reload-snapshot on already tracked device results in oops

[crawfxrd]

Steps to reproduce:

dbdctl setup-snapshot /dev/loop7 /tmp/dattobd/ext4/cow.snap 1
umount /dev/loop7
dbdctl reload-snapshot /dev/loop7 /cow.snap 0
mount /dev/loop7 /tmp/dattobd/ext4

Important that the minor for reload-snapshot is less than the minor for setup-snapshot.

Jun 14 12:33:04 fedora-23-srv-std kernel: datto: detected block device umount: /tmp/dattobd/ext4 : 0
Jun 14 12:33:04 fedora-23-srv-std kernel: datto: block device umount detected for device 1
Jun 14 12:33:04 fedora-23-srv-std kernel: datto: stopping cow thread
Jun 14 12:33:04 fedora-23-srv-std kernel: datto: destroying cow manager
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: post umount check succeeded
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: umount returned: 0
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: ioctl command received: 1075876098
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: received reload snap ioctl - 0 : /dev/loop7 : /cow.snap
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: allocating device struct
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: initializing tracer
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: minor range = 0 - 1
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: detected block device mount: /dev/loop7 -> /tmp/dattobd/ext4 : 0
Jun 14 12:33:05 fedora-23-srv-std kernel: EXT4-fs (loop7): mounted filesystem with ordered data mode. Opts: (null)
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: block device mount detected for unverified device 0
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: finding block device
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: checking block device is not already being traced
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: block device is already being traced: -22
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: error setting up base block device: -22
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: freeing base block device
Jun 14 12:33:05 fedora-23-srv-std kernel: datto: error transitioning snapshot tracer to active state: -22
Jun 14 12:33:05 fedora-23-srv-std kernel: BUG: unable to handle kernel paging request at ffffed3592708420
Jun 14 12:33:05 fedora-23-srv-std kernel: IP: [<ffffffff8121a447>] kfree+0x57/0x170
Jun 14 12:33:05 fedora-23-srv-std kernel: PGD 0 
Jun 14 12:33:05 fedora-23-srv-std kernel: Oops: 0000 [#1] SMP 
Jun 14 12:33:05 fedora-23-srv-std kernel: Modules linked in: dattobd(OE) loop ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfn
Jun 14 12:33:05 fedora-23-srv-std kernel: CPU: 1 PID: 1844 Comm: mount Tainted: G           OE   4.5.5-201.fc23.x86_64 #1
Jun 14 12:33:05 fedora-23-srv-std kernel: Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
Jun 14 12:33:05 fedora-23-srv-std kernel: task: ffff8800de04db80 ti: ffff8800dc0fc000 task.ti: ffff8800dc0fc000
Jun 14 12:33:05 fedora-23-srv-std kernel: RIP: 0010:[<ffffffff8121a447>]  [<ffffffff8121a447>] kfree+0x57/0x170
Jun 14 12:33:05 fedora-23-srv-std kernel: RSP: 0018:ffff8800dc0ffe18  EFLAGS: 00010286
Jun 14 12:33:05 fedora-23-srv-std kernel: RAX: 000077ff80000000 RBX: 000055649c210250 RCX: 000000000000e193
Jun 14 12:33:05 fedora-23-srv-std kernel: RDX: 000000000000e192 RSI: ffff88011fd1a1c0 RDI: ffffea0000000000
Jun 14 12:33:05 fedora-23-srv-std kernel: RBP: ffff8800dc0ffe30 R08: 000000000001a1c0 R09: ffffffffa0408ac4
Jun 14 12:33:05 fedora-23-srv-std kernel: R10: ffffed3592708400 R11: 00000000ffffffea R12: 0000000000000000
Jun 14 12:33:05 fedora-23-srv-std kernel: R13: ffffffffa0408851 R14: ffff88000252ca70 R15: ffff8800de02d200
Jun 14 12:33:05 fedora-23-srv-std kernel: FS:  00007ffadc86c840(0000) GS:ffff88011fd00000(0000) knlGS:0000000000000000
Jun 14 12:33:05 fedora-23-srv-std kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Jun 14 12:33:05 fedora-23-srv-std kernel: CR2: ffffed3592708420 CR3: 00000000023dd000 CR4: 00000000000406e0
Jun 14 12:33:05 fedora-23-srv-std kernel: Stack:
Jun 14 12:33:05 fedora-23-srv-std kernel:  000055649c210250 0000000000000000 ffff88000252c990 ffff8800dc0ffea0
Jun 14 12:33:05 fedora-23-srv-std kernel:  ffffffffa0408851 ffffffff811af587 0000000000000010 ffff8800dc0ffeb0
Jun 14 12:33:05 fedora-23-srv-std kernel:  ffffffeadc0ffe68 0000000000000000 000055649c210250 0000000032172a4b
Jun 14 12:33:05 fedora-23-srv-std kernel: Call Trace:
Jun 14 12:33:05 fedora-23-srv-std kernel:  [<ffffffffa0408851>] auto_transition_active+0x171/0x4a0 [dattobd]
Jun 14 12:33:05 fedora-23-srv-std kernel:  [<ffffffff811af587>] ? printk+0x57/0x73
Jun 14 12:33:05 fedora-23-srv-std kernel:  [<ffffffffa0408e07>] handle_bdev_mount_event+0x287/0x2c0 [dattobd]
Jun 14 12:33:05 fedora-23-srv-std kernel:  [<ffffffffa040908a>] mount_hook+0xaa/0x150 [dattobd]
Jun 14 12:33:05 fedora-23-srv-std kernel:  [<ffffffff817c90ee>] entry_SYSCALL_64_fastpath+0x12/0x71
Jun 14 12:33:05 fedora-23-srv-std kernel: Code: 00 00 00 80 ff 77 00 00 48 bf 00 00 00 00 00 ea ff ff 49 01 da 48 0f 42 05 d7 6b 9f 00 49 01 c
Jun 14 12:33:05 fedora-23-srv-std kernel: RIP  [<ffffffff8121a447>] kfree+0x57/0x170
Jun 14 12:33:05 fedora-23-srv-std kernel:  RSP <ffff8800dc0ffe18>
Jun 14 12:33:05 fedora-23-srv-std kernel: CR2: ffffed3592708420
Jun 14 12:33:05 fedora-23-srv-std kernel: ---[ end trace ddee4fa1131710ed ]---