Comment 0 for bug 1636517

if a zvol of an existing, already imported zpool is a vdev of another zpool, a call to "zpool import" will everything zfs related. the stack trace is as follows:

[<ffffffffc038d374>] taskq_wait+0x74/0xe0 [spl]
[<ffffffffc038d42b>] taskq_destroy+0x4b/0x100 [spl]
[<ffffffffc04a4afd>] vdev_open_children+0x12d/0x180 [zfs]
[<ffffffffc04ae6cc>] vdev_root_open+0x3c/0xc0 [zfs]
[<ffffffffc04a45f5>] vdev_open+0xf5/0x4d0 [zfs]
[<ffffffffc048f11e>] spa_load+0x39e/0x1c60 [zfs]
[<ffffffffc049170d>] spa_tryimport+0xad/0x450 [zfs]
[<ffffffffc04c42d4>] zfs_ioc_pool_tryimport+0x64/0xa0 [zfs]
[<ffffffffc04c770b>] zfsdev_ioctl+0x44b/0x4e0 [zfs]
[<ffffffff8122124f>] do_vfs_ioctl+0x29f/0x490
[<ffffffff812214b9>] SyS_ioctl+0x79/0x90
[<ffffffff818318b2>] entry_SYSCALL_64_fastpath+0x16/0x71
[<ffffffffffffffff>] 0xffffffffffffffff

I traced this back to 193fb6a2c94fab8eb8ce70a5da4d21c7d4023bee (merged in 4.4.0-6.21), which added a second parameter to lookup_bdev without patching the zfs module (which needs to special case the vdev-on-zvol case, and uses this exact method only in this special casing code path).

attached you can find the output of "zfs send -R" ing such a zvol ("brokenvol.raw"), running "zfs receive POOL/TARGET < FILE" followed by "zpool import" should reproduce the hang.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-45-generic 4.4.0-45.66
ProcVersionSignature: Ubuntu 4.4.0-45.66-generic 4.4.21
Uname: Linux 4.4.0-45-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Oct 25 15:46 seq
 crw-rw---- 1 root audio 116, 33 Oct 25 15:46 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
Date: Tue Oct 25 15:49:51 2016
HibernationDevice: RESUME=/dev/mapper/xenial--vg-swap_1
InstallationDate: Installed on 2016-10-25 (0 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb: Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
PciMultimedia:

ProcFB: 0 qxldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-45-generic root=/dev/mapper/hostname--vg-root ro
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-45-generic N/A
 linux-backports-modules-4.4.0-45-generic N/A
 linux-firmware 1.157.4
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-2.7
dmi.modalias: dmi:bvnSeaBIOS:bvrrel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-2.7:cvnQEMU:ct1:cvrpc-i440fx-2.7:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-2.7
dmi.sys.vendor: QEMU