0000 RCX: 0000000000000000
[ 62.045616] RDX: 0000000000000000 RSI: ffff8ae1b7b270a0 RDI: ffff8ae1b7403800
[ 62.049086] RBP: ffffae25012a3d40 R08: 00000000000270a0 R09: ffffffff9fcbc4cb
[ 62.052583] R10: ffffd2e708ce0bc0 R11: 0000000000000001 R12: ffff8ae1b20d0010
[ 62.056077] R13: ffff8ae1b20d0000 R14: ffff8ae1b20d0c30 R15: ffff8ae1b2180000
[ 62.059568] FS: 00007f1d4770e540(0000) GS:ffff8ae1b7b00000(0000) knlGS:0000000000000000
[ 62.063629] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 62.066534] CR2: 0000557518fb58f0 CR3: 00000002333fa000 CR4: 00000000000006e0
[ 62.070048] Call Trace:
[ 62.071530] bch_cached_dev_attach+0x356/0x480 [bcache]
[ 62.074201] ? bch_keybuf_init+0x70/0x70 [bcache]
[ 62.076666] __cached_dev_store+0x59f/0x8b0 [bcache]
[ 62.079234] bch_cached_dev_store+0x39/0xc0 [bcache]
[ 62.081786] sysfs_kf_write+0x3b/0x40
[ 62.083785] kernfs_fop_write+0x12e/0x1b0
[ 62.085921] __vfs_write+0x1b/0x40
[ 62.087809] vfs_write+0xab/0x1b0
[ 62.089649] ksys_write+0x5c/0xd0
[ 62.091497] __x64_sys_write+0x1a/0x20
[ 62.093520] do_syscall_64+0x5a/0x110
[ 62.095523] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 62.098106] RIP: 0033:0x7f1d4720e154
[ 62.100071] Code: 89 02 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 8d 05 b1 07 2e 00 8b 00 85 c0 75 13 b8 01 00 00 00
 0f 05 <48> 3d 00 f0 ff ff 77 54 f3 c3 66 90 41 54 55 49 89 d4 53 48 89 f5
[ 62.108931] RSP: 002b:00007ffddf05ed58 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 62.112761] RAX: ffffffffffffffda RBX: 0000557518fb37f0 RCX: 00007f1d4720e154
[ 62.116233] RDX: 0000000000000025 RSI: 0000557518fb37f0 RDI: 0000000000000001
[ 62.119751] RBP: 0000000000000025 R08: 000000000000f000 R09: 0000557518fb594d
[ 62.123233] R10: 00000000000001b6 R11: 0000000000000246 R12: 0000000000000001
[ 62.126711] R13: 0000000000000025 R14: 7fffffffffffffff R15: 00007ffddf060f39
[ 62.130191] ---[ end trace c47ed79da668a296 ]---
[ 62.135336] bcache: bch_cached_dev_attach() Caching loop0 as bcache0 on set 7abc55ca-0c2d-44bb-ae1e-dd51a6028769

# reboot
# # comment last line in script.

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
#

[ 30.327906] bcache: register_bdev() registered backing device loop0
[ 30.397705] bcache: run_cache_set() invalidating existing data
[ 30.409402] bcache: register_cache() registered cache device loop1
(wait a few seconds)
[ 38.603598] BUG: unable to handle kernel NULL pointer dereference at 0000000000000340
[ 38.607376] #PF error: [normal kernel read fault]
[ 38.609705] PGD 0 P4D 0
[ 38.611045] Oops: 0000 [#1] SMP PTI
[ 38.612835] CPU: 2 PID: 53 Comm: kworker/2:1 Not tainted 5.0.0-22-generic #23-Ubuntu
[ 38.616264] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 38.618928] Workqueue: events update_writeback_rate [bcache]
[ 38.620658] RIP: 0010:update_writeback_rate+0x36/0x330 [bcache]
[ 38.622436] Code: 56 41 55 41 54 53 48 83 ec 08 4c 8b af 10 f4 ff ff f0 80 8f 30 f4 ff ff 10 f0 83 44 24 fc 00 48 8b 87 30 f4 ff ff a8 0$
 74 59 <49> 8b 85 40 03 00 00 48 c1 e8 03 83 e0 01 49 89 c4 75 46 8b 47 e8
[ 38.627822] RSP: 0018:ffffb6fac0e23e50 EFLAGS: 00010202
[ 38.629401] RAX: 0000000000000018 RBX: ffff9fb637b22700 RCX: ffff9fb637b22720
[ 38.631481] RDX: 0000000000000001 RSI: ffff9fb63740aeb0 RDI: ffff9fb632120c70
[ 38.633587] RBP: ffffb6fac0e23e80 R08: 0000000000000000 R09: 000073746e657665
[ 38.635792] R10: 8080808080808080 R11: 00000008e9a3614c R12: ffff9fb637b27700
[ 38.637995] R13: 0000000000000000 R14: ffff9fb632120c70 R15: 0ffff9fb637b2770
[ 38.640215] FS: 0000000000000000(0000) GS:ffff9fb637b00000(0000) knlGS:0000000000000000
[ 38.642770] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 38.644586] CR2: 0000000000000340 CR3: 0000000234358000 CR4: 00000000000006e0
[ 38.646768] Call Trace:
[ 38.647705] ? __schedule+0x2d8/0x840
[ 38.648961] process_one_work+0x20f/0x410
[ 38.650297] worker_thread+0x34/0x400
[ 38.651539] kthread+0x120/0x140
[ 38.652670] ? process_one_work+0x410/0x410
[ 38.654051] ? __kthread_parkme+0x70/0x70
[ 38.655386] ret_from_fork+0x35/0x40
[ 38.656617] Modules linked in: bcache crc64 isofs nls_iso8859_1 kvm_intel kvm irqbypass input_leds joydev 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 raid4
56 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear psmouse virtio_net floppy
virtio_blk net_failover failover
[ 38.668111] CR2: 0000000000000340
[ 38.669245] ---[ end trace 583df0fb8c5b15fe ]---
[ 38.670752] RIP: 0010:update_writeback_rate+0x36/0x330 [bcache]
[ 38.672617] Code: 56 41 55 41 54 53 48 83 ec 08 4c 8b af 10 f4 ff ff f0 80 8f 30 f4 ff ff 10 f0 83 44 24 fc 00 48 8b 87 30 f4 ff ff a8 08
 74 59 <49> 8b 85 40 03 00 00 48 c1 e8 03 83 e0 01 49 89 c4 75 46 8b 47 e8
[ 38.678121] RSP: 0018:ffffb6fac0e23e50 EFLAGS: 00010202
[ 38.679761] RAX: 0000000000000018 RBX: ffff9fb637b22700 RCX: ffff9fb637b22720
[ 38.683382] RDX: 0000000000000001 RSI: ffff9fb63740aeb0 RDI: ffff9fb632120c70
[ 38.686852] RBP: ffffb6fac0e23e80 R08: 0000000000000000 R09: 000073746e657665
[ 38.690348] R10: 8080808080808080 R11: 00000008e9a3614c R12: ffff9fb637b27700
[ 38.693839] R13: 0000000000000000 R14: ffff9fb632120c70 R15: 0ffff9fb637b2770
[ 38.697305] FS: 0000000000000000(0000) GS:ffff9fb637b00000(0000) knlGS:0000000000000000
[ 38.701376] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 38.704236] CR2: 0000000000000340 CR3: 0000000234358000 CR4: 00000000000006e0