# bash -x test-bcache-byuuid-links.sh
+ mountpoint /mnt
/mnt is not a mountpoint
+ '[' -e /dev/bcache0 ']'
+ true
++ ls /sys/class/block/bcache0/bcache/stop
ls: cannot access '/sys/class/block/bcache0/bcache/stop': No such file or directory
+ bdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ true
++ ls '/sys/fs/bcache/*-*-*-*/stop'
ls: cannot access '/sys/fs/bcache/*-*-*-*/stop': No such file or directory
+ cdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdc
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdb
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ make-bcache --wipe-bcache -C /dev/vdc -B /dev/vdb
UUID:			02ec19a2-1eb3-4cb5-8825-365e2118b946
Set UUID:		d08c9f67-0b4a-4cda-95cc-dd8606ff446a
version:		0
nbuckets:		20480
block_size:		1
bucket_size:		1024
nr_in_set:		1
nr_this_dev:		0
first_bucket:		1
UUID:			5da87108-3a1c-4f16-8079-57f604b227e3
Set UUID:		d08c9f67-0b4a-4cda-95cc-dd8606ff446a
version:		1
block_size:		1
data_offset:		16
+ udevadm settle --exit-if-exists /dev/bcache0
+ '[' '!' -e /dev/bcache/by-uuid ']'
+ ls -al /dev/bcache/by-uuid/
total 0
drwxr-xr-x 2 root root 60 Apr  9 16:56 .
drwxr-xr-x 3 root root 60 Apr  9 16:56 ..
lrwxrwxrwx 1 root root 13 Apr  9 16:56 5da87108-3a1c-4f16-8079-57f604b227e3 -> ../../bcache0
+ echo 'Creating filesystems on bcache0'
Creating filesystems on bcache0
+ mkfs.ext4 /dev/bcache0
mke2fs 1.44.1 (24-Mar-2018)
Discarding device blocks: done
Creating filesystem with 2621438 4k blocks and 655360 inodes
Filesystem UUID: 5e3bcd5f-1c9d-4d10-b702-a25cf485463f
Superblock backups stored on blocks:
	32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

+ echo 'mounting bcache0 to /mnt'
mounting bcache0 to /mnt
+ mount /dev/bcache0 /mnt
+ udevadm settle --exit-if-exists /dev/bcache0
+ '[' '!' -e /dev/bcache/by-uuid ']'
+ ls -al /dev/bcache/by-uuid/
total 0
drwxr-xr-x 2 root root 60 Apr  9 16:56 .
drwxr-xr-x 3 root root 60 Apr  9 16:56 ..
lrwxrwxrwx 1 root root 13 Apr  9 16:56 5da87108-3a1c-4f16-8079-57f604b227e3 -> ../../bcache0
+ echo 'Everything OK'
Everything OK
+ exit 0
root@ubuntu:~# uname -r
4.15.0-20-generic

[   36.522788] bcache: run_cache_set() invalidating existing data
[   36.528680] bcache: register_cache() registered cache device vdc
[   36.529034] bcache: register_bdev() registered backing device vdb
[   36.532237] bcache: bch_cached_dev_attach() Caching vdb as bcache0 on set d08c9f67-0b4a-4cda-95cc-dd8606ff446a
[   36.777614] EXT4-fs (bcache0): mounted filesystem with ordered data mode. Opts: (null)


Second invocation:

root@ubuntu:~# bash -x test-bcache-byuuid-links.sh
+ mountpoint /mnt
/mnt is a mountpoint
+ umount -f /mnt
+ '[' -e /dev/bcache0 ']'
+ wipefs -a /dev/bcache0
/dev/bcache0: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
+ true
++ ls /sys/class/block/bcache0/bcache/stop
+ bdev_stop=/sys/class/block/bcache0/bcache/stop
+ '[' -n /sys/class/block/bcache0/bcache/stop -a -e /sys/class/block/bcache0/bcache/stop ']'
+ echo 'writing 1 to /sys/class/block/bcache0/bcache/stop'
writing 1 to /sys/class/block/bcache0/bcache/stop
+ echo 1
+ udevadm settle
+ sleep 1
+ true
++ ls /sys/class/block/bcache0/bcache/stop
ls: cannot access '/sys/class/block/bcache0/bcache/stop': No such file or directory
+ bdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ true
++ ls /sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop
+ cdev_stop=/sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop
+ '[' -n /sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop -a -e /sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop ']'
+ echo 'writing 1 to /sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop'
writing 1 to /sys/fs/bcache/d08c9f67-0b4a-4cda-95cc-dd8606ff446a/stop
+ echo 1
+ udevadm settle
+ sleep 1
+ true
++ ls '/sys/fs/bcache/*-*-*-*/stop'
ls: cannot access '/sys/fs/bcache/*-*-*-*/stop': No such file or directory
+ cdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdc
/dev/vdc: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdb
/dev/vdb: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ make-bcache --wipe-bcache -C /dev/vdc -B /dev/vdb
UUID:			e864408d-a9a7-4157-bf8e-b2c4c509d159
Set UUID:		89539960-f362-4c45-a5cb-169a6699afa1
version:		0
nbuckets:		20480
block_size:		1
bucket_size:		1024
nr_in_set:		1
nr_this_dev:		0
first_bucket:		1
UUID:			15f11834-3d90-4b38-b5c1-290ac132f350
Set UUID:		89539960-f362-4c45-a5cb-169a6699afa1
version:		1
block_size:		1
data_offset:		16
+ udevadm settle --exit-if-exists /dev/bcache0
+ '[' '!' -e /dev/bcache/by-uuid ']'
+ echo 'bcache0 present but /dev/bcache/by-uuid does not exist!'
bcache0 present but /dev/bcache/by-uuid does not exist!
+ exit 1

[  135.318976] bcache: bcache_device_free() bcache0 stopped
[  136.366439] bcache: cache_set_free() Cache set d08c9f67-0b4a-4cda-95cc-dd8606ff446a unregistered
[  137.478375] bcache: run_cache_set() invalidating existing data
[  137.484237] bcache: register_cache() registered cache device vdc

Notice the missing cached_dev_attach()  once the two bind, the kernel emits
the uevent which triggers the rules to generate the /dev/bcache/by-uuid
symlinks.

Third invocation:
# bash -x test-bcache-byuuid-links.sh
+ mountpoint /mnt
/mnt is not a mountpoint
+ '[' -e /dev/bcache0 ']'
+ true
++ ls /sys/class/block/bcache0/bcache/stop
ls: cannot access '/sys/class/block/bcache0/bcache/stop': No such file or directory
+ bdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ true
++ ls /sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop
+ cdev_stop=/sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop
+ '[' -n /sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop -a -e /sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop ']'
+ echo 'writing 1 to /sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop'
writing 1 to /sys/fs/bcache/89539960-f362-4c45-a5cb-169a6699afa1/stop
+ echo 1
+ udevadm settle
+ sleep 1
+ true
++ ls '/sys/fs/bcache/*-*-*-*/stop'
ls: cannot access '/sys/fs/bcache/*-*-*-*/stop': No such file or directory
+ cdev_stop=
+ '[' -n '' -a -e '' ']'
+ break
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdc
/dev/vdc: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ (( x=0 ))
+ (( x<50 ))
+ wipefs -a /dev/vdb
/dev/vdb: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+ rc=0
+ '[' 0 = 0 ']'
+ break
+ '[' 0 '!=' 0 ']'
+ make-bcache --wipe-bcache -C /dev/vdc -B /dev/vdb
UUID:			34afb042-e7ad-4953-9d72-761d71060fc1
Set UUID:		0eeec346-4cd1-49fe-91b4-6a04db5bef91
version:		0
nbuckets:		20480
block_size:		1
bucket_size:		1024
nr_in_set:		1
nr_this_dev:		0
first_bucket:		1
UUID:			b3a2e43f-667f-4870-950a-dea03e155116
Set UUID:		0eeec346-4cd1-49fe-91b4-6a04db5bef91
version:		1
block_size:		1
data_offset:		16
+ udevadm settle --exit-if-exists /dev/bcache0
+ '[' '!' -e /dev/bcache/by-uuid ']'
+ ls -al /dev/bcache/by-uuid/
total 0
drwxr-xr-x 2 root root 60 Apr  9 16:59 .
drwxr-xr-x 3 root root 60 Apr  9 16:59 ..
lrwxrwxrwx 1 root root 13 Apr  9 16:59 b3a2e43f-667f-4870-950a-dea03e155116 -> ../../bcache0
+ echo 'Creating filesystems on bcache0'
Creating filesystems on bcache0
+ mkfs.ext4 /dev/bcache0
mke2fs 1.44.1 (24-Mar-2018)
Discarding device blocks: done
Creating filesystem with 2621438 4k blocks and 655360 inodes
Filesystem UUID: 012e6ca9-c2b0-4a47-8631-67f2de7755cc
Superblock backups stored on blocks:
	32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

+ echo 'mounting bcache0 to /mnt'
mounting bcache0 to /mnt
+ mount /dev/bcache0 /mnt
+ udevadm settle --exit-if-exists /dev/bcache0
+ '[' '!' -e /dev/bcache/by-uuid ']'
+ ls -al /dev/bcache/by-uuid/
total 0
drwxr-xr-x 2 root root 60 Apr  9 16:59 .
drwxr-xr-x 3 root root 60 Apr  9 16:59 ..
lrwxrwxrwx 1 root root 13 Apr  9 16:59 b3a2e43f-667f-4870-950a-dea03e155116 -> ../../bcache0
+ echo 'Everything OK'
Everything OK
+ exit 0

[  235.637197] bcache: cache_set_free() Cache set 89539960-f362-4c45-a5cb-169a6699afa1 unregistered
[  236.773186] bcache: run_cache_set() invalidating existing data
[  236.778364] bcache: register_cache() registered cache device vdc
[  236.779896] bcache: register_bdev() registered backing device vdb
[  236.783588] bcache: bch_cached_dev_attach() Caching vdb as bcache0 on set 0eeec346-4cd1-49fe-91b4-6a04db5bef91
[  237.017778] EXT4-fs (bcache0): mounted filesystem with ordered data mode. Opts: (null)